Chapter 19: Oxidative Phosphorylation and Photophosphorylation

Question 1

Oxidative phosphorylation and photophosphorylation are mechanistically similar in three respects

Answer 1

1. Both processes involve the flow of electrons through a chain of membrane-bound carriers.
2. The free energy made available by this “downhill” (exergonic) electron flow is coupled to the “uphill” transport of protons across a proton-impermeable membrane, conserving the free energy of fuel oxidation as a transmembrane electrochemical potential
3. The transmembrane flow of protons back down their concentration gradient through specific protein channels provides the free energy for synthesis of ATP, catalyzed by a membrane protein complex (ATP synthase) that couples proton flow to phosphorylation of ADP.

Question 2

Mitochondria

Answer 2

• like gram-negative bacteria, have two membranes
• outer membrane
  
  • readily permeable to small molecules (Mr ,5,000) and ions
  • molecules move through porins
• inner membrane
  
  • impermeable to most small molecules and ions, including protons (H+)
  • species that cross, do so through specific transporters
  • bears components of the respiratory chain and the ATP synthase
• mitochondrial matrix
  
  • enclosed by the inner membrane
  • contains all pathways of fuel oxidation except glycolysis, which takes place in the cytosol
    
    • pyruvate dehydrogenase complex
    • citric acid cycle enzymes
    • fatty acid β-oxidation pathway
    • amino acid oxidation pathways
  • specific transporters carry pyruvate, fatty acids, and amino acids or their α-keto derivatives into the matrix
  • ADP and Pi are transported into the matrix
  • ATP is transported out

Question 3

• Oxidative phosphorylation begins with the entry of electrons into the chain of electron carriers called the ______ _____
• Most of these electrons arise from the action of dehydrogenases that collect electrons from catabolic pathways and funnel them into universal electron acceptors— _____ _____ or _____ _____

Answer 3

• respiratory chain
• nicotinamide nucleotides (NAD+ or NADP+), flavin nucleotides (FMN or FAD)

Question 4

Nicotinamide nucleotide–linked dehydrogenases

Answer 4

• catalyze reversible reactions of the following general types
  
  • Reduced substrate + NAD⁺ ⇔ oxidized substrate + NADH⁺ + H⁺
  • Reduced substrate + NADP⁺ ⇔ oxidized substrate + NADPH⁺ + H⁺
• Some dehydrogenases are in the cytosol, others in mitochondria, others in both
• Process
  
  • removes two hydrogen atoms from their substrates
  • One is transferred as a hydride ion (:H^-) to NAD⁺
  • Second is released as H⁺ in the medium

Question 5

NADH and NADPH

Answer 5

• water-soluble electron carriers that associate reversibly with Nicotinamide nucleotide–linked dehydrogenases
• Cells maintain separate pools of NADPH and NADH, with different redox potentials through ratios
• NADH
  
  • carries e^- from catabolic (break down) reactions into the respiratory chain
  • holds the ratio of [reduced form]/[oxidized form] relatively low
• NADPH
  
  • supplies e^- to anabolic (building up) reactions
  • holds the ratio of [reduced form]/[oxidized form] relatively high
• e^- they carry can be shuttled across indirectly

Question 6

Flavoproteins

Answer 6

• contain a very tightly, sometimes covalently, bound flavin nucleotide, either FMN or FAD
• The oxidized flavin nucleotide can accept either one e^- (yielding the semiquinone form) or two (yielding FADH2 or FMNH2)
• e^- transfer occurs because the flavoprotein has a higher reduction potential than the compound oxidized
• standard reduction potential of a flavin nucleotide, unlike that of NAD or NADP, depends on the protein with which it is associated
• The flavin nucleotide should be considered part of the flavoprotein’s active site rather than a reactant or product in the electron-transfer reaction
• can serve as intermediates between reactions

Question 7

Electrons Pass through a Series of Membrane-Bound Carriers

The mitochondrial respiratory chain consists of a series of sequentially acting electron carriers, most of which are _____ proteins with _____ _____ capable of accepting and donating either _____ or _____ electrons.

Answer 7

• integral
• prosthetic groups
• one
• two

Question 8

Three types of electron transfers

Answer 8

• direct transfer of electrons, as in the reduction of Fe³⁺ to Fe²⁺
• transfer as a hydrogen atom: H⁺ + e^-
• transfer as a hydride ion: :H^-, which bears two electrons

Question 9

reducing equivalent

Answer 9

designates a single electron equivalent transferred in an oxidation-reduction reaction

Question 10

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain:

Answer 10

• a hydrophobic quinone (ubiquinone)
• two different types of iron-containing proteins (cytochromes and iron-sulfur proteins)

Question 11

Ubiquinone, coenzyme Q, Q

Answer 11

• a lipid-soluble benzoquinone with a long isoprenoid side chain
• can accept one electron to become the semiquinone radical ^•QH or two electrons to form ubiquinol QH₂
• like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor
• small and hydrophobic
• freely diffusible within the lipid bilayer of the inner mitochondrial membrane
• can shuttle reducing equivalents between other, less mobile electron carriers in the membrane
• because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement

Question 12

cytochromes

Answer 12

• proteins with characteristic strong absorption of visible light, due to their iron-containing heme prosthetic groups
• three classes of cytochromes distinguished by differences in their light-absorption spectra
• Each type of cytochrome in its reduced (Fe2⁺) state has three absorption bands in the visible range
  
  • a: 600 nm absorption bands
  • b: 560 nm absorption bands
  • c: 550 nm absorption bands
• heme cofactors of a and b cytochromes are tightly, but not covalently, bound
• hemes of c-type cytochromes are covalently attached through Cys residues
• Prosthetic groups of cytochromes
  • Each group consists of four five-membered, nitrogen-containing rings in a cyclic structure called a porphyrin
  • four nitrogen atoms with a central Fe ion, either Fe²⁺ or Fe³⁺
  • The conjugated double-bond system of the porphyrin ring has delocalized π electrons that are relatively easily excited by photons with the wavelengths of visible light, accounting for strong absorption by hemes (and related compounds) in the visible region of the spectrum
• the standard reduction potential of the heme iron atom depends on its interaction with protein side chains and is therefore different for each cytochrome
• integral proteins of the inner mitochondrial membrane
  
  • cytochrome c is an exemption:
    
    • a soluble protein
    • associates through electrostatic interactions with the outer surface of the inner membrane

