Enzymes 3

What is enzyme regulation?

Changing enzyme activity to match cellular needs without rebuilding an entire pathway.

What are the three main types of reversible enzyme inhibition?

Competitive, noncompetitive, and uncompetitive inhibition.

What determines how an inhibitor affects Km and Vmax?

Where the inhibitor binds and which enzyme form it binds to.

What is competitive inhibition?

The inhibitor competes with the substrate for the free enzyme's active site.

How does competitive inhibition affect apparent Km?

It increases apparent Km because more substrate is needed to reach half of Vmax.

How does competitive inhibition affect Vmax?

Vmax remains unchanged because sufficiently high substrate concentration can overcome the competition.

What is noncompetitive inhibition?

The inhibitor binds at a site separate from the substrate-binding site and reduces productive catalysis.

How does pure noncompetitive inhibition affect Km and Vmax?

Km stays the same, while Vmax decreases.

Why does pure noncompetitive inhibition decrease Vmax?

The inhibitor reduces productive enzyme activity, so the maximum reaction rate decreases.

What is uncompetitive inhibition?

The inhibitor binds preferentially to the enzyme-substrate (ES) complex after the substrate binds.

How does uncompetitive inhibition affect Km and Vmax?

Both apparent Km and Vmax decrease.

Why does uncompetitive inhibition decrease Vmax?

The inhibitor traps ES in a nonproductive complex, reducing product formation.

How can you distinguish the three inhibition types using kinetic data?

Competitive: Km increases and Vmax stays the same. Pure noncompetitive: Km stays the same and Vmax decreases. Uncompetitive: both decrease.

What is a covalent inhibitor?

An inhibitor that forms a covalent bond with the enzyme, potentially inactivating it for a long time.

What is a mechanism-based inhibitor?

An inhibitor processed by the enzyme into a reactive form that then inactivates the enzyme.

What is allostery?

Binding at one site on a protein changes activity at another site through a conformational change.

What is an allosteric site?

A regulatory binding site physically distinct from the active site.

What is an allosteric activator?

A molecule that binds an allosteric site and stabilizes a more active or responsive enzyme conformation.

What is an allosteric inhibitor?

A molecule that binds an allosteric site and stabilizes a less active or less responsive conformation.

What does a sigmoidal enzyme-kinetics curve often indicate?

Cooperative behavior among binding sites or subunits.

How does cooperativity affect substrate response?

It can make enzyme activity change sharply over a narrow range of substrate concentrations.

How is a sigmoidal curve different from a hyperbolic curve?

A sigmoidal curve is S-shaped and often indicates cooperativity; a hyperbolic curve is typical of Michaelis-Menten behavior.

What is phosphorylation?

The covalent addition of a phosphate group to a protein, often by a protein kinase using ATP.

How can phosphorylation change enzyme activity?

The added negatively charged phosphate can alter ionic interactions, hydrogen bonding, protein conformation, and activity.

Does phosphorylation always activate an enzyme?

No. It can activate or inhibit an enzyme depending on the protein and context.

What is proteolytic activation?

Activation of an inactive precursor by cleavage of specific peptide bonds.

What is a zymogen?

An inactive enzyme precursor that requires activation, often through proteolytic cleavage.

Why is proteolytic activation often considered irreversible?

Cleavage of peptide bonds generally cannot simply be reversed to restore the original protein.

What is feedback inhibition?

A pathway's end product inhibits an earlier enzyme in that pathway, limiting further product synthesis.

How does enzyme regulation affect pathway flux?

Changing enzyme activity changes the rate at which metabolites move through a pathway and can alter the cell's overall response.