Unit 6 - Electron Transport by Respiratory Chain

Question 1

where is the respiratory chain located? what are its components?

Answer 1

enzymes are embedded in the inner mitochondrial membrane

• complex I - NADH dehydrogenase
• Q - quinone (dissolved in liquid phase)
• complex III - bc1 complex
• cytochrome c - bound to IMM in intermembrane space
• complex IV - cytochrome oxidase

Question 2

what are the advantages of a membrane-bound respiratory chain?

Answer 2

• not limited by the rate of diffusion
• no mobile carrier needed
• E stored in reduced fuels are converted to E stored in electrochemical gradient (needs 2 compartments provided by membrane)

Question 3

what are the 5 REDOX centers of respiratory chain?

Answer 3

1. flavins
2. Fe-S centers
3. ubiquinone
4. hemes
5. copper centers

Question 4

what are flavins? and examples?

Answer 4

2-electron donor/acceptor + 2H+ via 1,4 addition

• includes FMN (flavin mononucleotide; oxidized), FMNH2 (reduced), and FAD (flavin adenine dinucleotide; oxidized)
• -FAD has adenine bound, so needs specific binding site

Question 5

what REDOX center does NADH dehydrogenase have?

Answer 5

tightly bound FMN

Question 6

what REDOX center does succinate dehydrogenase have?

Answer 6

covalently bound FAD

Question 7

what REDOX center does FA-CoA dehydrogenase have?

Answer 7

FAD

Question 8

what REDOX center does glycerol phosphate dehydrogenase have?

Answer 8

FAD

Question 9

what are Fe-S centers? examples?

Answer 9

accept electrons from flavins and Q

• no matter how many Fe, only accept/donate 1 electron
• Fe2S2 of complex III
• -2 Fe+++ + e- –> 2 Fe2.5+
• Fe4S4 of succinate dehydrogenase
• -2 Fe+++ + 2 Fe+++ e- –> Fe+++ + 3 Fe++
• -3 Fe+++ + Fe++ + e- –> 2 Fe+++ + 2 Fe++

Question 10

what happens if Fe-S centers are acidified?

Answer 10

makes H2S gas (like what happens in old eggs)

Question 11

what is ubiquinone? (also Q, Ub-Q, CoQ, etc.)

-why is it unique?

Answer 11

two electron donor/acceptor + 2H+

• 1,6 addition
• 10 isoprenoid chains in mammals
• very hydrophobic
• 10-fold excess over other components
• function as electron buffer
• unlike the other REDOX centers, it’s not a prosthetic group
• unique b/c excess mobile electron carrier between early dehydrogenases and later part of respiratory chain

Question 12

what is heme as a REDOX center?

Answer 12

one-electron donor/acceptor

• Fe+++ + e- –> Fe++
• covalently attached to 2 cysteine side chains in cytochromes c and c1
• -Fe atom coordinates to met 80 and his 18 side chains

Question 13

how is the structure of cytochrome:c, cytochrome b, and heme c?

Answer 13

• cytochrome c has highly conserved structure
• cytochrome b hemes are 1 protein with 2 cytochrome complexes bound to same IMM walls
• heme C has thioester linkages to a protein

Question 14

what are copper centers as REDOX centers?

Answer 14

one electron donor/acceptors in cytochrome oxidase (complex IV)

• CC A: has 2 Cu
• -2 Cu++ + e- Cu++ + Cu+
• CC B: along with heme of cyt a3 forms O2 binding site at end of respiratory chain

Question 15

how are electron carriers arranged along respiratory chain?

Answer 15

increasing affinity for electrons (Eo’)

• important for efficiency of extracting useful energy
• NAD –> Q –> bc1 complex –> cytochrome c –> cytochrome a
• when electron passes thru large Eo’ drop, it’s passing from REDOX center of low affinity to greater affinity
• -stabilization of electron is an exergonic process that could be coupled to endergonic generation of electrochemical gradient

Question 16

what is the only heme group in the respiratory chain that binds O2?

Answer 16

cytochrome a3 of cytochrome oxidase

-it’s the last thing to be reduced, thus maintaining ferric Fe

Question 17

what are the components of cytochrome oxidase? why are these components necessary?

Answer 17

cytochrome c –> Cu A –> heme A –> heme a3 (bound to O2 and Cu B)
-participation of 2 Cu centers and 2 heme irons in cytochrome oxidase are required to rapidly deliver 4 electrons to O2, and avoid production of H2O2 and superoxides

Question 18

ATP synthase (FoF1) structure

Answer 18

• F1 has 3 catalytic sites for ATP synthesis (juts into matrix)
• Fo is hydrophobic complex that traverses membrane and carries H+ from one site to another (sits on membrane)
• connected by central stalk (rotor) and external stalk (stator)

Question 19

overall reaction for oxidative phosphorylation

-how is this related to respiratory control?

Answer 19

NADH + H+ + 1/2 O2 + 3 ADP + 3 Pi –> NAD+ + 3 ATP + 4 H2O

• IOW: 1 NADH ~ 3 ATP (actually about 2.7)
• obligatory linkage of endergonic ATP synthesis to exergonic REDOX RXNs

Question 20

respiratory control and what causes it

Answer 20

electrochemical gradient functions as a common intermediate linking oxidation to phosphorylation

• O2 consumption is coupled to ATP synthesis
• rate of respiration is controlled by availability of ADP

Question 21

what is the P/O ratio?

Answer 21

ADP consumed / O consumed = ATP formed per pair of electrons from substate to O2

Question 22

what is the overall job of the respiratory chain?

Answer 22

break up the 52 kcal/mol available from flow of 2 electrons from NADH to O2 into 3 equal amounts, used to drive formation of ATP