Question 13

iron-sulfur proteins

Answer 13

• iron is present not in heme but in association with inorganic sulfur atoms or with the sulfur atoms of Cys residues in the protein, or both.
• These iron-sulfur (Fe-S) centers range from simple structures with a single Fe atom coordinated to four Cys —SH groups to more complex Fe-S centers with two or four Fe atoms
• Rieske iron-sulfur proteins are a variation: one Fe atom is coordinated to two His residues rather than two Cys residues
• participate in one-electron transfers
• one iron atom of the iron-sulfur cluster is oxidized or reduced
• At least eight Fe-S proteins function in mitochondrial electron transfer
• The reduction potential of Fe-S proteins varies from 20.65 V to 10.45 V, depending on the microenvironment of the iron within the protein

PDF pg 767

Question 14

in the overall reaction catalyzed by the mitochondrial respiratory chain, electrons move from

Answer 14

NADH, succinate, or some other primary electron donor through flavoproteins, ubiquinone, iron-sulfur proteins, and cytochromes, and finally to O₂

Question 15

electron carriers of the respiratory chain are organized into _____-_____ supramolecular complexes that can be physically separated

Answer 15

membrane-embedded

Question 16

• Complexes I and II catalyze e^- transfer to _____ from two different electron donors:
  
  • _____ (Complex I)
  • _____ (Complex II)
• Complex III carries e^- from reduced _____ to _____ _____
• Complex IV completes the sequence by transferring e^- from _____ to _____

Answer 16

• ubiquinone, NDH, succinate
• ubiquinone, cytochrome c
• cytochrome c, O₂

Question 17

Path of electrons from NADH, succinate, fatty acyl–CoA, and glycerol 3-phosphate to ubiquinone

• Ubiquinone (Q) is the point of entry for electrons derived from reactions in the cytosol, from _____ _____ _____, and from _____ _____ (in the citric acid cycle)
• Electrons from NADH pass through a flavoprotein with the cofactor ______ to a series of _____ centers (in Complex I) and then to Q.
• Electrons from succinate pass through a flavoprotein with the cofactor _____ and several _____ centers (in Complex II) on the way to Q.
• Glycerol 3-phosphate donates electrons to a flavoprotein on the _____ face of the inner mitochondrial membrane, to Q.
• Acyl-CoA dehydrogenase (the first enzyme of oxidation) transfers electrons to electron-transferring flavoprotein (_____), to _____ _____ _____ and then to Q

Answer 17

• fatty acid oxidation, succinate oxidation
• FMN, Fe-S
• FAD, Fe-S
• outer
• ETF, ETF : ubiquinone oxidoreductase

Question 18

Complex I: NADH to Ubiquinone

aka NADH:ubiquinone oxidoreductase or NADH dehydrogenase

Answer 18

• large enzyme composed of 42 different polypeptide chains
  
  • an FMN-containing flavoprotein
  • at least six ironsulfur centers
• L-shaped, with one arm of the L in the membrane and the other extending into the matrix
• Catalysis two process which are simultaneous and coupled
• Catalyic Processes
  
  1. exergonic transfer of hydride from NADH to ubiquinone and a proton from the matrix
     
     • transfers hydride ion from NADH to FMN
     • two e^- pass through a series of Fe-S centers to the Fe-S center N-2 in the matrix arm of the complex
     • e^- transfer from N-2 to ubiquinone on the membrane arm & forms QH₂
     • QH₂ diffuses into the lipid bilayer
  2. This electron transfer drives the endergonic expulsion of 4 protons from the matrix to the intermembrane space, per pair of e^-
• ^​a proton pump driven by the energy of e^- transfer
  
  • reaction catalyzed is vectorial: moves protons in a specific direction from one location (matrix → becomes negatively charged) to another (intermembrane space → becomes positively charged)
    
    • P for the positive side
    • N for the negative side
• NADH + 5H⁺_N + Q → NAD⁺ + QH₂ + 4H⁺_P
• PDF pg. 770

Question 19

Complex II: Succinate to Ubiquinone

Answer 19

• Complex II is also known as succinate dehydrogenase
• only membrane-bound enzyme in the citric acid cycle
• smaller and simpler than Complex I
• contains 5 prosthetic groups of two types and four different protein subunits
  
  • Subunits C and D
    
    • integral membrane proteins, each with 3 transmembrane helices (transmembrane)
    • contains
      
      • heme b
        
        • sandwiched between subunits C and D
        • not in the direct path of electron transfer
        • may reduce e^- “leaks” out of the system
      • binding site for ubiquinone
    • Subunit D
      
      • has 2 tightley bound phosphatidylethanolamine molecules
  • Subunits A and B
    
    • extend into matrix (cytoplasmic extension)
    • subunit A
      
      • binding site for succinate, behind FAD
    • Subunit B
      
      • three 2Fe-2S centers
      • ubiquinone is bound
• path of electron transfer
  
  • more than 40 Å long
  • individual electron-transfer distances dont’ exceed ≈ 11 Å
  • Electrons move (blue arrows)
    
    • from succinate to FAD
    • then through the three Fe-S centers
    • to ubiquinone
• PDF pg. 771

Question 20

• Other substrates for mitochondrial dehydrogenases pass electrons into the respiratory chain to ubiquinone, but not through Complex II. Descrive the other two pathways.
• The effect of each of these electron-transferring enzymes is to
• QH₂ from all these reactions is reoxidized by _____ _____

Answer 20

• Two ways e^- are passed to ubiquonne, bypassing Complex II

1. β oxidation of fatty acyl–CoA
   
   • catalyzed by the flavoprotein acyl-CoA dehydrogenase
   • transfers e^- from the substrate to FAD of the dehydrogenase
   • then to electron-transferring flavoprotein, ETF
   • ETF passes its e^- to ETF : ubiquinone oxidoreductase
   • ETF : ubiquinone oxidoreductase transfers e^- into the respiratory chain by reducing ubiquinone
2. Glycerol 3-phosphate
   
   • formed from glycerol released by triacylglycerol breakdown or by the reduction of dihydroxyacetone phosphate from glycolysis
   • oxidized by glycerol 3-phosphate dehydrogenase
     
     • a flavoprotein located on the outer face of the inner mitochondrial membrane
     • it channels e^- into the respiratory chain by reducing ubiquinone

• contribute to the pool of reduced ubiquinone
• Complex III

Question 21

Complex III: Ubiquinone to Cytochrome c

Answer 21

• also called cytochrome bc1 complex or ubiquinone : cytochrome c oxidoreductase
• couples the transfer of e^- from ubiquinol (QH₂) to cytochrome c with the vectorial transport of protons from the matrix to the intermembrane space
• structure
  
