Enzymes: Classification, Kinetics, Inhibition and Regulation
What this domain tests
Enzymes questions test the six IUBMB classes, active-site specificity, Michaelis-Menten kinetics, inhibition patterns, and physiological regulation.
Core rules
- Six classes: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases. Kinases are transferases; dehydrogenases are oxidoreductases.
- Km is the substrate concentration at half Vmax. Low Km means high apparent affinity. Vmax reflects enzyme amount and catalytic constant.
- Competitive inhibitor binds free enzyme; Km increases, Vmax unchanged. Noncompetitive inhibitor binds enzyme or enzyme-substrate complex; Vmax falls, Km unchanged. Uncompetitive inhibitor binds ES only; both Km and Vmax fall.
- Allosteric enzymes show sigmoidal kinetics and are regulated by effectors. Covalent modification includes phosphorylation. Feedback inhibition uses end product to inhibit an early enzyme.
- Cofactors include metal ions and coenzymes; apoenzyme plus cofactor gives holoenzyme.
Worked example
An enzyme has Km = 2 mM. At [S] = 2 mM, v = Vmax/2. A competitive inhibitor is added so that [I] = Ki. Apparent Km = Km(1 + [I]/Ki) = 2(1+1) = 4 mM. At the same [S] = 2 mM, v/Vmax = 2/(4+2) = 1/3, lower than 1/2. Increasing [S] can restore Vmax because the inhibitor is overcome.
Traps
Km is not a direct measure of affinity for every enzyme; it is an operational constant. Competitive inhibition does not lower Vmax. Noncompetitive inhibition cannot be reversed by excess substrate. A cofactor is not the same as a substrate.