electron transport

Question 1

why do we breath oxygen?

Answer 1

in eukaryotes, oxygen is the ultimate electron acceptor for electrons passed from NADH and FADH2 into ETC

Question 2

what is oxidative phosphorylation?

Answer 2

the process where electrons from reduced cofactors NADH and FADH2 are passed to proteins in the respiratory chain and the energy of oxidation is used to phosphorylate ADP

Question 3

how does photophosphorylation work?

Answer 3

light causes charge separation between a pair of chlorophyll molecules and energy of the oxidized and reduced chlorophyll molecules is used to drive synthesis of ATP. uses water as electron source and uses NADP+ as ultimate electron acceptor.

Question 4

how does chemiosmotic theory explain the formation of ATP despite its unfavorable thermodynamics

Answer 4

phosphorylation of ADP is not a result of a direct reaction between ADP and some high energy phosphate carrier (such is the case in glycolysis) but instead the energy needed to phosphorylate ADP is provided by the flow of protons down the electrochemical gradient. this gradient is created by electron transport energy release

Question 5

what features of membranes allow the proton gradient to be established and used to make ATP?

Answer 5

membrane is impermeable to ions (H+), includes plasma membrane in bacteria, inner mitochondrial membrane, and thylakoid membrane in chloroplasts.
membrane must contain proteins that couple downhill electron flow with uphill proton flow
membrane must contain a protein that couple downhill proton flow to ADP phosphorylation

Question 6

where is the proton gradient located in mitochondria and chloroplasts?

Answer 6

mitochondria: high [H] outside inner membrane
chloroplast: high [H] inside thylakoid membrane

Question 7

structure of mitochondria

Answer 7

outer membrane: porous membrane allows passage of metabolites
inter membrane space: similar to cytosol, high [H] and lower pH
inner membrane: impermeable with proton gradient. location of ETC. has convolutions called cristae to increase surface area
matrix: location of CAC and parts of lipid and amino acid metabolism. lower [H] and higher pH

Question 8

each electron transport complex contains redox centers consisting of

Answer 8

FMN or FAD (flavin cofactors carry electrons one at a time)
cytochromes a, b, or c
iron sulfur cluster

Question 9

what are cytochromes?

Answer 9

one electron carries capable of reduction by 1 electron and then oxidation by 1 electron (Fe3+ -> Fe2+). consists of iron coordinating porphoryin ring derivatives
the types (a, b, c) differ by ring additions and have different reduction potentials

Question 10

what are iron sulfur centers?

Answer 10

one electron carriers. consists of coordinated cysteine and contains equal number iron and sulfur atoms (greater number of irons/sulfurs doesn’t change how many electrons can carry, still just one electron)

Question 11

what is coenzyme Q (ubiquinone)

Answer 11

a lipid soluble conjugated dicarbonyl compound that readily accepts electrons. functions as a mobile electron carrier transporting electrons from complexes I and II to couples III

Question 12

what does ubiquinone become upon accepting electrons?

Answer 12

it accepts two electrons and then picks up two protons to give an alcohol called ubiquinol. Ubiquinol can freely diffuse in the membrane and carry electrons from one side of membrane to another

Question 13

structure of ubiquinone, ubiquinol, and semiquinone

Answer 13

slide 14

probably not the most important thing, but recognize structures just in case

Question 14

Complex I

Answer 14

NADH:ubiquinone oxidoreductase
one of the largest macro-molecular assemblies in mammalian cell, over 40 diff polypeptide chains encoded by both nuclear and mitochondrial genes
noncovalently bound FMN accepts two electrons from NADH
several iron-sulfur centers pass one electron at a time toward the ubiquinone binding site
slide 19

Question 15

what is the net reaction/result of complex I?

Answer 15

transfer of two electrons from NADH to ubiquinone, accompanied by transfer of protons from matrix to intermembrane space. 4 protons are transported per one NADH. 2 protons are picked up by coenzyme Q. total of 6 protons removed from matrix

Question 16

how are protons transported across membrane?