  • dimer of identical monomers, each with 11 different subunits
  • The functional core of each monomer is three subunits:
    
    • cytochrome b (green) with two hemes: bH bL
    • Rieske iron-sulfur protein (purple) with its 2Fe-2S centers
    • cytochrome c1 (blue) with its heme
  • cytochrome c1 and the Rieske iron-sulfur project from the P side and interact with cytochrome c (not part of the functional complex) in the intermembrane space
  • has two distinct binding sites for ubiquinone, QN and QP
  • interface between monomers forms two caverns
    
    • each containing a QP site from one monomer and a QN site from the other
    • ubiquinone intermediates
      
      • move w/in caverns from the matrix side of the membrane Qn to the intermembrane space Qp
      • it shuttles electrons and protons across the inner mitochondrial membrane

Question 22

Q cycle

Answer 22

• proposed for the passage of electrons and protons through the Complex III
• The net equation for the redox reactions of the Q cycle is
  
  • QH₂ + 2 cyt c₁ (oxidized) + 2H_N⁺ → + 2 cyt c₁ (reduced) + 4H_P⁺
• Pathway
  
  • On the N side of the membrane:
    
    • First stage:
      
      • Q on the N side is reduced to the semiquinone radical (^•Q^-), which moves back into position to accept another electron
    • Second stage:
      
      • Semiquinone radical, ^•Q^- is reduced to QH₂
      • reduction consumes 2 protons, taken from the matrix (N side)
      • reduction is catalyzed by the oxidation of QH₂ on the P side, see bullet in red.
  • _​On the P side of the membrane:
    
    • two molecules of QH₂ are oxidized to Q
    • Each QH2
      
      • releases 2H⁺ (protons), 4 protons in all, into the intermembrane space
      • donates 2 e^-
        
        • one (via Rieske Fe-S center) to cytochrome c1, near P side
        • one (via cytochrome b) to Q near N side, reducing it in two steps to QH₂, see bullet in red.
  • Reduced cyt c1 passes electrons one at a time to cyt c
  • cty c
    
    • soluble protein of the intermembrane space
    • After its single heme accepts the electron it dissociates and carries electrons to Complex IV
    • it donates the electron to a binuclear copper center
• net effect of the transfer:
  
  • uptake of two protons on the N side
  • release of four protons on the P side per pair of electrons passing through Complex III to cytochrome c
  • QH2 is oxidized to Q
  • 2 molecules of cytochrome c are reduced
  • protons are moved from the P side to the N side

Question 23

Complex IV: Cytochrome c to O2

Answer 23

• final step of the respiratory chain
• also called cytochrome oxidase
• carries electrons from cytochrome c to molecular oxygen, reducing it to H2O
• structure
  
  • a large enzyme (13 subunits; M_r 204,000) of the inner mitochondrial membrane
  • has four subunits in each of two identical units of a dimer
  • three subunits are critical to the function
  • Subunit I (yellow)
    
    • has two heme groups, a and a₃ near a single copper ion, CuB
    • Heme a3 and CuB form a binuclear Fe-Cu center
      
      • accepts electrons from heme a and transfers them to O₂ bound to heme a3
  • Subunit II (purple)
    
    • has a binuclear center, CuA
      
      • two Cu ions complexed with the —SH groups of two Cys residues
      • Cu ions (green spheres) share electrons equally.
      • When the center is reduced, the ions have the formal charges Cu¹⁺Cu¹⁺; when oxidized, Cu^1.5+Cu^1.5+
      • Six amino acid residues are ligands around the Cu ions: two His, two Cys, Glu, and Met
      • resembles the 2Fe-2S centers of iron-sulfur proteins
    • This binuclear center and cytochrome c–binding site are located in a domain that protrudes from the P side of the inner membrane, into the intermembrane space
  • Subunit III (blue)
    
    • essential for rapid proton movement through subunit II
  • subunit IV (green)
    
    • role is not yet known

Question 24

Path of electrons through Complex IV

Answer 24

• Pathway
  
  • Two molecules of reduced cytochrome c
    
    • each donate an e^- to the binuclear center CuA
  • e^- pass through heme a
  • e^- pass to the Fe-Cu center (heme a3 and CuB)
  • Oxygen binds to heme a3 and is reduced to its peroxy derivative O₂^2- by the two e^- from the Fe-Cu center
  • two more e^- from cytochrome c, converts O₂^2- to 2 molecules of water, consuming 4 “substrate” protons from the matrix
  • At the same time, four protons are pumped from the matrix to the intermebrane by an unknown mechanism
• For every four electrons passing through
  
  • four “substrate” H+ are consumed from the matrix (N side) when converting O₂ to 2H₂O.
  • energy of this redox reaction pumps one proton outward into the intermembrane space (P side) for each electron that passes through
    
    • adding to the electrochemical potential produced by redox-driven proton transport through Complexes I and III
  • overall reaction catalyzed
    
    • 4 cyt c (reduced) + 8H_N⁺ + O₂ → 4 cyt c (oxidized) + 4H_P⁺
• ^​This four-electron reduction of O2
  
  • involves redox centers that carry only one electron at a time
  • must occur without the release of incompletely reduced intermediates such as hydrogen peroxide or hydroxyl free radicals

Question 25

• in the intact mitochondrion, the respiratory complexes tightly associate with each other in the inner membrane to form _____, functional combinations of two or more different electron-transfer complexes
• _____, the lipid that is especially abundant in the inner mitochondrial membrane, may be critical to the integrity of respirasomes; its removal with detergents, or its absence in certain yeast mutants, results in defective mitochondrial electron transfer and a loss of affinity between the respiratory complexes.

Answer 25

• respirasomes
• Cardiolipin

Question 26

• The transfer of two electrons from NADH through the respiratory chain to molecular oxygen can be written as
• It is highly _____
• For each pair of electrons transferred to O2, _____ protons are pumped out by Complex I, _____ by Complex III, and _____ by Complex IV
• The vectorial equation for the process is

Answer 26

• NADH + H⁺ + ½O₂ → NAD⁺ + H₂O
• exergonic
• four, four, two
• NADH + 11H_N⁺ + ½O₂ → NAD⁺ + 10H_P⁺ + H₂O

Question 27

Proton-motive force

Answer 27

• The inner mitochondrial membrane separates two compartments of different [H⁺], resulting in differences in chemical concentration (ΔpH) and charge distribution (Δψ) across the membrane
• The net effect is the proton-motive force (ΔG), which can be calculated with:
  
  • ΔG = RT ln (C2/C1) + Z JΔψ
• energy stored in such a gradient has two components:
  
  • the chemical potential energy due to the difference in concentration of a chemical species (H⁺) in the two regions separated by the membrane
  • electrical potential energy that results from the separation of charge when a proton moves across the membrane without a counterion

Question 28

In actively respiring mitochondria, the measured Δψ is 0.15 to 0.20 V and the pH of the matrix is about 0.75 unit more _____ than that of the intermembrane space.

