Enzyme Kinetics and Inhibition Practice Questions

Correct answer: Biological catalysts

Enzymes are biological catalysts that accelerate biochemical reactions by lowering activation energy. They are not consumed or permanently altered during the reaction and can catalyze the same reaction many times.

Correct answer: Proteins

The vast majority of enzymes are proteins with specific three-dimensional structures that determine substrate specificity. A small class of RNA molecules called ribozymes also exhibit catalytic activity.

Correct answer: Oxidoreductases

Oxidoreductases catalyze oxidation-reduction reactions involving the transfer of electrons between molecules. Common examples include dehydrogenases, oxidases, and reductases, which are central to cellular respiration.

Correct answer: Transferases

Transferases move functional groups — such as methyl, amino, or phosphate groups — from one molecule to another. They are essential in metabolic pathways including amino acid biosynthesis and carbohydrate metabolism.

Correct answer: Active site of the enzyme

The active site is the specific region of an enzyme where the substrate binds and catalysis occurs. Its precise three-dimensional shape, determined by amino acid residues, confers the enzyme's substrate specificity.

Correct answer: Maximum velocity at substrate saturation

Vmax is the maximum reaction rate achieved when all active sites are occupied and the enzyme is fully saturated with substrate. It depends on the total enzyme concentration and the catalytic rate constant (kcat).

Correct answer: Velocity equals half of Vmax

Km is the substrate concentration at which the reaction velocity equals half of Vmax. It serves as a measure of enzyme-substrate affinity — a lower Km indicates higher affinity.

Correct answer: High affinity for substrate

A low Km value means the enzyme reaches half-maximal velocity at low substrate concentrations, indicating high affinity for its substrate. Conversely, a high Km reflects weaker enzyme-substrate affinity.

Correct answer: 1/V versus 1/[S]

The Lineweaver-Burk plot is a double-reciprocal plot of 1/V against 1/[S], linearizing the Michaelis-Menten equation. The x-intercept gives −1/Km and the y-intercept gives 1/Vmax.

Correct answer: Competes with substrate for the active site

Competitive inhibitors structurally resemble the substrate and compete directly for binding at the active site. The inhibition is reversible and can be overcome by increasing substrate concentration; Vmax remains unchanged but Km increases.

Correct answer: Unchanged

In competitive inhibition, Vmax remains unchanged because excess substrate can displace the inhibitor from the active site. However, apparent Km increases, reflecting reduced enzyme-substrate affinity in the presence of the inhibitor.

Correct answer: Altering enzyme conformation

Non-competitive inhibitors bind to an allosteric site separate from the active site, inducing a conformational change that reduces catalytic efficiency. Because substrate binding is unaffected, Km remains unchanged while Vmax decreases.

Correct answer: Unchanged

Non-competitive inhibition does not affect substrate binding affinity; therefore Km remains unchanged. Vmax decreases because the inhibitor reduces the enzyme's catalytic efficiency regardless of substrate concentration.

Correct answer: Non-competitive inhibition

Non-competitive inhibition cannot be reversed by increasing substrate concentration because the inhibitor binds at a separate allosteric site. It decreases Vmax while leaving Km unchanged.

Correct answer: Covalent bonds

Irreversible inhibitors form stable covalent bonds with the enzyme, permanently inactivating it. A classic example is organophosphate compounds, which covalently modify the serine residue in the active site of acetylcholinesterase.

Correct answer: Hydrolases

Hydrolases catalyze the cleavage of chemical bonds using water molecules. Common examples include proteases (cleave peptide bonds), lipases (cleave ester bonds), and glycosidases (cleave glycosidic bonds).

Correct answer: Show sigmoidal kinetics

Allosteric enzymes exhibit sigmoidal (S-shaped) substrate saturation kinetics due to cooperative binding — binding of one substrate molecule increases affinity at remaining subunits. They possess separate regulatory sites that modulate activity through conformational changes.

Correct answer: Enzyme adjusts shape upon substrate binding

The induced-fit model proposes that the enzyme undergoes a conformational change upon substrate binding to achieve a precise catalytic fit. This contrasts with the rigid lock-and-key model and better explains the flexibility observed in enzyme catalysis.

Correct answer: Light intensity

Enzyme activity is governed by temperature, pH, and substrate concentration, which affect enzyme structure and substrate availability. Light intensity does not directly influence enzymatic reactions under standard physiological conditions.

Correct answer: Pepsin

Pepsin has an optimal pH of approximately 1.5–2.0, well-suited to the acidic environment of the stomach where it degrades proteins. Trypsin and amylase function best at neutral to slightly alkaline pH, while salivary lipase works at a slightly acidic range.

Correct answer: Hydrolases

Hydrolases break chemical bonds by adding water. This process is common in digestion and metabolism.

Correct answer: Transferases

Transferases catalyze the transfer of functional groups such as methyl or phosphate groups between molecules.

Correct answer: Ligases

Ligases join two molecules together using energy from ATP. They are also known as synthetases.

Correct answer: Region where substrate binds

The active site is the region of the enzyme where the substrate binds. It is highly specific to the substrate.

Correct answer: Binds to the active site

Competitive inhibitors compete with substrate for the active site. This inhibition can be overcome by increasing substrate concentration.

Correct answer: Unchanged

In non-competitive inhibition, Km remains unchanged because substrate binding is not affected. Only Vmax decreases.

Correct answer: Binding to another site on enzyme

Non-competitive inhibitors bind to an allosteric site, changing enzyme shape and reducing activity.

Correct answer: Covalent bonds

Irreversible inhibitors form covalent bonds with enzymes, permanently inactivating them.

Correct answer: Induced fit model

The induced fit model suggests that the enzyme changes shape upon substrate binding to form a better fit.

Correct answer: Stomach

Enzymes active at pH 2 are adapted to acidic environments like the stomach, such as pepsin.