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

Step-by-step solution

- Identify Key Terms

First, identify the key terms in the question: enzyme, nonprotein molecule, catalytically, and devoid.

- Understand the Definition

An apoenzyme is an enzyme that requires a cofactor (a nonprotein molecule) to be active, but it is in its inactive form when the cofactor is not present.

- Analyze Each Choice

Consider which term fits the description of an enzyme without its necessary cofactor:(A) Holoenzyme: an enzyme with its cofactor and active.(B) Apoenzyme: an enzyme without its cofactor and inactive.(C) Coenzyme: the nonprotein molecule that helps the enzyme function.(D) Zymoenzyme: an inactive precursor of an enzyme.

- Select the Correct Answer

Based on the analysis, the enzyme devoid of its necessary cofactor is an apoenzyme.

Key concepts

Apoenzyme

An apoenzyme is the protein part of an enzyme, which is inactive on its own because it lacks a crucial nonprotein molecule.
This nonprotein molecule, required for the enzyme's catalytic activity, is known as a cofactor. Without its cofactor, the apoenzyme cannot facilitate chemical reactions.
This is why the apoenzyme is often considered the 'inactive' form of the enzyme.

• An example of an apoenzyme would be the protein part of an enzyme before it binds to its cofactor.

Understanding this concept is key to grasping how enzymes work.
Once the apoenzyme binds with the cofactor, it becomes active and transforms into a holoenzyme.

Cofactor

A cofactor is the nonprotein molecule or ion that is necessary for the biological activity of an enzyme.
Cofactors can be metal ions like iron, magnesium, or zinc, or organic molecules.
They bind to the apoenzyme to form a functional enzyme. There are two types of cofactors:

• Inorganic Cofactors: Metal ions that aid in enzyme activity. For example, magnesium ions are vital for ATP-utilizing enzymes.
• Organic Cofactors: Also known as coenzymes, these are organic molecules that assist enzymes during the catalysis of reactions.

Cofactors precisely aid the enzyme in the chemical reactions by stabilizing the structure of the enzyme or participating directly in the chemical reaction.
Their role is critical in the enzyme’s ability to function properly.

Holoenzyme

A holoenzyme is the complete, active form of an enzyme, composed of the apoenzyme and its necessary cofactor.
When the apoenzyme binds to its specific cofactor, the resulting complex is termed a holoenzyme.
This complete structure is essential for the enzyme to become catalytically active.

• For instance, DNA polymerase requires a metal ion cofactor to become a functional holoenzyme.
• The bond between the apoenzyme and its cofactor can be either weak and reversible, or strong and permanent.

Without the cofactor, the enzyme remains only partially functional as an apoenzyme, which can catalyze reactions.
Understanding the holoenzyme concept helps in realizing how some enzymes require extra components beyond their protein structure for full functionality.

Coenzyme

A coenzyme is a special type of cofactor that is an organic molecule.
These coenzymes participate directly in enzymatic reactions, often acting as carriers for chemical groups or electrons from one enzyme to another.

• Examples of coenzymes include NAD+ (Nicotinamide adenine dinucleotide), which acts as an electron carrier in reactions.
• Another example is Coenzyme A, which transfers acyl groups in metabolic processes.

Coenzymes are usually derived from vitamins such as niacin and riboflavin, which are essential nutrients that organisms must obtain from their diet.
These organic molecules bind to the apoenzyme either permanently (as a prosthetic group) or temporarily during the reaction.
The role of coenzymes is key in allowing enzymes to perform a wide variety of complex biochemical reactions efficiently.