Answer 28

alkaline

Question 29

When protons flow spontaneously down their electrochemical gradient, energy is made available to do work. In mitochondria, chloroplasts, and aerobic bacteria, the electrochemical energy in the proton gradient drives the synthesis of ATP from _____ and _____

Answer 29

• ADP
• Pi

Question 30

Reactive Oxygen Species (ROS)

• The passage of electrons from QH₂ to Complex III and the passage of electrons from Complexes I and II to QH₂ involve the radical ^•Q^- as an intermediate which can, with a low probability, pass an electron to O2 in the reaction O₂ + e- → ^•O₂^- creating a ______ ______ _____ which is highly reactive
• its formation also leads to production of the even more reactive

Answer 30

• superoxide free radical
• hydroxyl free radical, ^•OH

Question 31

• formation of ROS is favored when two conditions are met:
• In these situations, the mitochondrion is under oxidative stress because

Answer 31

• two conditions
  
  • mitochondria are not making ATP (for lack of ADP or O2) and therefore have a large proton-motive force and a high ratio of QH2/Q
  • there is a high NADH/NAD⁺ ratio in the matrix
• more electrons are available to enter the respiratory chain than can be immediately passed through to oxygen

Question 32

Although overproduction of ROS is clearly detrimental, low levels of ROS may be used by the cell as a _____ reflecting the insufficient supply of oxygen (hypoxia)

Answer 32

• signal

Question 33

• To prevent oxidative damage by •O₂^-, cells have several forms of the enzyme _____ _____
• This enzyme catalyzes the reaction:
• The hydrogen peroxide generated is rendered harmless by the action of _____ ______
• Glutathione reductase recycles the oxidized glutathione to its reduced form, using electrons from the _____ generated by nicotinamide nucleotide transhydrogenase or by the pentose phosphate pathway

Answer 33

• superoxide dismutase
• 2 •O₂^- + 2H⁺ → H₂O₂ + O₂
• glutathione peroxidase
• NADPH

Question 34

• _____ _____ produces most of the ATP made in aerobic cells
• Complete oxidation of a molecule of glucose to CO₂ yields _____ or_____ ATP
• By comparison, glycolysis under anaerobic conditions (lactate fermentation) yields only _____ ATP per glucose

Answer 34

• Oxidative phosphorylation
• 30, 32
• 2

Question 35

_____ _____ pathways that result in electron transfer to O₂ accompanied by ______ _____ therefore account for the vast majority of the ATP produced in catabolism

Answer 35

• Aerobic oxidative
• oxidative phosphorylation

Question 36

Oxidative Phosphorylation Is Regulated by Cellular Energy Needs

• The rate of respiration (O2 consumption) in mitochondria is tightly regulated; it is generally limited by the availability of _____
• In some animal tissues, the _____ _____ _____, which is the ratio of the maximal rate of ADP-induced O₂ consumption to the basal rate in the absence of ADP, is at least 10
• Another, related measure is the _____ _____ _____ of the ATP-ADP system, [ATP]/([ADP][Pi]). Usually this ratio is very _____, so the ATP-ADP system is almost fully _____
• the [ATP]/([ADP][Pi]) ratio fluctuates _____ in most tissues, even during extreme variations in energy demand
• ATP is formed only as _____ as it is used in energy-requiring cellular activities

Answer 36

• ADP
• acceptor control ratio
• mass-action ratio, high, phosphorylated
• slightly
• fast

Question 37

An Inhibitory Protein Prevents ATP Hydrolysis during Hypoxia

• When a cell is hypoxic (deprived of oxygen), as in a heart attack or stroke, electron transfer to oxygen slows, and so does the pumping of protons. The _____ _____ soon collapses
• Under these conditions, the _____ _____ could operate in reverse, hydrolyzing ATP to pump protons outward and causing a disastrous drop in ATP levels
• This is prevented by a small protein inhibitor, _____, which binds to two _____ _____ molecules, inhibiting their ATPase activity
• IF1 is inhibitory only in its ______ form, which is favored at pH _____ than 6.5
• In a cell starved for oxygen, the main source of ATP becomes ______, and the pyruvic or lactic acid thus formed lowers the pH in the cytosol and the mitochondrial matrix. This favors IF1 ______, leading to inhibition of the ATPase activity of ATP synthase

Answer 37

• proton-motive force
• ATP synthase
• IF1, ATP synthase
• dimer, lower
• glycolysis, dimerization

Question 38

Hypoxia Leads to ROS Production and Several Adaptive Responses

• In hypoxic cells there is an imbalance between the input of electrons from fuel oxidation in the mitochondrial matrix and transfer of electrons to molecular oxygen leading to increased formation of _____ _____ _____
• In addition to the glutathione peroxidase system, cells have two other lines of defense against ROS:
• these two other lines of defense are mediated by _____ which accumulates in hypoxic cells, acts as a _____ _____, triggers increased synthesis of PDH kinase, COX4-2, and a protease that degrades COX4-1

Answer 38

• reactive oxygen species
• two other lines of defense against ROS
  
  • regulation of pyruvate dehydrogenase (PDH)
    
    • PDH kinase phosphorylates & inactivates mitochondrial PDH
    • this slows the delivery of FADH2 and NADH from the citric acid cycle to the respiratory chain
  • replacement of one subunit of Complex IV
    
    • subunit COX4-1 is replaced with COX4-2, that is better suited to hypoxic condition
• HIF-1, transcription factor
  *

Question 39

PHOTOSYNTHESIS: HARVESTING LIGHT ENERGY

The overall equation for photosynthesis in plants describes an oxidation-reduction reaction in which H₂O donates electrons (as _____) for the reduction of CO₂ to carbohydrate (_____):
CO₂ + H₂O → O₂ + (CH₂O)

Answer 39

• hydrogen
• CH₂O

Question 40

General Features of Photophosphorylation

• H2O is a poor _____ of electrons
• Photophosphorylation differs from oxidative phosphorylation in requiring the input of _____ in the form of _____ to create a good electron donor and a good electron acceptor
• electrons flow through a series of _____-_____carriers including cytochromes, quinones, and iron-sulfur proteins, while protons are pumped across a membrane to create an electrochemical potential
• Photosynthesis in plants encompasses two processes

