Lecture 32

Question 1

what is Pyruvate Dehydrogenase

Answer 1

Pyruvate dehydrogenase is the enzyme that catalyzes the reaction
pyruvate + CoA + NAD+ → acetyl-CoA + CO2 + NADH

Question 2

what does pdc stand for

Answer 2

pyruvate dehydrogenase complex

Question 3

where does pdc reaction occur

Answer 3

in mitochondrion
glucose to pyruvate in cytoplasm
pdc converts pyruvate to acetyl co1 in mito - mpc bings in first
acetyl coa has thioester bond = high energy - slp

Question 4

how does pyruvate get into mito

Answer 4

through pyruvate translocase - h+ symport
mitochondrial pyruvate carrier =how gets into mito
mpc needs symport with h+
adp/atp antiport
pyruvic acid = neg so need h+ to maintain electrochem gradient

Question 5

how many carbons generate citrate

Answer 5

2 carbons of acetyl coa + 4 carbons of oxaloacetate = generate citrate - makes cycle

Question 6

describe pdc structure

Answer 6

massive multi enzyme compled
size = 9.5 megadaltons in eukaryotes
pyruvate = 88 daltons

Question 7

describe protein composition of pdc

Answer 7

E1 pyruvate dehydrogenase → 30 heterotetramers
E2 dihydrolipoyl transacetylase →60 core monomers
E3 dihydrolipoyl dehydrogenase →12 homodimers
E3 binding protein →12
pyruvate dehydrogenase kinase →1-3
pyruvate dehydrogenase phosphatase → 1-3

Question 8

name coenzymes needed for pdc

Answer 8

TPP
lipoic acid
coenzyme a
FAD
NAD+

Question 9

summary of Pyruvate Dehydrogenase Reaction - 5 steps

Answer 9

1 = e1 step = decarboxylate where red portion of pyruvate gets put onto molecules tpp (accepts pyruvate), releaes co2 = now committed
regenerate atp to continue moving forwards
2 = e2 - lipoamide, acetyl dihydrolipoamide intermediate
3 = acetyl coa made
4 = e3, fad gets electrons put on - sulfur atoms reduced
5 = nad+ to nadh = reoxidizes

Question 10

summary of Pyruvate Dehydrogenase Reaction - facts

Answer 10

pyruvate + CoA + NAD+ –> acetyl-CoA+CO +NADH
irrev since step 1 irrev = co2 removed from mito
generates acetyl coa for cac and nadh for op
liberated co2 diffuses out of mito

Question 11

What are the mechanistic advantages of multi-enzyme complexes?

Answer 11

quicker kinetics, efficient, not many unfavourable side reactions, easier to regulate complex
Minimized distances for substrates in between active sites= increased reaction rate without having to maintain large pools of intermediates
Metabolic intermediates are channeled between successive enzyme sites = 2 important effects: side reactions are minimized & protection for chemically labile intermediates
Coordinated control of reactions= shutting off one enzyme effectively shuts the system down

Question 12

what does E2 do

Answer 12

generates acetyl coa

Question 13

describe overall reaction of E2

Answer 13

pyruvate + CoA + NAD+ acetyl-CoA + CO2 + NADH

Question 14

describe function of coenzyme a - E2

Answer 14

functions as a carrier of acetyl and other acyl groups

Question 15

describe function of acetyl coa - E2

Answer 15

coa = massive
thioester bond to acetyl group
acetyl thioester (R-C=O-S-R’)
high energy compound (hydrolysis = -31.5)
has high acyl group transfer potential and can donate acetyl group to several acceptors
entry point of cac

Question 16

how is pdc regulated - 2

Answer 16

allostery
phosphorylation

Question 17

describe pdc regulation by allostery

Answer 17

usually metabolites trigger regulation
feedback inhibition = products of pdc, acetyl coa and nadh = inhibit pdc if allowed to accumulate
Causes enzyme to go in reverse = acetyl coa made by e2 so can bind as metabolite and cause to go in reverse
shut down e1 through product inhibiton
prevents useless comsumption of pyruvate

Question 18

describe pdc regulation by phosphorylation

Answer 18

covalent reg on e1
kinase and phosphatase inactive and activate e1 of pdh complex by phosphorylation and dephosphorylation
magnesium and calcium insulin = activate
pyruvate and adp inhibit
if adp levels high = need to make more atp
if pyruvate levels high = suggest lots of glycolysis so need more energy = inhibits inhibitors

Question 19

what is cac

Answer 19

series of 8 enzymatic reactions
combine acteyl coa (2c) and oxaloacetate (4c) to generate co2, nadh, fadh2
regenerates starting product oxaloacetate
aka tricarboxylic acid cycle (tca) or krebs cycle
3nadh and 2fadh generated

Question 20

describe discovery of cac

Answer 20

1937
2 main contributors =
1) Albert Szent-Györgyi (Nobel prize 1937, respiration) 2) Hans Krebs (Nobel prize 1953, CAC)
(Krebs also discovered the urea cycle)

Question 21

what is cac part of

Answer 21

hub of aerobic metabolism in eukaryotes
cac is part of aerobic metabolism - but does not directly consume oxygen

Question 22

what is cac - characteristic

Answer 22

amphibolic = site of anabolism and catabolism

Question 23

describe anabolism of cac

Answer 23

cac intermediates are starting point of anabolic pathways
ex = gluconeogensis, fatty acid and aa synthesis
endergonic
cata= emptying - deplete cac intermediates
build

Question 24

describe catabolism of cac

Answer 24

cac intermediates are end point
aerobic catabolism of carbs, lipid and aas merge into cac
ex = oxaloacetate from pyruvate carboxylase aa degradation
ana = filling up, replenish depleted cac intermediates
oxidation to smaller

Question 25

main functions of cac

Answer 25

to produce intermediates for biosynthesis

Question 26

is cac anaerobic or aerobic

Answer 26

anaerobic
part of aerobic metabolism but does not need oxygen tho

Question 27

does cac harvest energy

Answer 27

yes it does
generates electron carriers

Question 28

Describe overall reaction of cac

Answer 28

3NAD+ +FAD+GDP+Pi+acetyl-CoA→3NADH+FADH2 +GTP+CoA+2CO2

Question 29

describe carbon count of cac

Answer 29

double everything since glucose makes 2 pyruvate

Question 30

describe high Energy intermediates of cac

Answer 30

many
succinyl coa etc

Question 31

describe net energy production of cac

Answer 31

net production/turn
3nadh: 2.5 atp/nadh = 7.5 atp
1fadh2: 1.5atp/fadh2 = 1.5 atp
1gtp=1 atp
total = 10 atps, but double so 20 atps per glucose
3 reactions with low delta g which gives directionality

Question 32

how is cac regulated - gen

Answer 32

allosteric reg
either upstream or things associated with using atp
activate = calcium, insulin, magnesium, adp, pyruvate
inhibit = acetyl coa, nadh, atp, succinyl coa (inhibits citrate synthase)
if citric acid cycle stopped = all inhibitors will build up

Question 33

what is cac flux responsive to = 3 things

Answer 33

1 - energy state of cell through allosteric activation (idh by adp)
2 - redox state of cell through mitochondrial nadh/nad ratio
3 - availability of energy rich compounds (acetyl coa, succinyl coa) that inhibit cac enzymes (cs and alpha-kgdh)

Question 34

what gives directionality - 3 reactions

Answer 34

citrate synthase
isocitrate dehydrogenase
alpha-ketodehydrogenase complex