PDH Coenzymes — BCH Exam 2

What is a coenzyme in this lecture?

A nonprotein helper that lets an enzyme carry out a reaction its amino acids alone cannot perform.

What makes a coenzyme a prosthetic group?

It is tightly bound to its enzyme and does not dissociate during the reaction.

Which coenzyme is the prosthetic group on PDH E1?

TPP (thiamin pyrophosphate).

Which coenzyme is the prosthetic group on PDH E2?

Lipoamide, attached to a flexible arm of E2.

Which coenzyme is the prosthetic group on PDH E3?

FAD (flavin adenine dinucleotide).

Which two PDH coenzymes can enter and leave rather than staying tightly bound?

Coenzyme A (CoA) and NAD+.

Which vitamin gives rise to TPP?

Thiamin (vitamin B1).

What is TPP's main job in PDH?

It helps E1 decarboxylate pyruvate, releasing CO2 and temporarily holding the remaining two-carbon fragment.

Why can TPP help decarboxylate pyruvate?

Its unusually acidic ring carbon can lose a proton and participate in the bond changes needed for decarboxylation.

Which precursor gives rise to lipoamide?

Lipoic acid.

How is lipoamide attached in PDH?

It is covalently linked to a lysine residue on E2 and remains part of a flexible arm.

What two kinds of jobs can lipoamide do?

It transfers an acyl group and participates in oxidation-reduction through its sulfur groups.

Why is E2's lipoamide arm flexible?

It carries the two-carbon group between active sites so CoA can receive it.

Which vitamin gives rise to CoA?

Pantothenic acid (vitamin B5).

What does CoA do in PDH?

It receives the acetyl group from lipoamide to form acetyl-CoA, then leaves.

Which vitamin gives rise to FAD?

Riboflavin (vitamin B2).

What does FAD do on E3?

It accepts reducing equivalents from reduced lipoamide, restoring the lipoamide disulfide and becoming FADH2.

Which vitamin gives rise to NAD?

Niacin (vitamin B3).

What does NAD+ do near the end of PDH?

It accepts reducing equivalents from FADH2, regenerates FAD, and leaves as NADH.

What is the two-carbon handoff in PDH?

TPP on E1 → lipoamide on E2 → CoA, producing acetyl-CoA.

What is the electron handoff during PDH reset?

Reduced lipoamide → FAD on E3 → NAD+.

When is acetyl-CoA formed relative to the final PDH steps?

It forms when CoA receives the acetyl group; the later reactions reset lipoamide and FAD.

What is the core distinction between CoA and lipoamide in PDH?

Both handle the acetyl group, but lipoamide stays attached to E2 while CoA comes in, accepts acetyl, and leaves.