Answer 40

• donor
• energy, light
• membrane-bound
• 2 processes
  
  • light-dependent reactions, or light reactions
    
    • occur only when plants are illuminated
    • chlorophyll and other pigments of photosynthetic cells absorb light energy and conserve it as ATP and NADPH
  • carbon-assimilation reactions, carbonfixation reactions, dark reactions
    
    • driven by products of the light reaction
    • ATP and NADPH are used to reduce CO2 to form triose phosphates, starch, and sucrose, and other products derived from them

Question 41

In photosynthetic eukaryotic cells, both the light dependent and the carbon-assimilation reactions take place in the ______

Answer 41

chloroplasts

Question 42

chloroplasts

Answer 42

• intracellular organelles
• variable in shape
• generally a few micrometers in diameter
• surrounded by two membranes
• the aqueous phase enclosed by the inner membrane is called the stroma,
  
  • contains most enzymes required for carbon-assimilation reactions
• contains many thylakoids
  
  • ​flattened, membrane-surrounded vesicles
  • arranged in stacks called grana
  • the thylakoid membranes, lamellae, are embedded with photosynthetic pigments and enzyme complexes that carry out the light reactions and ATP synthesis

Question 43

Light Absorption

The energy of a single photon (a quantum of light) is greater at the ______ end of the spectrum than at the _____ end; shorter wavelength (and higher frequency) corresponds to _____ energy

Answer 43

• violet
• red
• higher

Question 44

Planck equation

Answer 44

• gives the energy, E, in a single photon of visible light
• E = hv = hc/λ
• h: Planck’s constant: 6.626 3 10234 J•s
• v: frequency of the light in cycles/s
• c: speed of light 3.00 × 10⁸ m/s
• λ: wavelength in meters
• PDF pg. 802

Question 45

Light Absorption

• When a photon is absorbed, an electron in the absorbing molecule (_____) is lifted to a higher energy level
• Absorbtion is and _____-_____-_____ event
• to be absorbed, the photon must contain a quantity of energy (a _____) that exactly _____ the energy of the electronic transition
• A molecule that has absorbed a photon is in an _____ state, which is generally _____
• An electron lifted into a higher-energy orbital usually returns rapidly to its lower-energy orbital; the excited molecule ______ to the _____ ground state, giving up the absorbed quantum as _____ or ____ or using it to do chemical work
• Light emission accompanying decay of excited molecules (called _____) is always at a _____ wavelength (______ energy) than that of the absorbed light

Answer 45

• chromophore
• all-or-nothing
• quantum, matches
• excited, unstable
• decays, stable, light, heat
• fluorescence, longer, lower

Question 46

exciton transfer

Answer 46

• An alternative mode of decay important in photosynthesis involves direct transfer of excitation energy from an excited molecule to a neighboring molecule
• exciton is a quantum of energy passed from an excited molecule to another molecule i

Question 47

chlorophylls

Answer 47

• most important light-absorbing pigments in the thylakoid membranes
• green pigments with polycyclic, planar structures resembling the protoporphyrin of hemoglobin
• Mg²⁺ occupies the central position
• four inward-oriented nitrogen atoms of chlorophyll are coordinated with the Mg²⁺
• have a long phytol side chain, esterified to a carboxyl-group substituent in ring IV
• have a fifth five-membered ring
• The heterocyclic five-ring system that surrounds
  the Mg²⁺ has an extended polyene structure, with alternating single and double bonds
  
  • polyenes show strong absorption in the visible region of the spectrum
• have unusually high molar extinction coefficients so are well suited for absorbing visible light during photosynthesis

Question 48

• Chloroplasts always contain chlorophyll _____ and ______
• Although both are green, their absorption spectra are sufficiently different that they
• Most plants contain about twice as much chlorophyll ______ as chlorophyll _____
• Plants are green because their pigments absorb light from the ______ and ______ regions of the spectrum, leaving primarily green light to be reflected
• the combination of chlorophylls (a and b) and accessory pigments enables plants to harvest

Answer 48

• a, b
• complement each other’s range of light absorption in the visible region
• a, b
• red, blue
• most of the energy available in sunlight

Question 49

Chlorophyll is always associated with specific binding proteins, forming _____-______ ______ in which chlorophyll molecules are fixed in relation to each other, to other protein complexes, and to the membrane

Answer 49

light-harvesting complexes (LHCs)

Question 50

phycobilins

Answer 50

• Cyanobacteria and red algae employ phycobilins as their light-harvesting pigments used
• they have the extended polyene system found in chlorophylls, but not their cyclic structure or central Mg²⁺
• covalently linked to specific binding proteins, forming phycobiliproteins, which associate in highly ordered complexes called phycobilisomes

Question 51

carotenoids

Answer 51

• secondary light-absorbing pigments in the thylakoid membranes
• accessory pigments
• yellow, red, or purple
• most important
  
  • β-carotene: a red-orange isoprenoid
  • lutein: yellow
• absorb light at wavelengths not absorbed by the chlorophylls
• supplementary light receptors

Question 52

action spectrum

Answer 52

useful in identifying the pigment primarily responsible for a biological effect of light

Question 53

Photosystem

Answer 53

• In the thylakoid membrane, chlorophyll molecules are organized along with other small organic molecules and proteins into complexes called photosystems
• Compose of a reaction-center complex (organized association of proteins holding a special pair of chlorophyll a molecules)
• Reaction-center complex is surrounded by several light-harvesting complexes (consists of various pigment molecules bound to proteins)
  
  • Act as an antenna for the reaction-center complex
  • When a pigment molecule absorbs a photon, the energy is transferred from pigment to pigment w/in a light-harvesting complex until it’s passed into the reaction-center complex, to a special pair of chlorophyll a pigments
  • The special pair of chlorophyll a pigments, in the reaction-center complex, hands off the electrons to the primary electron acceptor (this is the first step of the light reactions)
  • The photosystem converts light energy to chemical energy
• Two types of photosystems
  
  • Photosystem II (PS II) and photosystem I (PSI)
  • Name in order of their discovery, however PSII functions first in the light reactions
  • Reaction center chlorophyll a of PSII is known as P680
    
    • Best at absorbing light w/wavelength of 680nm (red part of the spectrum)
  • Reaction center chlorophyll a of PSI is known as P700
    
