Question

. Some enzymes require the presence of a nonprotein molecule to behave catalytically. An enzyme devoid of this molecule is called a(n) A. holoenzyme. B. apoenzyme. C. coenzyme. D. zymoenzyme.

Short answer

B. apoenzyme

Step-by-step solution

- Understand the terminologies

First, understand the terms provided in the options. A holoenzyme is the complete, active form of an enzyme with its cofactor. An apoenzyme is the protein part of the enzyme, without its cofactor. A coenzyme is a nonprotein molecule that aids the enzyme. A zymoenzyme, also known as a zymogen, is an inactive precursor of the enzyme.

- Identify what the question asks

The question asks for the name of the enzyme that is devoid of its required nonprotein molecule. Based on the terminologies from Step 1, we need to find which term refers to an enzyme that is not complete because it lacks its cofactor.

- Eliminate incorrect options

Eliminate incorrect answers by checking each term against the definition. Holoenzyme (A) refers to the complete form with its cofactor, so it is incorrect. Coenzyme (C) is the nonprotein molecule itself, so it is incorrect. Zymoenzyme (D) refers to an inactive enzyme precursor, so it is incorrect.

- Select the correct option

The term that correctly matches the definition of an enzyme without its required nonprotein molecule is apoenzyme (B).

Key concepts

Holoenzyme

A holoenzyme is the fully functional form of an enzyme. To be active, enzymes often require additional nonprotein components called cofactors.
When these cofactors are bound to the enzyme, they form the holoenzyme.
This is the complete, active version of the enzyme, which is capable of carrying out its catalytic function.

• Example: DNA polymerase is a holoenzyme that requires a magnesium ion as a cofactor to function properly.

Coenzyme

Coenzymes are a type of cofactor, specifically organic molecules that assist enzymes in their catalytic activities.
They often act as carriers for chemical groups or electrons during the reaction. Unlike metal ion cofactors, coenzymes are typically organic molecules.

• Example: NAD+ (Nicotinamide Adenine Dinucleotide) is a common coenzyme involved in redox reactions.

Zymogen

A zymogen is an inactive precursor of an enzyme.
These precursors require a biochemical change to become active, usually involving a change in structure or the removal of a segment of the protein.
Zymogens help regulate enzyme activity, ensuring that the enzyme is activated only when needed.

• Example: Pepsinogen is a zymogen that converts to the active enzyme pepsin in the acidic environment of the stomach.

Enzyme Catalysis

Enzyme catalysis is the process by which enzymes increase the rate of a chemical reaction.
Enzymes lower the activation energy required for reactions, making them occur more rapidly and efficiently.
They achieve this through various mechanisms, including the formation of enzyme-substrate complexes and stabilizing transition states.

• Example: The enzyme catalase accelerates the breakdown of hydrogen peroxide into water and oxygen.

Enzyme Structure

The structure of an enzyme is crucial to its function. Enzymes are made up of long chains of amino acids that fold into specific three-dimensional shapes.
The active site of the enzyme, where the substrate binds, is typically a small pocket or groove.
This precise structure allows enzymes to interact specifically with their substrates.

• Example: The structure of the enzyme lysozyme allows it to specifically bind and break down bacterial cell walls.