W4 Electron Transport and Oxidative Phosphorylation

Question 1

how are the 12 electron pairs involved in glucose oxidation transferred to O2

Answer 1

transferred to coenzymes NAD+ and FAD > form 10 NADH and 2FADH2 > become oxidised in electron transport chain

Question 2

anatomy of mitochondria

Answer 2

inner membrane highly to form cristae

heart muscle cells have high rates of respiration > densely packed cristae

liver cells with lower respiration rates > sparsely distributed cristae

electrochemical gradient generated by protons across membrane by complex I, III and IV

Question 3

what is the malate-aspirate shuttle

Answer 3

electrons in NADH are used to reduce cytosolic oxalacetate into malate > malate transported into mitochondria via malate-alpha-ketoglutarate carrier > oxidised back into oxalacetate to release NADH again

Question 4

what is the glycerophosphate shuttle

Answer 4

1. cytosolic NADH from glycolysis transfers its electrons to DHAP > converted to glycerol-3-phosphate, catalysed by glycerol-3-phosphate dehydrogenase
2. glycerol-3-phosphate enters mitochondria > reoxidation of FADH2 with the passage of electrons into the ETC

Question 5

how does atp-adp translocate mediate movements of atp and adp

Answer 5

has single nucleotide binding site facing matrix > binds to atp > reorients to face inter membrane space > exchange for adp > face back matrix

outside of inner membrane more positive than inside due to pumping of protons > atp more negative than adp > outward movement of atp is favoured

Question 6

concept of electron transport chain

Answer 6

most of chain’s components are proteins

carriers alternate reduced and oxidised states as they accept and donate electrons

electrons drop in free energy as they go down the chain and pass to final e acceptor, O2, forming H2O

it breaks the large free energy drop from food to O2 into smaller steps that release energy in manageable amounts

Question 7

what does it mean to have a more positive standard reduction potential

Answer 7

more positive > greater the tendency for the redox couple’s oxidized form to accept electrons and become reduced

Question 8

how many ATP is produced per oxidation of one NADH

Answer 8

3

Question 9

structure and function of complex I (NADH dehydrogenase)

Answer 9

consists of about 45 subunits > core structure includes flavin mono nucleotide and several iron-sulfur clusters

NADH donates two electrons to FMN > reduced to FMNH2 > e passed through series of Fe-S clusters in complex > transferred t o coenzyme Q

Question 10

properties of coenzyme Q (ubiquinone)

Answer 10

a lipophilic EC with benzoquinone linked to an isoprene-containing tail

can transfer two electrons in one electron step via a stable semiquinone intermediate

provides a link between two-electron carriers and one-electron carriers

Question 11

structure and function of complex II (succinate dehydrogenase)

Answer 11

made up of 4 subunits including FAD and heme, not involved in pumping of protons

catalyse oxidation of succinate to fumarate in TCA > FAD reduced to FADH2

electrons from FADH2 transferred through Fe-S clusters in complex II to coenzyme Q > reduce it to ubiquinol > ubiquinol carries these electrons to complex III

Question 12

structure and function of complex III (cytochrome bc1 complex)

Answer 12

11 subunits; cytochrome b consisting of 2 heme groups, low and high spin, cytochrome c1 and Fe-S

catalyse transfer of electrons from ubiquinol to cytochrome c in intermembrane space > pumps two protons into IMS for every 2 electrons transferred

about 250kDa and functions as a dimer with each monomer composed of 10 or 11 protein chains

Question 13

what happens in the Q cycle

Answer 13

first e transfer: ubiquinol donates one e to cytochrome c via Fe-S and another to cytochrome b > reduces a semiquinone at Qi site

second round: another ubiquinol repeats this process > fully reducing semiquinone back to ubiquinol > pump total of 4 protons into inter membrane space

Question 14

structure and function of cytochrome c

Answer 14

contains a heme group with iron that can alternate between Fe2+ and Fe3+ > transfer electrons from Fe-S of complex III > migrates along membrane surface in reduced state > carry electrons to complex IV (cytochrome c oxidase)

Question 15

structure and function of complex IV (cytochrome c oxidase)

Answer 15

accept electrons from cytochrome c > transferred through two heme and two copper centers > catalyze reduction of O2 to H2O with the use of 4 electrons from cytochrome c and 4 protons from matrix

Question 16

what is the final outcome of electron transfer

Answer 16

free energy stored in proton gradient is called proton motive force (PMF) > has 2 components:

1. chemical potential energy due to difference in proton concentration (expressed as pH difference)
2. electrical potential energy due to separation of charge across membrane (expressed as electrical potential difference)

Question 17

what is the chemiosmotic hypothesis

Answer 17

free energy of electron transport is conserved by pumping H+ from matrix to IMS > create electrochemical H+ gradient across inner mitochondrial membrane > electrochemical potential of gradient harnessed to synthesise ATP

Question 18

how is electron transport chain regulated through coupling

Answer 18

more atp used > increase in adp > increase in proton influx > electrochemical gradient decreases > proton pumps respond by pumping more to maintain electrochemical gradient > result in increased O2 consumption

Question 19

uncoupling in oxidative phosphorylation

Answer 19

uncouplers have a hydrophobic character and a dissociable proton on cytosolic surface of membrane

they acquire protons on cytosolic surface of membrane > carry into matrix > destroy proton gradient

protons still driven out of matrix but bc of uncouplers > leak back in rapidly > atp synthesis does not occur > energy released in ETC dissipated as heat

Question 20

what is the brown fat story

Answer 20

newborn mammals have brown fat > cells have larger number of mitochondria containing uncoupling protein > energy of gradient dissipated as heat > maintain body temperature