    • Best at absorbing light w/wavelength of 700nm (far red part of the spectrum)
  • Both chlorophyll a’s are nearly identical except for their slight differences in their light absorbing properties

Question 54

• light-absorbing pigments of thylakoid or bacterial membranes are arranged in functional arrays called ______
• All the pigment molecules in a photosystem can absorb photons, but only a few chlorophyll molecules associated with the ______ _____ _____ are specialized to transduce light into chemical energy
• The other pigment molecules in a photosystem are called _____-______ or ______ ______. They absorb light energy and transmit it rapidly and efficiently to the reaction center
• chlorophyll molecules in _____-______ complexes have light-absorption properties that are subtly different from those of free chlorophyll

Answer 54

• photosystems
• photochemical reaction center
• light-harvesting, antenna molecules
• light-harvesting

Question 55

Exciton and electron transfer

describe the steps

Answer 55

1. Light excites an antenna molecule (chlorophyll or accessory pigment), raising an electron to a higher energy level
2. the excited antenna chlorophyll transfers energy directly to a neighboring chlorophyll molecule (exciton transfer) which becomes excited as the first molecule returns to its ground state
3. This exciton transfer, extends to a third, fourth, or subsequent neighbor, until it is transferred to a reaction-center chlorophyll a molecule, at the photochemical reaction center
4. The excited reaction-center chlorophyll passes an electron to a higher-energy orbital (electron acceptor).
   
   • This electron then passes to a nearby electron acceptor that is part of the electron-transfer chain
   • this leaves the reaction-center chlorophyll with a missing electron, an “electron hole,” denoted by +
5. The electron acceptor acquires a negative charge. The electron lost by the reaction-center chlorophyll is replaced by an electron from a neighboring electron-donor molecule, becoming positively charged

In this way, excitation by light causes electric charge separation and initiates an oxidation-reduction chain.

Question 56

• Photosynthetic bacteria have one of two general types of reaction center.
• Cyanobacteria and plants have

Answer 56

• two types
  
  • One type (found in purple bacteria) passes electrons through pheophytin (chlorophyll lacking the central Mg²⁺ ion) to a quinone.
  • The other (in green sulfur bacteria) passes electrons through a quinone to an iron-sulfur center
• two photosystems (PSI, PSII), one of each type, acting in tandem

Question 57

The Pheophytin-Quinone Reaction Center

(Type II Reaction Center)

Answer 57

• The photosynthetic machinery in purple bacteria consists of three basic modules:
  
  • a single reaction center (P870)
  • a cytochrome bc1 electrontransfer complex
    
    • similar to Complex III of the mitochondrial
  • an ATP synthase
• Powered by the resulting proton gradient
• Pathway
  
  • Illumination drives e^- flow from reaction-center P870 through pheophytin
  • through a quinone
  • through cytochrome bc₁ complex
    
    • causes proton pumping
    • creates an electro-chemical potential that powers ATP synthesis
  • electrons then flow through cytochrome c₂ and back to the reaction center, restoring its preillumination state
• Structure
  
  • PDF pg 809
• Patway for excitation of the special pair of bacteriochlorophylls (Chl)₂
  
  • antenna chlorophyll, surrounding the reaction center, absorbs energy from a photon and reaches (Chl)₂ by exciton transfer
  • (Chl)₂ absorbs the exciton, and its redox potential is shifted, by an amount equivalent to the energy of the photon, converting the special pair to a very strong electron donor
  • (Chl)₂ donates an electron that passes through a neighboring chlorophyll monomer to pheophytin (Pheo) producing 2 radicals:
    
    • one positively charged (the special pair of chlorophylls)
    • one negatively charged (the pheophytin)
  • pheophytin radical passes its electron to a tightly bound molecule of quinone (QA),
  • QA is converted to a semiquinone radical and donates its extra electron to a second, loosely bound quinone (QB).
  • Two electron transfers convert QB to its fully reduced form, QBH2, hydroquinone
  • QBH2
    
    • has some energy of the photons that originally excited P870
    • diffuses in the membrane bilayer, away from the reaction center
    • enters pool of reduced quinone (QH2)
    • moves through the lipid phase of the bilayer to the cytochrome bc₁ complex
  • cytochrome bc₁ complex
    
    • carries electrons from a quinol donor (QH2) to an electron acceptor, P870 via cytochrome c2
    • uses energy of electron transfer to pump protons across the membrane, producing a protonmotive force
    • e^- path flow through is similar to mitochondrial Complex III, involving a Q cycle
  • electron-transfer process completes the cycle, returns reaction center to its unbleached state, ready to absorb another exciton from antenna chlorophyll
• all chemistry occurs in the solid state, reacting species are held close together in the right orientation for reaction, resulting in a very fast and efficient series of reactions

Question 58

The Fe-S Reaction Center (Type I Reaction Center)

Answer 58

• Photosynthesis in green sulfur bacteria involves the same three modules as in purple bacteria
• pathway
  
  • Excitation causes an electron to move from the reaction center to the cytochrome bc₁ complex via a quinone carrier
  • Electron transfer through this complex powers proton transport and creates the proton-motive force used for ATP synthesis
  • in contrast to the cyclic flow of electrons in purple bacteria, some electrons flow from the reaction center to an iron-sulfur protein, ferredoxin
  • ferredoxin passes electrons via ferredoxin:NAD reductase to NAD⁺, producing NADH.
  • The electrons taken from the reaction center to reduce NAD⁺ are replaced by oxidation of H₂S to elemental S, then to SO²₄^-
    
    • this reaction defines the green sulfur bacteria
    • oxidation of H₂S is analogous to oxidation of H₂O by oxygenic plants.

Question 59

Kinetic and Thermodynamic Factors Prevent the Dissipation of Energy by Internal Conversion

• Reaction centers are constructed to prevent the inefficiency that would result from internal conversion, which is
• The proteins of the reaction center hold the ______, ______, and ______ in a fixed orientation relative to each other, allowing photochemical reactions to take place in a virtually solid state accounting for high efficiency and rapidity of the reactions
• electrontransfer reactions are thermodynamically “downhill”: the excited special pair (Chl)* is a very good _____ _____ and each successive electron transfer is to an acceptor of substantially less negative E’°. The standard free-energy change for the process is therefore _____ and _____
• The overall energy yield (the percentage of the photon’s energy conserved in QH2 is > _____, with the remainder of the energy dissipated as _____ and _____.

