Enzyme Catalysis Quiz Questions

Competitive inhibition can be overcome by the addition of large amounts of substrate to a reaction. Both the substrate and the competitive inhibitor bind to the active site of the enzyme. Consequently a competitive inhibitor will have no effect at infinite substrate concentration since the substrate will completely win in the competition to bind to the active site. Competitive inhibitors can only bind to the enzyme at the active site. They cannot bind to the enzyme-substrate complex.

For a biochemical reaction to occur, an energy barrier has to be overcome even if the reaction has a strong negative G (the free energy difference between the reactant(s) and product(s)). A substrate or reactant is first activated to form a higher energy transition state intermediate that is very unstable and rapidly converts to product(s). The energy required for this is the activation energy. In an uncatalysed reaction, this energy is supplied by molecular collisions. In a catalysed reaction, the enzyme positions the substrate molecule(s) in the most favourable relative orientation to form the transition state intermediate. The latter binds to the enzyme more tightly than the substrate (in its ground state) and stabilises it. An enzyme can therefore increase the rate of the reaction it catalyses by lowering the energy of activation but it cannot affect the equilibrium of the reaction.

Competitive inhibition occurs when a molecule, known as a competitive inhibitor, competes with the substrate for the active site of the enzyme. This inhibitor binds to the active site and prevents the substrate from binding, thus blocking the enzyme's action. The inhibitor and substrate are structurally similar, allowing them to compete for the same binding site. In competitive inhibition, increasing the concentration of the substrate can overcome the inhibition by outcompeting the inhibitor for the active site.

An enzyme that displays hyperbolic kinetics (single substrate enzyme) is referred to as a Michaelis-Menten enzyme. For such an enzyme, the plot of velocity of the reaction against substrate concentration is hyperbolic. The Michaelis-Menten equation describes the kinetics of such an enzyme at initial rates before any product is formed. The Michaelis-Menten equation can be used to calculate the Michaelis constant (Km). This is defined as that concentration of substrate at which the enzyme is working at half maximum velocity. Km is expressed in units of molar concentration and is independent of the enzyme concentration. Km is also a measure of the affinity that the enzyme has for its substrate. The higher the Km, the lower the affinity of the enzyme for its substrate.

Competitive inhibitors have a similar structure to the substrate and compete with the latter for binding to the active site of an enzyme. A competitive inhibitor binds reversibly to the active site. It will have no effect on the reaction at infinite substrate concentration since the substrate will win in the competition to bind to the enzyme active site. Consequently a competitive inhibitor and substrate cannot bind simultaneously to the enzyme. A comparison of the double reciprocal plots of an enzyme reaction with and without competitive inhibitor shows that:
• Vmax, the maximum velocity for the reaction, remains unchanged in the presence of a competitive inhibitor
• Km, the Michaelis constant for the reaction, increases in the presence of a competitive inhibitor. This is because the affinity that the enzyme has for its substrate is reduced in the presence of a competitive inhibitor.

Allosteric enzymes are multi-subunit proteins which have allosteric sites ('allo' means other) separate from the active site to which bind allosteric modifiers (activators or inhibitors). These cause conformational changes in the enzyme, giving sigmoidal kinetics rather than the hyperbolic kinetics of classical enzymes. This affects the affinity of the enzyme for its substrate and hence the activity of the enzyme. An allosteric activator increases the affinity whereas an inhibitor decreases it. The sigmoid kinetics means that over a critical range, the enzyme is more sensitive to changes in substrate concentration than is the case for those with hyperbolic kinetics.

The active or catalytic site of an enzyme is a very small part of the overall molecule. It is a three-dimensional pocket or cleft where the substrate(s) are aligned and bind reversibly via several weak, noncovalent bonds. These bonds are short range, directional, and confer specificity. The substrate molecule(s) form a transition state intermediate which rapidly converts to products because it is unstable. The amino acid groups in the active site help to stabilise the electron distribution of the transition state. The latter binds to the enzyme (active site) more tightly than the substrate in its ground state liberating energy. The active site of the enzyme is therefore complementary to the transition state intermediate. This lowers the energy of activation for the reaction thus increasing the reaction rate. The active site may also position a metal group that may facilitate the reaction.

The rate of formation of the transition state intermediate determines the overall reaction rate. For a biochemical reaction to occur, an energy barrier has to be overcome even if the reaction has a strong negative G (the free energy difference between the starting and final products). The reactants must be activated to a higher energy transition state intermediate which is very unstable and rapidly converts to products. In a catalysed reaction, the enzyme stabilises the transition state intermediate favouring its formation and lowering the energy of activation. In an uncatalysed reaction this energy is supplied by the collision between molecules.
The active site of an enzyme is perfectly complementary to the transition state, which is an intermediate between the reactant molecules and products. The active site of an enzyme is not complementary to the substrate in its ground state.
Natural substrates do not bind to enzymes more tightly than transition state analogues. Enzymes position the substrate molecules in the most favourable relative orientations for the reaction to occur.

Competitive inhibition occurs when a molecule similar in structure to the substrate binds to the active site of an enzyme, preventing the substrate from binding. In this case, malonic acid is a structural analog of succinic acid, the substrate for succinic dehydrogenase. Malonic acid binds to the active site of succinic dehydrogenase, blocking the binding of succinic acid and inhibiting the enzyme's activity. This is an example of competitive inhibition.

Carbonic anhydrase is the correct answer because it is known to be one of the fastest enzymes in the human body. This enzyme catalyzes the conversion of carbon dioxide and water into carbonic acid, which is important for maintaining the pH balance in various tissues and organs. Its high catalytic efficiency and rapid reaction rate make it essential for processes such as respiration and acid-base regulation. DNA gyrase and DNA polymerase are enzymes involved in DNA replication and repair, while pepsin is a digestive enzyme. However, none of these enzymes are known for their speed or efficiency compared to carbonic anhydrase.