Ch 19: Harvesting electrons from the cycle

What generates ATP?

Oxidative-phosphorylation

Reducing equivalent

reduced form of a redox coenzyme related to a dehydrogenase (NADH, FADH2)

Where does the citric acid cycle occur?

Mitochondria

What are the major reactants of the citric acid cycle?

• CoA

• NAD+

• GDP

• FAD

Step 1

Reactants: Acetyl-CoA, Oxaloacetate, H2O

Products: CoASH, Citrate

Enzyme: Citrate synthase

Step 2

Reactant: Citrate

Product: Isocitrate

Enzyme: Aconitase

Step 3

Reactant(s): Isocitrate, NAD+

Product(s): a-ketoglutarate, NADH + H+

Enzyme: isocitrate dehydrogenase

Step 4

Reactants: a-ketoglutarate, CoA-SH, NAD+

Products: Succinyl-CoA, NADH, H+, CO2

Enzyme: a-ketoglutarate dehydrogenase complex

Step 5

Reactants: Succinyl-CoA, GDP, Pi

Products: Succinate, CoASH, GTP

Enzyme: Succinyl-CoA synthetase

Step 6

Reactants: Succinate, FAD

Products: Fumarate, FADH2

Enzyme: Succinate dehydrogenase

Step 7

Reactants: Fumarate, H2O

Products: Malate

Enzyme: Fumarase

Step 8

Reactants: Malate, NAD+

Products: Oxaloacetate, NADH + H+

Enzyme: Malate dehydrogenase

What regulates metabolic flow in TCA cycle?

when the standard Gibbs free energy is negative (exergonic) , inhibitors and activators

• automatic regulation to the ratio of P/S

• excess of products, endergonic

• excess of substrate, exergonic

Negative inhibitors

Coenzymes that are also products in the reaction

Positive activators

Coenzymes that are reactants in the reactions

Homotropic regulation

Allosteric regulation that is linked directly the substrate and product

Heterotrophic regulation

binding of effector that isn’t a direct substrate or product of net reaction of the targeted enzyme (ex: Ca2+)

Amphibolic

both sides of metabolic pathway

What steps are committed?

Step 1, 3, and 4

Committed steps

Steps that are exergonic and are rate limiting

What type of regulation is used for committed steps?

Kinetic (enzymes regulate)

FAD

exists in hydrophobic environment, buried in the interphase and inner membrane of mitochondria

What is a competitive inhibitor of succinate dehydrogenase?

Malonate (both have similar structures)

Products of TCA cycle

• 4 reduced coenzymes

• 2 CO2

• 1 ATP