Answer 59

• a very rapid process (10 picoseconds) in which the energy of the absorbed photon is converted to heat
• bacteriochlorophylls, bacteriopheophytins, quinones
• electron donor (E’° = -1V), negative, large
• 30%, heat, entropy

Question 60

• In modern cyanobacteria, algae, and vascular plants, two _____ _____ act in tandem
• The thylakoid membranes of chloroplasts have two different kinds of photosystems, each with its own type of photochemical _____ ______ and set of ______ molecules.
• The two systems have distinct and complementary functions. They are _____ _____ and ______ _____

Answer 60

• reaction centers
• reaction center, antenna
• Photosystem II (PSII), Photosystem I (PSI)

Question 61

photosystems I and II in chloroplasts

Answer 61

• two reaction centers in plants act in tandem to catalyze the light-driven movement of electrons from H₂O to NADP⁺
• Photosystem II (PSII)
  
  • pheophytin-quinone type
    
    • like the single photosystem of purple bacteria
  • contains
    
    • roughly equal amounts of chlorophylls a and b
    • two very similar proteins, D1 and D2, form an almost symmetric dimer, to which all the electroncarrying cofactors are bound
  • Path
    
    • Excitation of reaction-center P680 drives electrons through the cytochrome b₆f complex with concomitant movement of protons across the thylakoid membrane
• Photosystem I (PSI)
  
  • structurally and functionally related to the type I reaction center of green sulfur bacteria
  • contains
    
    • reaction-center P700
    • high ratio of chlorophyll a to chlorophyll b
  • Path
    
    • Excited P700 passes electrons to the Fe-S protein ferredoxin, then to NADP⁺, producing NADPH
• plastocyanin
  
  • protein that carries electrons between the two photosystems
  • one-electron carrier functionally similar to cytochrome c

Question 62

photosystems I and II in chloroplasts

Linear/Noncyclic Electron Flow

and Cyclic Electron Flow

Answer 62

Linear/Noncyclic Electron Flow

1. Photon of light hits one pigment in PS II
   
   • It passes this energy/electron off to the next pigment until it excites P680
   • Excitation of P680 produces P680*, an excellent e^- donor
2. After excitation, the high-energy electrons “falls” down an ETC from PSII to PSI
3. ETC is made of electron carrier plastoquinone (Pq) a cytochrome complex and a protein called plastocyanin (Pc)
   
   • e^- is transferred from P680* to to pheophytin
     
     • P680* becomes a radical cation, P680⁺
     • pheophytin gets a negative charge ^•Pheo^-
   • ^•Pheo^- passes its extra e^- to protein-bound plastoquinone, PQ_A
   • PQ_A passes its e^- to a more loosely bound plastoquinone, PQ_B
   • When PQ_B acquireds 2 e^- from PQ_A and two protons from the solvent water, it is in its fully reduced quinol form, PQ_BH₂
   • electrons in PQ_BH₂ pass through the cytochrome b₆f complex
4. H₂O is split
   
   • e^- transferred from H atoms to P680⁺ and reduced to P680
   • O₂ is released as a by-product
5. The fall of the e^- and water splitting cause the creation of ATP into stroma
   
   • As the electrons pass through (Pc) and H₂O is split, H+ is pumped into the thylakoid space, contributing to the proton gradient used in chemiosmosis
6. PS I gets electrons from ETC or light hitting one pigment in PS II
   
   • energy/electron excites P700*
   • e^- is transferred from P700* to e^- acceptor, A₀
   • P700 becomes P700⁺ and A₀ becomes A₀^-
   • P700⁺ acquires an e^- from plastocyanin, a soluble Cu-containing electron-transfer protein
   • P700 is now ready to accept another electron from PSII
7. PS I pass the electrons from the primary electron acceptor down another ETC through the protein ferredoxin (E)
   
   • A₀^- passes its e^- to phylloquinone, A₁
   • A₁ passes its e^- to an iron-sulfur protein (through three Fe-S centers)
   • the e^- then moves to ferredoxin (Fd), another iron-sulfur protein
   • Does not product a proton gradient
   • Does not product ATP
8. Electron is transferred to NADP⁺
   
   • NADP⁺ is catalyzed by NADP+ reductase to NADPH
   • NADPH is released into stroma to go to the Calvin Cycle

Alternative Path: Cyclic Electron Flow

• In certain cases, photoexcited electrons can take an alternative path called cyclic electron flow, which uses PSI and not PSII
  
  • electron flow
    
    • electrons cycle back from ferredoxin (Fd) to cytochrome b₆f complex
    • cytochrome b₆f complex transfers it to plastocyanin
    • plastocyanin transfer it to a P700 chlorophyll in the PSI
    • and then back to ferredoxin
  • electrons are repeatedly recycled through cytochrome b₆f complex and the reaction center of PSI
  • each electron is propelled around the cycle by the energy of one photon
  • There is no release of oxygen or net formation of NADPH
  • cyclic photophosphorylation
    
    • produces a proton gradient to drive ATP synthesis
    • produces more ATP and less NADPH than noncyclic
• occurs to varying degrees depending primarily on the light conditions: color/wavelength and quantity
• Several of the currently existing groups of photosynthesis bacteria are known to have a single photosystem related to either PSII or PSI
  
  • In these bacteria, cyclic electron flow is the one and only means of generating ATP
  • Evolutionary biologists hypothesize they are descendants of ancestrial bacteria

Question 63

oxygenic photosynthesis

Answer 63

cyanobacteria and plants oxidize H₂O producing O₂ to replace the electrons that move from PSII through PSI to NADP⁺​

Question 64

All O₂-evolving photosynthetic cells—those of plants, algae, and cyanobacteria— contain _____ and _____; organisms with one do not evolve _____

Answer 64

• PSI, PSII, O₂

Question 65

Integration of photosystems I and II in chloroplasts

• For every ______ photons absorbed (one by each photosystem), one electron is transferred from H2O to NADP⁺.
• To form one molecule of O₂, a total of ______ photons must be absorbed, four by each photosystem
  
  • requires transfer of four electrons from two H₂O to two NADP+

Answer 65

• two
• eight

Question 66

The electron-carrying cofactors of PSI and the lightharvesting complexes are part of a supramolecular complex that consists of

Answer 66

• three identical complexes, each composed of 11 different proteins
• antenna chlorophyll and carotenoid molecules are positioned around the reaction center
• reaction center’s electron-carrying cofactors are tightly integrated with antenna chlorophylls

Question 67

Electrons temporarily stored in plastoquinol as a result of the excitation of P680 in PSII are carried to P700 of PSI via the _____ _____ complex and the soluble protein _____

