PHSC 208, L27

Pyruvate Dehydrogenase (PDH)

• converts pyruvate (3 Carbons) to acetyl-CoA (2 Carbons) for the citric acid cycle

• exothermic reaction therefore irreversible in cells

• 3 subunits: E1, E2, E3

• uses 5 different co-enzymes

Coenzyme A (CoASH)

• acts as acyl carrier without being oxidized

E1 group of Pyruvate Dehydrogenase

Cofactor: TPP

decarboxylates pyruvate and forms hydroxyethyl-TPP

releases CO2 in the process

E2 group

Cofactor: Lipoic acid, CoA

the dihydropolipoyl acetyltransferase converts pyruvate into acteyl-CoA

• gets reduced to hold 2 carbons

??

E3 group

Cofactor: FAD, NAD+

dihydrolipoyl dehydrogenase?

• FAD reduced by lipoamide

• NAD+ reduced by FADH2

8 stages in the citric acid cycle

citrate

isocitrate

alpha-Ketoglutarate

succinyl-CoA

succinate

fumarate

malate

oxaloacetate

Citrate → Isocitrate enzyme

Aconitase

• brings H2O in and forms alcohol?

Isocitrate → alpha-Ketoglutarate enzyme

Isocitrate dehydrogenase, releases CO2 and NADH

1. oxidation to a ketone turns NAD+ to NADH

2. Decarboxylation releases CO2 (6C → 5C)

alpha-Ketoglutarate → Succinyl-CoA enzyme

alpha-ketoglutarate dehydrogenase, releases CO2 and NADH

1. oxidation to a ketone turns NAD+ to NADH

2. Decarboxylation releases CO2 (5C → 4C)

Succinyl-CoA→ Succinate enzyme

Succinyl-CoA synthetase, releases GTP

• uses energy by alpha-KGDH to make one GDP directly

• CoA phosphorylated off succinate, and phosphate transferred to GDP

• GDP converts spontaneously to ATP

Succinate→ Fumarate enzyme

Succinate dehydrogenase, releases FADH2

• oxidation of C-C to C=C has less energy

• FAD is hydrogen acceptor

Fumarate→ Malate enzyme

fumarase

• water enzyme (lyase class)

• highly reversible

Malate → Oxaloacetate enzyme

Malate dehydrogenase, releases NADH

Oxaloacetate → Acetyl-CoA

citrate synthase

Oxaloacetate transport

• an intermediate in TCA cycle

• there are no transporters for oxaloacetate on the mitochondrial membrane due to its charged nature, it must be converted to malate or aspartate for transport into or out of the mitochondria

  • Aspartate- aspartate aminotransferase

  • Malate- malate dehydrogenase

iClicker: Citrate is also a regulator in glycolysis, which glycolysis enzyme does it regulate?

Phosphofructokinase-1 (PFK-1)

activity of the citric acid cycle is determined by ____

the need for energy

• high levels of ATP and NADH indicate that there is plenty of energy available “respiratory control”

Define anaplerotic reactions

reactions that add intermediate molecules

Key anaplerotic reaction?

Pyruvate carboxylase

Pyruvate→ Oxaloacetate

Citrate synthase is regulated by ____

the availability of oxaloacetate

Excess citrate transported to the cytosol to…

1. have role in oxaloacetate, malate, pyruvate formation

2. lead to NADPH production for fatty acid biosynthesis

3. inhibit glycolysis (inhibits PFK-1)

What happens when we have too much citrate?

pump citrate back into cytoplasm, which gets acetyl CoA back from citrate and oxaloacetate using citrate liase

• conserves energy

Role of citrate synthase in citric acid cycle?

Citrate synthase catalyzes the first step of the citric acid cycle, by condensing acetyl CoA and oxaloacetate to form citrate.

What does citrate do when it is transported to the cytosol?

• helps with oxaloacetate, malate, pyruvate production

• produces NADH for fatty acid biosynthesis

• feed back into glycolysis to inhibit PFK-1