Lecture 32 Flashcards
what is Pyruvate Dehydrogenase
Pyruvate dehydrogenase is the enzyme that catalyzes the reaction
pyruvate + CoA + NAD+ → acetyl-CoA + CO2 + NADH
what does pdc stand for
pyruvate dehydrogenase complex
where does pdc reaction occur
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
how does pyruvate get into mito
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
how many carbons generate citrate
2 carbons of acetyl coa + 4 carbons of oxaloacetate = generate citrate - makes cycle
describe pdc structure
massive multi enzyme compled
size = 9.5 megadaltons in eukaryotes
pyruvate = 88 daltons
describe protein composition of pdc
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
name coenzymes needed for pdc
TPP
lipoic acid
coenzyme a
FAD
NAD+
summary of Pyruvate Dehydrogenase Reaction - 5 steps
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
summary of Pyruvate Dehydrogenase Reaction - facts
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
What are the mechanistic advantages of multi-enzyme complexes?
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
what does E2 do
generates acetyl coa
describe overall reaction of E2
pyruvate + CoA + NAD+ acetyl-CoA + CO2 + NADH
describe function of coenzyme a - E2
functions as a carrier of acetyl and other acyl groups
describe function of acetyl coa - E2
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
how is pdc regulated - 2
allostery
phosphorylation
describe pdc regulation by allostery
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
describe pdc regulation by phosphorylation
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
what is cac
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
describe discovery of cac
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)
what is cac part of
hub of aerobic metabolism in eukaryotes
cac is part of aerobic metabolism - but does not directly consume oxygen
what is cac - characteristic
amphibolic = site of anabolism and catabolism
describe anabolism of cac
cac intermediates are starting point of anabolic pathways
ex = gluconeogensis, fatty acid and aa synthesis
endergonic
cata= emptying - deplete cac intermediates
build
describe catabolism of cac
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
main functions of cac
to produce intermediates for biosynthesis
is cac anaerobic or aerobic
anaerobic
part of aerobic metabolism but does not need oxygen tho
does cac harvest energy
yes it does
generates electron carriers
Describe overall reaction of cac
3NAD+ +FAD+GDP+Pi+acetyl-CoA→3NADH+FADH2 +GTP+CoA+2CO2
describe carbon count of cac
double everything since glucose makes 2 pyruvate
describe high Energy intermediates of cac
many
succinyl coa etc
describe net energy production of cac
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
how is cac regulated - gen
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
what is cac flux responsive to = 3 things
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)
what gives directionality - 3 reactions
citrate synthase
isocitrate dehydrogenase
alpha-ketodehydrogenase complex
