What is the primary reason for the specificity of an enzyme toward its substrate?

• A. The presence of metal ions as cofactors
• B. The overall size of the enzyme molecule
• C. The concentration of substrate in the cell
• D. The unique three-dimensional structure of the enzyme's active site

Answer: (D)

Verified step by step guidance

1. Understand the concept of enzyme specificity: Enzyme specificity refers to the ability of an enzyme to selectively bind to a particular substrate and catalyze its reaction. This specificity is crucial for the enzyme's function in biochemical processes.
2. Recognize the role of the active site: The active site of an enzyme is a specific region where the substrate binds. It is shaped in a way that complements the substrate's structure, allowing for precise interaction.
3. Learn about the three-dimensional structure: The unique three-dimensional structure of the enzyme's active site is determined by the folding of the enzyme's polypeptide chain. This structure creates a specific shape and chemical environment that matches the substrate.
4. Understand why other factors are less relevant: While metal ions as cofactors, enzyme size, and substrate concentration can influence enzyme activity, they do not directly determine the enzyme's specificity. Specificity is primarily governed by the active site's structure.
5. Conclude that the correct answer is based on the active site's unique structure: The enzyme's specificity is due to the precise fit between the substrate and the active site, often described as a 'lock and key' or 'induced fit' model.