MDCAT XI Biology Enzymes Online Test 2026

Competitive inhibitors compete with substrate for the active site, preventing substrate binding.

Enzyme–substrate interaction is stabilized by multiple weak non-covalent forces.

Below maximum temperature, mild heat inactivation can be reversible, and enzymes regain activity at minimum temperature.

Aldolase is a lyase that cleaves fructose-1,6-bisphosphate into two 3-carbon molecules.

Zn²⁺ activates carbonic anhydrase, while Mg²⁺ commonly activates phosphatases and kinases.

The active site is formed by a small number of critical amino acids that provide specificity and catalytic activity.

RUBISCO functions as both carboxylase and oxygenase and is the most abundant enzyme on Earth.

NAD⁺ is an organic non-protein coenzyme that assists enzymes in redox reactions.

Km reflects the substrate concentration required to achieve half of the maximum reaction velocity.

Allosteric enzymes possess regulatory sites distinct from the active site, allowing control of enzyme activity.

Enzymes do not change the equilibrium constant; they only speed up the rate at which equilibrium is reached.

RNAse P and the peptidyl transferase activity of ribosomal RNA are classic examples of ribozymes.

The correct mechanism involves enzyme–substrate complex formation, product formation, and release of enzyme unchanged.

Anselme Payen discovered the first enzyme, diastase, from malt, marking the beginning of enzymology.

Ribozymes are RNA molecules capable of catalyzing biochemical reactions without protein involvement, proving that RNA can have enzymatic activity.

Non-competitive inhibitors reduce the number of functional enzyme molecules, lowering reaction rate without altering activation energy.

Enzymes are biological catalysts, but many catalysts (e.g., metals) are not enzymes.

Small molecules efficiently regulate metabolic pathways by reversible enzyme inhibition.

In uncompetitive inhibition, the inhibitor binds exclusively to the ES complex, reducing both Vmax and Km because it prevents product formation after substrate binding.

Restriction enzymes cleave DNA by breaking phosphodiester bonds in the sugar-phosphate backbone.

Metal ions such as Zn²⁺ or Mg²⁺ are inorganic cofactors essential for catalytic activity.

According to Michaelis–Menten kinetics, rate rises with substrate concentration until all enzymes are saturated, reaching Vmax.

Competitive inhibitors resemble the substrate and compete for the active site, which normally acts as a template to bind and orient molecules.

The reaction involves rearrangement of atoms within the same molecule, characteristic of isomerases.

Reaction rate increases with temperature due to higher kinetic energy until optimum temperature, after which enzyme denaturation causes rapid decline.

Pepsin functions best in highly acidic conditions (around pH 1.4) and remains active up to about pH 2.5.

The active site is complementary in shape and chemical properties to the substrate, allowing specific binding.

All active sites are saturated; the number of enzyme molecules now limits the reaction rate.

High temperature disrupts hydrogen bonds and tertiary structure, denaturing the enzyme and destroying the active site.

Non-competitive inhibitors bind to an allosteric site and reduce Vmax without changing Km, because substrate binding affinity remains unchanged.

Competitive inhibitors compete with the substrate for the active site. Adding more substrate can overcome inhibition, restoring the maximum reaction rate (Vmax).

In the induced-fit model, the enzyme undergoes conformational changes upon substrate binding, unlike the lock-and-key model where the active site is rigid.

Most human enzymes have an optimum temperature around 37–40°C, which allows maximum catalytic activity. Higher temperatures lead to denaturation.

Enzymes stabilize the high-energy transition state, reducing the activation energy needed, which speeds up the reaction without changing ΔG or equilibrium.

Enzymes accelerate reactions by lowering the activation energy required for the transition state, without altering the overall free energy (ΔG) or equilibrium of the reaction.

Pepsinogen is activated by HCl, which cleaves a specific peptide segment, exposing the active site and converting it into active pepsin.

Cofactors bind either permanently or temporarily to the enzyme, usually at the time of substrate binding, to assist in proper catalysis.

Active sites often contain charged amino acid residues that facilitate electrostatic interactions, stabilizing the substrate and assisting in catalysis.

Once all enzyme active sites are occupied (saturation), adding more substrate does not increase the reaction rate. The enzyme concentration becomes the limiting factor.

Non-competitive inhibitors bind to an allosteric site (different from the active site), causing a conformational change that reduces enzyme activity without directly blocking substrate binding.

High temperatures disrupt hydrogen bonds, ionic interactions, and hydrophobic forces that maintain the enzyme’s tertiary structure. This denaturation alters the active site and abolishes catalytic activity.

pH changes disrupt ionic bonds, temperature affects hydrogen bonds, and heavy metals can break disulfide bonds, destabilizing the enzyme’s structure.

Many enzymes are attached to organelles or membranes, enabling compartmentalization and efficient regulation of metabolic pathways in cells.

Absolute specificity means the enzyme catalyzes only a single substrate; even minor structural differences prevent binding and reaction.

Extreme pH alters the ionization of amino acid residues at the active site, disrupting substrate binding and catalysis, even though the overall enzyme structure is still intact.

Denaturation primarily disrupts tertiary structure, altering the active site and enzyme activity. Primary structure (sequence of amino acids) usually remains intact.

The lock-and-key model assumes a rigid active site. It cannot account for conformational changes that occur in the enzyme when the substrate binds (as explained by the induced-fit model).

Non-competitive inhibitors bind to a site other than the active site, reducing the maximum velocity (Vmax) of the enzyme-catalyzed reaction without affecting substrate binding (Km).

In the induced-fit model, the enzyme changes shape upon substrate binding, stabilizing the transition state. This flexibility allows regulatory control, unlike the rigid lock-and-key model.

Enzyme specificity depends on the 3D shape of the active site, which ensures only complementary substrates can bind effectively. This is the basis for selective catalysis.