Enzymes Practice Questions

Correct answer: Ribozymes

While most enzymes are proteinaceous, certain RNA molecules possess catalytic activity and are known as ribozymes. This discovery highlighted that not all biological catalysts are proteins.

Correct answer: Active site

The active site is a pocket or crevice in the enzyme's tertiary structure into which the substrate fits. This proximity allows the enzyme to catalyze reactions at high rates.

Correct answer: By lowering the activation energy needed

Enzymes function by lowering the activation energy barrier required for a transition state to form. This allows more substrate molecules to convert into products in a shorter time.

Correct answer: Ligases

Ligases are enzymes that catalyze the joining of C-O, C-S, C-N, or P-O bonds, often using ATP. An example is DNA ligase, which joins DNA fragments.

Correct answer: Lyases

Lyases catalyze the breakage of various chemical bonds by means other than hydrolysis and oxidation, often forming a new double bond or a new ring structure.

Correct answer: Dehydrogenases

Oxidoreductases catalyze oxido-reduction between two substrates. They are frequently called dehydrogenases or oxidases depending on the specific reaction.

Correct answer: Prosthetic group

Prosthetic groups are organic compounds that are distinguished from other cofactors by being tightly or permanently bound to the apoenzyme. For example, 'haem' is the prosthetic group in peroxidase.

Correct answer: The maximum velocity at substrate saturation

Vmax is the maximum rate of reaction reached when all enzyme active sites are occupied by substrate. At this point, adding more substrate will not further increase the rate.

Correct answer: Michaelis constant

The Km value reflects the affinity of the enzyme for its substrate. A lower Km value indicates that the enzyme reaches half-maximal velocity at a lower substrate concentration, implying higher affinity.

Correct answer: Binds to the active site

Competitive inhibitors compete with the substrate for binding at the active site. This inhibition can often be overcome by increasing the concentration of the actual substrate.

Correct answer: Malonate

Malonate closely resembles succinate in structure. It competes for the active site of succinic dehydrogenase, thereby inhibiting the conversion of succinate to fumarate.

Correct answer: They denature and lose activity

High temperatures destroy enzymatic activity because proteins are denatured by heat. The tertiary structure of the enzyme unfolds, destroying the active site.

Correct answer: Niacin

Many co-enzymes are derived from vitamins. For instance, the co-enzymes Nicotinamide Adenine Dinucleotide (NAD) and NADP contain the vitamin Niacin.

Correct answer: Lock and key or Induced fit hypothesis

These hypotheses describe how a substrate fits into the active site of an enzyme. The formation of the E-S complex is a mandatory intermediate step in any enzymatic catalysis.

Correct answer: Isomerases

Isomerases are a class of enzymes that rearrange the atoms within a molecule. They catalyze changes that turn one isomer into another without changing the chemical formula.

Correct answer: Noble gas concentration

Enzyme activity is highly sensitive to pH, temperature, and concentrations of substrate and inhibitors. Noble gases are chemically inert and do not influence biological catalysts.

Correct answer: Carboxypeptidase

Many enzymes require metal ions for their activity. Carboxypeptidase, a digestive enzyme that breaks down proteins, requires zinc ions as a cofactor.

Correct answer: Cofactor

In enzymes that require a cofactor, the apoenzyme (protein part) is inactive on its own. Removal of the cofactor renders the enzyme non-functional.

Correct answer: The binding of the substrate to the active site

Catalysis begins with the substrate diffusing to and binding with the active site of the enzyme. This binding induces the enzyme to alter its shape, leading to the formation of the transition state.