Enzyme regulation

What are the two primary ways a cell can regulate enzymes?

By controlling the amount of enzyme available (altering synthesis/degradation) or by controlling the enzyme's activity (changing its conformation).

What limits the reaction speed when substrate concentration is extremely high ([S]»K_M)?

The total amount of enzyme present, because every active site is fully saturated (the enzyme is operating at V_max

What happens to the reaction rate when [S]«K_M?

The reaction rate increases almost linearly with added substrate because the enzyme has many open active sites.

What happens to the reaction rate when [S]=K_M?

The enzyme operates at half its maximum velocity (1/2 V_max) and still responds significantly to increases in substrate concentration.

What is Allosteric Regulation?

The regulation of an enzyme's activity by other compounds (like inhibitors, coenzymes, or products) that bind to the protein and change the shape of the active site.

What is a Homotropic effector?

An allosteric regulator where the substrate and the effector are the exact same molecule. It binds to the active site of one subunit and structurally forces other active sites to open.

What is a Heterotropic effector?

An allosteric regulator that is a different molecule from the substrate. It regulates the enzyme by binding to a separate, dedicated regulatory site.

What is the T State of an allosteric enzyme?

The "Tense" state; a rigid structural conformation that has a low binding affinity for the substrate.

What is the R State of an allosteric enzyme?

The "Relaxed" state; a shifted structural conformation that has a high binding affinity for the substrate.

What is Feedback Inhibition?

A form of negative regulation where the final product of an enzymatic pathway allosterically inhibits an enzyme used early in that same pathway.

What is the characteristic shape of a kinetics graph (Velocity vs. Substrate) for an enzyme with homotropic cooperativity?

Sigmoidal (S-shaped), which shows a slow start followed by rapid acceleration as the enzyme shifts from the T state to the R state.

What is Covalent Modification?

The regulation of an enzyme by reversibly altering its amino acids with covalent bonds (such as phosphorylation or methylation) to change its activity.