MCQs

Km is a measure of an enzyme's affinity for its substrate, defined as the substrate concentration at which the reaction rate is one-half of the maximum velocity (Vmax).

At high substrate concentrations, all enzyme active sites are occupied. The reaction velocity reaches a maximum (Vmax), and further substrate addition cannot increase the rate.

The Induced Fit model states the active site is not rigid; substrate binding induces a conformational change that properly positions catalytic groups for optimized catalysis.

A coenzyme is a non-protein organic molecule that binds transiently to an apoenzyme, allowing it to be separated by dialysis, unlike a prosthetic group.

Substrate binding is mediated by multiple weak, non-covalent forces which are reversible, essential for both binding and product release.

Enzymes lower activation energy by providing an alternative reaction pathway where specific R-groups orient and stress substrates, stabilizing the transition state.

The active site is a specific, flexible 3D pocket formed by a few amino acids that binds the substrate and catalyzes its conversion to product via weak interactions.

A prosthetic group is a non-protein unit that is covalently or very tightly bound to an apoenzyme, making it a permanent part of the functional holoenzyme.

The tertiary structure of an enzyme, which dictates the shape of the active site, is stabilized by hydrogen bonds, ionic interactions, hydrophobic interactions, van der Waals forces, and covalent disulfide bonds.

Enzymes are biological catalysts that accelerate reactions by decreasing the activation energy. They do not provide energy, alter the equilibrium constant, or change the free energy change (ΔG) of the overall reaction.

When substrate is in excess, the reaction rate is directly proportional to enzyme concentration because every additional enzyme molecule can contribute to the product formation rate.

The discovery of ribozymes (RNA catalysts) disproved the long-held belief that all enzymes are proteins.

Compartmentalization separates catabolic and anabolic pathways to prevent futile cycles and concentrates reactants to increase reaction efficiency.

A competitive inhibitor competes for the active site, requiring higher substrate concentrations to reach Vmax, thus increasing apparent Km.

High temperatures disrupt non-covalent bonds (e.g., hydrogen bonds) stabilizing protein structure, causing irreversible unfolding (denaturation) and loss of active site shape.

Irreversible inhibitors covalently modify essential residues or cofactors, leading to permanent enzyme inactivation.

The triad allows histidine to act as a powerful general base catalyst, abstracting a proton from the serine hydroxyl group to make it a nucleophile.

Non-allosteric enzymes follow Michaelis-Menten kinetics, where the plot of V₀ vs. [S] is a rectangular hyperbola: first-order at low [S] and zero-order at high [S].

An enzyme accelerates both forward and reverse reactions equally by lowering activation energy without changing the equilibrium point or free energy.