L2

What is the function of Pyruvate dehydrogenase?

converts pyruvate to acetyl-CoA,

producing CO₂ and NADH

What type of reaction does PDH catalyse?

oxidative decarboxylation reaction

Where does the PDH catalysing reaction occur?

mitochondria

Why is the PDH reaction metabolically important?

irreversible

commits pyruvate to energy production via the citric acid cycle

Why is pyruvate described as being at a central point in metabolism?

links glycolysis

to citric acid cycle and other metabolic cycles

What pathway does acetyl-CoA enter after PDH?

citric acid cycle

Is PDH a single enzyme or complex?

a large multi enzyme complex

between 5 and 10 ×10⁶ Da

What are the 3 enzyme components of PDH?

E1 = Pyruvate dehydrogenase

E2 = Dihydrolipoyl transacetylase

E3 = Dihydrolipoyl dehydrogenase

Why does PDH require multiple enzymes for one reaction?

To allow coordinated catalysis

increase efficiency

prevent side reactions

How many cofactors are required for PDH activity?

5

many are derived from vitamins

Why do vitamin deficiencies impair PDH activity?

Because several of the PDH cofactors are derived from vitamins meaning there will be a reduced energy production when deficient

What is the role of lipoic acid in PDH?

It forms a flexible, movable arm

transfers intermediates between active sites

Which PDH subunit contains the lipoamide arm?

E2 (dihydrolipoyl transacetylase)

Why is the lipoamide arm important?

allows substrate channelling

increasing reaction speed and efficiency

What reaction does E1 catalyse?

the decarboxylation of pyruvate

hydroxyethyl-TPP

What happens to the hydroxyethyl group after decarboxylation?

hydroxyethyl group transferred to lipoamide

AND it is oxidised to an acetyl group

Two electrons left behind from decarboxylation transferred to E2

What is the role of E2 in PDH?

transfer of the acetyl group to a CoA

forming acetyl CoA

(joining them)

What is the role of E3 in PDH?

regeneration of oxidised lipoamide via FAD and NAD⁺

producing NADH - electron carrier

(reduction of FAD)

• Electrons move from FADH2 to NAD to make NADH and FAD

Why are PDH intermediates not released into the solution?

They are highly bound

transfer between enzymes, reducing side reactions

What advantages does the PDH complex structure provide?

Maximised efficiency, speed, and control of the overall reaction

Why must PDH be regulated?

Because it catalyses an irreversible commitment to energy production

Which molecules inhibit PDH activity?

NADH

acetyl-CoA

ATP

(reaction products)

Which molecules activate PDH activity?

NAD⁺

ADP

AMP

CoA

(reactants)

How is PDH primarily regulated?

By phosphorylation and dephosphorylation of the E1 subunit.

What is the effect of PDH kinase activity?

Phosphorylation → PDH inactivation

What activates PDH kinase?

ATP

NADH

acetyl-CoA

What inhibits PDH kinase?

NAD⁺

ADP/AMP

CoA

What is the role of phosphatase?

Dephosphorylates E1

→ activates PDH

What stimulates PDH phosphatase in muscle?

Ca²⁺

especially during muscle contraction

What is beriberi and how does it relate to PDH?

thiamine deficiency- vitamin B₁

that impairs PDH

reducing ATP production

What metabolic changes are seen in PDH impairment?

Elevated pyruvate and lactate in the blood

How do mercury and arsenite poisoning affect metabolism?

They inhibit PDH

causing beriberi-like symptoms.

Why is pyruvate described as as a metabolic pathways?

it can be metabolised by five different pathways

depending on cellular conditions

What happens to pyruvate under anaerobic conditions in muscle?

It is reduced to lactate by lactate dehydrogenase

to regenerate NAD⁺

so glycolysis can continue

Why does lactate accumulation cause muscle cramping?

Lactate is a metabolic dead end in muscle

accumulates when oxygen is limited

How can pyruvate contribute to glucose or glycogen replenishment?

converted to oxaloacetate by pyruvate carboxylase

in glucose-depleted conditions

How is pyruvate linked to amino acid metabolism?

Pyruvate can be transaminated to alanine

reaction is reversible and depends on amino acid availability

What happens to pyruvate in microorganisms during fermentation?

It can be converted to ethanol

Which PDH subunit uses thiamine pyrophosphate (TPP)?

E1

Which vitamin is required to make TPP?

Vitamin B₁ -thiamine

Which PDH subunit uses Coenzyme A?

E2

Which vitamin is Coenzyme A derived from?

Vitamin B₅

Which PDH subunit contains lipoamide?

E2

Is lipoamide dietary or synthesised by the body?

Synthesised

→ non-dietary

Which PDH subunit uses FAD?

E3

Which vitamin is FAD derived from?

Vitamin B₂

Which PDH subunit interacts with NAD⁺?

E3

E3

Which vitamin is NAD⁺ derived from?

Vitamin B₃

Why is TPP essential for pyruvate decarboxylation?

It forms a reactive carbanion

that attacks the carbonyl carbon of pyruvate

What happens when the TPP carbanion reacts with pyruvate?

A carbon–carbon bond forms

triggering CO₂ release

What happens to the electrons left after decarboxylation?

They remain on the hydroxyethyl-TPP intermediate

Why is the hydroxyethyl-TPP intermediate unstable?

It is electron-rich

readily transfers electrons

What role does lipoamide play in PDH?

It accepts electrons and the acetyl group, then transfers them to other subunits

Why is lipoamide described as a “flexible arm”?

It is attached to E2 via a long (~14 Å) arm that reaches multiple active sites

Trace the path of electrons during the PDH reaction.

Pyruvate → TPP → lipoamide → FAD → NAD⁺ → electron transport chain

Why does thiamine deficiency cause lactic acidosis?

PDH is impaired, so pyruvate is shunted to lactate to regenerate NAD⁺

How do mercury and arsenite inhibit PDH?

They bind to lipoamide

rendering it inactive via chelation

What is PDH phosphatase stimulated by?

cytosolic Ca²⁺

important in muscles