Answer 67

• cytochrome b₆f
• plastocyanin

Question 68

cytochrome b₆f complex

Answer 68

• structure
  
  • b-type cytochrome with two heme groups: b_H and b_L
  • Rieske iron-sulfur protein
    
    • Fe-S center lies just outside the bilayer on the P side
  • cytochrome f (heme f)
    
    • on protein domain that extends into the thylakoid lumen
  • fourth (heme x) near heme b_H
  • β-carotene of unknown function
  • Two sites bind plastoquinone:
    
    • PQH2 site near the P side of the bilayer
    • PQ site near the N side
  • homodimer creates a cavern
    
    • connects PQH2 and PQ sites
    • allows plastoquinone to move between the sites of its oxidation and reduction
• Electrons flow from PQ_BH₂ → cytochrome b₆f → cytochrome f → plastocyanin → P700
  
  • PSII generates Plastoquinol (PQH₂)
  • PQH₂ binds to cytochrome b₆f
  • cytochrome b₆f oxidizes PQH₂ in a series of steps like in the Q cycle in the cytochrome bc₁ complex (Complex III of mitochondria)
    
    • electrons pass, one at a time
    • cycle results in pumping of protons across the membrane
    • in chloroplasts direction of proton movement is from the stromal compartment to the thylakoid lumen
    • up to four protons moving for each pair of electrons
    • results in the production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI
  • One e^- from PQH2 passes to the Fe-S center of the Rieske protein, the other to heme b_L of cytochrome b6
  • e^- from Fe-S center transfers to cytochrome f → plastocyanin → PSI P700
• PDF pg. 813-814

Question 69

PSI and PSII placement

• energy required to excite PSI (P700) is _____ than that needed to excite PSII (P680).
• If PSI and PSII were physically contiguous, excitons originating in the antenna system of PSII would migrate to the reaction center of PSI, leaving PSII chronically _____
• Imbalance is prevented by separation of the two photosystems in the _____ _____
• PSII is located almost exclusively in the _____ membrane stacks of thylakoid grana; while PSI and _____ _____ are located almost exclusively in the _____ thylakoid membranes, where they have access to the contents of the ____, including _____ and _____
• he cytochrome b6 f complex is present primarily in the _____

Answer 69

• less: longer wavelength, lower energy
• underexcited
• thylakoid membrane
• appressed, ATP synthase, nonappressed, stroma, ADP, NADP⁺
• grana

Question 70

• state transitions change the distribution of ______-_____ _____ between the two photosystems
• association of LHCII with PSI and PSII depends on ______ _____ and _____ which lead to state transitions

Answer 70

• light-harvesting complex LHCII
• light intensity, wavelength

Question 71

Appressed vs. Non-appressed region of Chloroplast

Answer 71

• Non-apreessed region / stroma lamella
  
  • located on exposed part of grana thylakoid membranes
  • located on the outside of the stack
• Appressed region / granal lamellae
  
  • located on internal part of grana thylakoid membranes
  • the stacks on the inside

Question 72

state transitions

Pathway

Answer 72

• state 1
  
  • conditions of intense or blue light favor absorption by PSII
  • a critical Ser residue in LCHII is not phosphorylated
  • LHCII associates with PSII
  • photosystem reduces plastoquinone to plastoquinol (PQH2) faster than PSI can oxidize it.
  • PQH2 accumulates and activates a protein kinase
  • Protein kinase phosphorylates a Thr residue on LHCII triggering transition to state 2
  • the path of electrons is primarily noncyclic
• state 2
  
  • Phosphorylation weakens interaction of LHCII with PSII
  • some LHCII dissociates and moves to the stromal lamellae and captures photons (excitons) for PSI
  • This speeds oxidation of PQH2
  • Imbalance between electron flow in PSI and PSII is reversed
• in less intense light (shade, with more red light)
  
  • PSI oxidizes PQH₂ faster than PSII can make it
  • Increase in [PQ] triggers dephosphorylation of LHCII
  • this reverses the effect of phosphorylation
  • the path of electrons is primarily cyclic

Question 73

• The ultimate source of the electrons passed to NADPH in plant (oxygenic) photosynthesis is _____. Photosynthetic bacteria use a variety of electron donors for this purpose.
• Having given up an electron to pheophytin, P680⁺ (of PSII) must acquire an electron to return to its _____ ______
• Two water molecules are split, yielding
• A single photon of visible light does not have enough energy to break the bonds in water; _____ photons are required
• The four electrons abstracted from water do not pass directly to P680⁺, which can accept ______ electron at a time

Answer 73

• water
• ground state
• four electrons, four protons, and molecular oxygen
  
  • 2H₂O → 4H⁺ + 4e^- + O₂
• _​four
• one

Question 74

oxygen-evolving complex, water-splitting complex

Answer 74

Pathway

• passes four electrons one at a time to P680⁺
• immediate electron donor to P680⁺ is a Tyr residue (Z or Tyr_Z)
  
  • in subunit D1 of the PSII reaction center
• Tyr residue loses both a proton and an electron, generating the electrically neutral Tyr free radical, ^•Tyr
• Tyr radical regains its missing electron and proton by oxidizing a cluster of four manganese ions and one calcium ion in the water-splitting complex, one at a time
  
  • With each single-electron transfer, the Mn₄Ca cluster becomes more oxidized
  • four single-electron transfers, each corresponding to the absorption of one photon, produce a charge of 4+ on the Mn₄Ca cluster
  • 4 ^•Tyr + [Mn₄Ca]⁰ → 4 Tyr + [Mn₄Ca]⁴⁺
• Mn₄Ca cluster can take four electrons from a pair of water molecules, releasing 4H⁺ and O₂
  
  • _​ [Mn₄Ca]⁴⁺ + 2H₂O → [Mn₄Ca]⁰ + 4H⁺ + 0₂
• the four protons produced are released into the thylakoid lumen
  
  • the oxygen-evolving complex acts as a proton pump, driven by electron transfer

Structure

• metal cluster takes the shape of a chair
• seat and legs of the chair are made up of three Mn ions, one Ca ion, and four O atoms
• the fourth Mn and another O form the back of the chair
• has four water molecules
  
  • two associated with one of the Mn ions
  • two with the Ca ion
• metal cluster is associated with a peripheral membrane protein on the lumenal side of the thylakoid membrane stabilizing the cluster
• Tyr residue (Z) is part of a network of hydrogenbonded water molecules that includes the four associated with the Mn₄Ca cluster
• PDF pg 816