Answer 16

through proton wires. a series of amino acids that undergo protonation and deprotonation to get a net transfer of a proton from one side of a membrane to another

Question 17

complex II

Answer 17

succinate dehydrogenase
FAD accepts two electrons from succinate (forms fumarate)
electrons are passed one at a time via iron-sulfur centers to ubiquinone, which becomes ubiquinol
no proton transfer occurs
((same enzyme as in CAC))
slide 22

Question 18

complex III

Answer 18

ubiquinone:cytochrome c oxidoreductase
uses 2 electrons from ubiquionol to reduce 2 molecules of cytochrome c
contains iron-sulfur clusters, cytochrome b and c
Q cycle results in 4 additional protons transported to inter membrane space
slide 23

Question 19

describe the Q cycle

Answer 19

4 protons are transported across membrane per 2 electrons that reach cytochrome c. 2 protons come from QH2. the Q cycle explains the other 2 protons
2 molecules of QH2 become oxidized, releasing the 4 protons. Then one molecule becomes rereduced, thus a net transfer of 4 protons per reduced Q
slide 26

Question 20

complex IV

Answer 20

cytochrome oxidase
contains two heme groups and copper ions. CuA: two ions that accept electrons from Cyt c. CuB: bonded to heme forming a binuclear center that transfers four electrons to oxygen
4 electrons are used to reduce one oxygen molecule into two water molecules. 4 protons are picked up from the matrix and 4 additional protons are passed from the matrix to the intermembrane space
slide 29

Question 21

what is a respirasome?

Answer 21

the associated complexes of ETC. they are gathered close together to decrease traveling distance

Question 22

draw the simplified summary of electron flow in the respiratory chain. account for proton gradient flow and electron transporters from NADH to water

Answer 22

slide 31

Question 23

what is the net result of electron transport from both complex I and complex II?

Answer 23

complex I: 2NADH + 22H+(N) + O2 –> 2NAD+ + 20H+(P) + 2H2O
complex II: 2FADH2 + 12H+(N) + O2 –> 2FAD + 12H+(P) + H2O
can be simplified by dividing by two so uses 1/2O2
complex II moves less protons than complex I

Question 24

it is possible to generate reactive oxygen radicals during electron transport. how are these radicals dealt with?

Answer 24

the radical is converted to hydrogen peroxide via superoxide dismutase. H2O2 converts to H20 via glutathione peroxidase, converting GSH to GSSG in the process. the GSSG converts back to GSH by glutathione reductase and the use of NADPH which is restored by using NADH. the expense of this process is one NADH

Question 25

the proteins in the ETC create the proton gradient by three different means:

Answer 25

actively transport protons across the membrane (I and IV)
chemically remove protons from the matrix (CoQ reduction and oxygen reduction)
release protons into the intermembrane space (QH2 oxidation)

Question 26

how is the proton gradient taken advantage of? structure of ATP synthase complex

Answer 26

mitochondrial ATP synthase complex uses the proton gradient to synthesize ATP from ADP
two functional units
F1: in the matrix, catalyzes hydrolysis of ATP
F0: integral membrane. tranports protons from intermembrane space to matrix, dissipating proton gradient, transfers energy to F1 via rotation

Question 27

what is the structure of the F1 unit?

Answer 27

a hexamer arranged in three aB dimers which exist in three conformations: open (empty), loose (ADP bound), and tight (catalyzes ATP formation)

Question 28

how is rotation of the F0 subunit accomplished?

Answer 28

proton translocation causes rotation of F0 and the gamma central shaft that causes conformational changes within all 3 aB pairs. the change promotes condensation of ADP and Pi to ATP

Question 29

how does ADP and Pi get into the matrix to be used for making ATP?

Answer 29

an adenine nucleotide translocase (antiporter) gets ADP into the matrix and moves ATP out. the phosphate transolcase (symporter) movies Pi in the matrix with the proton gradient

Question 30

what does the malate-aspartate shuttle do? draw cycle

Answer 30

moves NADH from cytoplasm to mitochondrial matrix. malate goes through a malate-a-ketoglutarate transporter (antiporter) then converts to oxaloacetate, then aspartate which moves through glutamate-aspartate transporter (antiporter) and converts to oxaloacetate and then malate. draw cycle slide 40

Question 31

what tissues use the malate-aspartate shuttle?

Answer 31

liver, kidney, heart

Question 32

what does the glycerol-3-phosphate shuttle do? draw cycle

Answer 32

moves NADH from cytoplasm into mitochondrial matrix. dihydroxyacetone phosphate converts to glycerol-3-phosphate, which converts FAD to FADH2 which becomes QH2 and enters complex III
slide 41

Question 33

what tissues use the glycerol-3-phosphate shuttle?

Answer 33

skeletal muscle and brain

Question 34

how do the two NADH shuttles differ in ATP yield?

Answer 34

the malate-aspartate shuttle yields 5 ATP as it enters the ETC via complex I
the glycerol-3-phosphate shuttle yields 3 ATP as it enters ETC via complex III

Question 35

how is ETC regulated?

Answer 35

primarily regulated by substrate availability (NADH and ADP/Pi)
Inhibitor of F1 prevents hydrolysis of ATP during low oxygen, only active at low pH in matrix when electron transport is stalled
Inhibition of oxidative phosphorylation leads to accumulation of NADH which causes inhibition cascade up to PFK-1