A10 PDH complex

What is the overall reaction catalyzed by the pyruvate dehydrogenase (PDH) complex?

Pyruvate → acetyl-CoA + CO₂; reduces NAD+ → NADH. (Net: decarboxylation + transfer of 2-carbon acetyl to CoA and generation of NADH.)

Where does conversion of pyruvate to acetyl-CoA occur?

In the mitochondrial matrix; pyruvate is transported there via the mitochondrial pyruvate carrier (MPC).

Name the three catalytic components (enzymes) of the PDH complex and their basic activities.

E1 — pyruvate dehydrogenase (decarboxylase; requires TPP)

E2 — dihydrolipoyl transacetylase (transacylase; uses lipoate and CoA)

E3 — dihydrolipoyl dehydrogenase (oxidizes reduced lipoate, uses FAD and NAD+).

Which coenzyme is required by E1 and what vitamin is it derived from?

Thiamine pyrophosphate (TPP), derived from vitamin B1 (thiamine).

What is the essential reactive group of CoA and what vitamin provides CoA?

A reactive thiol (-SH) group; CoA is derived from pantothenic acid (vitamin B5).

What chemical feature of lipoic acid makes it useful in PDH?

A disulfide bond that can cycle between oxidized (disulfide) and reduced (two thiols) forms, allowing reversible acyl transfer.

What do FAD and FMN have in common and what vitamin are they derived from?

They are flavin nucleotides (flavin cofactors) derived from riboflavin (vitamin B2) and are usually tightly bound to enzymes (flavoproteins).

How does NAD+ accept electrons compared to FAD?

NAD+ accepts a hydride ion (2 electrons at once, H⁻) to form NADH

FAD accepts electrons one at a time (via semiquinone intermediate) to form FADH and FADH₂.

Step 1 of the PDH mechanism — what happens and which enzyme/cofactor does it use?

Oxidative decarboxylation: pyruvate loses CO₂ and forms hydroxyethyl-TPP; catalyzed by E1 with TPP stabilizing the carbanion intermediate.

Step 2 of PDH — how is the hydroxyethyl group processed?

Hydroxyethyl is oxidized to an acetyl group and transferred to the lipoic acid arm on E2, reducing its disulfide to thiols and forming an acetyl-lipoate thioester.

Step 3 of PDH — how is acetyl-CoA formed?

CoA-SH attacks the acetyl-lipoate thioester via E2, generating acetyl-CoA and leaving reduced lipoate on E2.

How is the reduced lipoic acid reoxidized and what cofactors are involved?

E3 uses FAD to accept electrons from reduced lipoate (forming FADH₂), then FADH₂ transfers electrons to NAD+ → NADH, regenerating oxidized FAD and lipoate.

Which PDH step is rate-limiting?

The decarboxylation step catalyzed by E1 (formation of hydroxyethyl-TPP) is rate-limiting.

Name five key coenzymes required by the PDH complex.

TPP (B1), CoA (B5), lipoic acid, FAD (B2), NAD+ (from niacin, B3).

What allosteric signals inhibit the PDH complex?

High energy indicators and products: ATP, NADH, acetyl-CoA, and long-chain fatty acids.

What allosteric signals activate the PDH complex?

Low energy/status signals: AMP (or ADP), NAD+, and CoA-SH.

How is PDH covalently regulated (phosphorylation)? Which hormones affect it?

PDH is inactivated by phosphorylation (PDH kinase) and activated by dephosphorylation (PDH phosphatase).

Insulin stimulates PDH phosphatase (activates PDH in liver and adipose).

Glucagon does not directly regulate PDH kinase; PDH kinase is activated by high ATP, NADH, and acetyl-CoA.

How does insulin influence PDH activity?

Insulin activates the PDH phosphatase which dephosphorylates and activates PDH, promoting acetyl-CoA production for TCA and lipogenesis (in liver and adipose).

Which tissues respond to glucagon versus epinephrine in glycogen and glucose regulation?

Liver responds to both glucagon and epinephrine

muscle responds to epinephrine but not to glucagon (muscle lacks glucagon receptors in the same metabolic context).

Why should hepatic glycolysis be downregulated during hepatic glycogenolysis?

Because liver’s role is to release glucose into blood (via G6P → glucose)

not to consume it; glycolysis should be low during glycogenolysis to allow export.

What role does GSK3 play in glycogen synthase regulation?

GSK3 (glycogen synthase kinase 3) phosphorylates (inhibits) glycogen synthase; insulin signaling inhibits GSK3 to promote glycogen synthase activity.