Lecture 21 - MIDTERM 3 Flashcards
How do we get energy?
– glycolysis and CAC produce little ATP, but we do generate reduced electron carriers, NADH and FADH2
– these molecules are re-oxidized in the mitochondria during cellular respiration by passing electrons through a series of electron carriers, called the electron transport chain
– finally reducing O2 to H2O
– free energy released during these redox reactions is coupled to ATP synthesis, called oxidative phosphorylation
Describe oxidative phosphorylation.
– the majority of ATP recycling occurs via oxidative phosphorylation
– Ox-Phos is the process by which ATP is formed as a result of transfer of electrons from NADH or FADH2 to O2 by a series of electron carriers
– Complete oxidation of a glucose to CO2 and H2O generates 32-36 molecules of ATP, and ox-phos is responsible for 26
How do NADH and FADH contribute to the proton gradient?
– NADH and FADH2 generated during glycolysis the TCA cycle, and other pathways are reduced O2 to H2O
– this creates a proton gradient across the membrane
– the energy in the gradient, the proton-motive force, powers the phosphorylation of ADP to ATP
Where does ox-phos take place?
– in the mitochondria
Describe Mitochondria organization.
– two membranes; Outer membrane surrounds the organelle. Inner membrane is folded in an elaborate manner to create cristae –> folding increases surface area of inner membrane, more sites for ox-phos
– space between outer and innner membrane = intermembrane space
– space within inner membrane = matrix
– outer membrane –> permeable to most small molecules and ions
– inner membrane –> site of ox-phos reactions; highly impermeable
Describe mitochondria organization and location of respiratory chain.
– NADH or FADH2 are oxidized on the inner mitochondrial membrane by enzyme complexes of the respiratory chain
– 5 separate complexes, I-IV are involved in redox reactions (ETC) and complex V is an ATP synthase (phosphorylation)
What is the purpose of Complex V?
– it takes energy from proton gradient and that drives ATP synthesis
What is the mobile that carries electrons from complexes 1, 2 and 3?
– Coenzyme Q
T or F, 8 electrons from CAC move to inner membrane of mitochondria
True; 6 from NADH and 2 from FADH2 ( 3 NADH and 1 FADH2 each of which carry 2 electrons)
T or F, NADH and FADH2 enter in different places in the ETC
True, NADH goes to Complex I and FADH2 goes to Complex II
What is the key concept of the ETC?
– electron transfer through the chain is driven by differences in the electron-transfer potential of the individual components
– NADH and FADH2 donate these electrons to carriers that ultimately transfer them to O2 to form water
What is the difference between strong reducing agents and strong oxidizing agents?
– strong reducing agent (e.g. NADH) is inclined to donate e- and has neg E’0 value
– strong oxidizing agent (e.g. O2) is inclined to accept e- and has pos E’0 value
How many ATP can 1 molecule NADH drive in the synthesis of ATP?
– 2.5 molecules of ATP
– electrons on NADH drive synthesis of ATP
How does the reduction of oxygen to water by NADH occur?
– it happens gradually and involves several carriers
– electrons are transported down a chain
– NADH –> Q –> cytochrome c –> O2
– note that E’O gets increasingly positive as electrons move down the chain
– the more positive the value, the more able a substance is to accept electrons
– redox potential thus determines the direction of electron flow
What happens to the redox potential as electrons from NADH move down ETC?
– the potential becomes increasingly positive
– the more positive the value the easier it is to accept those electrons which helps move/drive electrons in a unidirectional fashion
T or F, large enzyme complexes catalyze electron transfer and harness the released energy to pump protons
True; electron transfer is catalyzed by enzymes located in the inner mitochondrial membrane
– these are massive complexes
What role do prosthetic groups have in the ETC?
– they help the transfer of electrons to ultimately pump protons
What is the key concept of electron transfer?
– electron transfer drives the pumping of protons across the inner membrane, from the matrix to the intermembrane space, against a concentration gradient.
Describe Step 1 of NADH oxidation
– coenzyme Q reduction – proton pumping
– catalyzed by NADH Q oxidoreductase (Complex I)
– electrons transferred to coenzyme Q (Q) – a quinone derivative that exists in several different oxidation states
– another important feature of Q is that is it mobile within the inner membrane of the mitochondria
What is the electron transport route through complex I?
– NADH enters complex I and is picked up by FMN which then transfers them to iron sulfur complexes; protons reduce FMN to FMNH2 in the process
– Quinon (Q) picks them up from iron-sulfur complexes and becomes reduced to QH2 and then exits complex I
– as electrons exit there is a conformational change in Complex I
What is FMN and what is it’s role in complex I in ETC?
– FMN is Flavin mononucleotide (oxidized) (FMN)
– NADH reduces it to FMNH2 (w proton)
– it is involved with the first transfer step –> it picks up electrons from NADH
– 2 electrons are transferred
What are the Iron- Sulfur clusters?
– Iron boun to Cysteine’s sulfhydral groups
– also have Iron bound to Cys sulfhydral groups and inorganic sulfides
– the sulfhydryl groups help to bind iron together
– iron-sulfur clusters are important bc they help electrons move along through being good accepters and transferers
T or F, after Q exits complex I the protons diffuse back into the intermembrane space
False; mitochondrial matrix
Describe coenzyme Q as an electron carrier.
– Coenzyme Q is a versatile cofactor because it is a soluble electron carrier in the hydrophobic bilipid layer of the inner mitochondrial membrane.
– Like flavoproteins, CoQ can accept/donate electrons one at a time or two at a time
– Long hydrophobic tail anchors Q in the membrane, but it’s mobile
– exists in different oxidative states
What is the importance of Q’s varied redox states?
– can exist stabling a semiquinone state ( can pick up 1 electron)
– important if the need arises to transfer one electron at a time
– can also pick up one or two electrons at one time
– also high energy electron carrier; picking up electrons but not being reduced
– Q can pick up one or two electrons depending on who is donating
T or F, Q accepts protons exclusively from the matrix
True
What happens when there is a conformational change of Complex I?
– Another 4 protons are “pumped,” due to the conformation change which comes from electrons exiting complex I
What is the net reaction of NADH oxidation in complex 1?
– electron transfer from NADH through complex I provides QH2 for the Q pool and pumps protons
– NADH + Q + 5protons matrix –> NAD+ +QH2 + 4protons intermembrane space
T or F, another source of QH2 are reactions catalyzed by Complex II, or succinate-Q reductase
True
Describe the role succinate in complex II.
– succinate dehydrogenase is part of complex II with succunate-Q reductase
– FADH2 never leaves the complex instead it is used to reduce Q to QH2 and thereby increase QH2 content in the Q pool
– Although QH2 is generated, no protons are pumped in this reaction
How does FADH2 drop off its electrons?
– FADH drops off electrons at Q-cytocrhome c oxidoreductase
– Q comes and picks it up and this is a different Q that is also then reduced to QH2
T or F, Q being reduced to QH2 induces conformational change?
True
T or F, Coenzyme Q can also receive electrons from other sources
True; but all enter the same “Q pool”
– from here QH2 transfers electrons to Complex III
T or F, FAD functions as 2 electron acceptor and a 1 electron donor
true; final step of this Complex is transfer of 2 electrons one at a time to coenzyme Q to produce CoQH2
