CAC Pyruvate Dehydrogenase Complex

Thiamine Pyrophosphate (TPP) location

Found tightly Bound to E1

Thiamine Pyrophosphate (TPP) Function

Decarboxylates pyruvate, yields hydoethyl-TPP

Lipoic acid (lipoamide) location

Covalently bound to E2 via lysine "swinging arm"

Lipoic acid (lipoamide) function

Accepts hydroxyethyl carbanion from TPP ass acetyl group

Co enzyme A (CoA) function

Accepts acetyl group from lipoamide

Co enzyme A (CoA) location

Dissociable substrate for E2

Flavin Adenine Dinucleotide (FAD) function

Accepts pair of electrons from reduced lipoamide

Flavin Adenine Dinucleotide (FAD) location

Tightly bound to E3

Nicotinamide adenine dinucleotide (NAD+) location

Dissociable substate for E3

Nicotinamide adenine dinucleotide (NAD+) function

Accepts a pair of electrons from reduced FADH2

Three enzymes that are apart of the pyruvate dehydrogenase complex (PDH complex)

pyruvate dehydrogenase (E1), dihydrolipoamide transacetylase (E2), and dihydrolipoamide dehydrogenase (E3)

E1

Pyruvate Dehydrogenase

E2

Dihydrolipoamide transacetylase

E3

Dihydrolipoamide Dehydrogenase

Thiamine pyrophosphate (TPP) is the coenzyme used

for all decarboxylations of α-keto acids, which require a cofactor to help stabilize the carbanion intermediate that develops during the reaction.

Thiamine pyrophosphate (TPP) vitamin

Vitamin B1

Riboflavin vitamin

Vitamin B2

Reaction of PDH step 1

Decarboxylation of Pyruvate: E1 catalyzes the decarboxylation of pyruvate, forming hydroxyethyl-TPP.

Reaction of PDH step 2

Transfer to Lipoamide: The hydroxyethyl group is transferred to a lipoamide moiety on E2, forming acetyl-dihydrolipoamide.

Reaction of PDH step 3

Formation of Acetyl-CoA: E2 transfers the acetyl group to CoA, producing acetyl-CoA.

Reaction of PDH step 4

Regeneration of Lipoamide: E3 reoxidizes dihydrolipoamide, transferring electrons to NAD+, forming NADH.

Niacin Vitamin

Vitamin B3

Pantothenic Acid Vitamin

Vitamin B5

Which enzyme in the PDH complex is vitamin B1 associated with?

E1

What is the function of the cofactor lipoamide?

Transfers acyl groups

What would be the effect of losing the cofactor NAD+ on the function of PDH? 

The Cys sulfhydryl groups could not be regenerated to the oxidized from, preventing the enzyme complex from resetting to catalyze another reaction.

Why would losing the cofactor NAD+ prevent the PDH complex from resetting to catalyze another reaction?

The function of NAD+ in PDH complex is to accept electrons from the Cys sulfhydryl groups in the lipoamide arm of E2. This reduces NAD+ to NADH and oxidizes the Cys groups from two SH groups to S-S. This oxidation step is what allows the lipoamide arm of E2 to “reset” and catalyze the next substrate. A would be a result of losing some function in E1 or the TPP cofactor, and B would result from loss of CoA.