Making Acetyl CoA

Question 1

enzymes involved

Answer 1

3 enzymes held by noncovalent interactions

E1: pyruvate dehydrogenase

E2: dihydrolipolyl transacetylase

E3: dihydrolipolyl dehydrogenase

Question 2

overall rxn

Answer 2

enzyme name: oxidative decarboxylation

enzyme cofactors: TPP, lipoic acid, FAD

pyruvate + CoASH + NAD+ –> acetyl CoA + CO2 + NADH + H+

Question 3

cofactors involved

Answer 3

1. TPP
2. lipoic acid
3. FAD

Question 4

substrate channeling

Answer 4

evolutionary advantage of having substrates passed from 1 subunit to another in this 3 enzyme complex

prevents intermediates from being used in other rxns

Question 5

step 1

Answer 5

decarboxylation by E1 using TPP

TPP has carbanion from low pKa (adjacent to + charge on nitrogen and environment of enzyme)

uses 2 HA

Question 6

step 2

Answer 6

don’t have to memorize

TPP reformed

HA 1 reformed

Question 7

step 3

Answer 7

CoASH attacks

HA 2 reformed

HA 3 used

Question 8

step 4

Answer 8

enzyme 3 does this

FAD + NADH + H+ –> FADH2 + NAD+

lipoic acid reformed

Question 9

overall reactions in words

Answer 9

step 1: decarboxylation with TPP (E1)

step 2: oxidation/reduction + transfer to lipoic acid (E2)

step 3: thioester cleavage by CoASH –> CoA (E3)

step 4: oxidize lipoic acid and reduce FAD to FADH2