L16: C6 Enzyme Inhibition and Control

Biochemistry I

Allosteric effector

A small molecule whose binding to one site of a protein affects the function of another site of the protein

Irreversible Inhibitors

Bind to the enzyme so tightly or permanently (ex: covalent bind) that they permanently block the enzyme’s activity (inactivators)

Reversible Inhibitors

Binds to the enzyme reversibly, temporarily diminishing the enzyme apparent activity

• have different kinetics, depending on how they bind to the enzyme

Competitive Inhibitor

Inhibitor binds to the enzyme substrate binding site, and competes with the substrate

• increases the apparent K_M for the reaction

Uncompetitive Inhibitor

Inhibitor binds directly to the enzyme/substrate (ES) complex, not to the free enzyme

• decrease both the apparent V_max and K_M

Mixed inhibitor (noncompetitive)

Inhibitor binds to both the free enzyme and the enzyme/substrate (ES) complex

• decrease V_max and the apparent K_M might increase, decrease, or stay the same

Control of enzyme availability

Enzyme V_max is impacted by the concentration of enzyme units

• can be regulated by the rates of enzyme synthesis and enzyme degradation

Control of substrate availability

• without the substrate, the enzyme cannot catalyze the reaction

• therefore, in a metabolic pathway, preventing the production of a substrate can be an effective mechanism to regulate the activity of an enzyme

Control of enzyme activity

• Enzyme activity can be regulated by both competitive inhibitors and allosteric effectors

• both of these changes are reversible

• alternatively, enzyme activity can be controlled by covalent modifications, such as phosphorylation and dephosphorylation

Aspartate transcarbamoylase (ATCase)

• catalyzes the formation of carbamoyl aspartate, an early step in pyrimidine biosynthesis

• enzyme is controlled by allosteric regulation and feedback inhibition

Pyrimidines (single ring)

• Thymine

• Uracil

• Cytosine

Feedback Inhibition

Where the product of a metabolic pathway inhibits its own synthesis at the beginning or first committed step in the pathway

ATCase has two subunit types

• c subunit (catalytic subunit 34 kD) which forms trimers

• r subunit (regulatory subunit, 17 kD) which forms dimers

• active complex: c6r6