Which statement best explains how pH affects enzyme activity under optimal enzyme conditions?

• A. pH affects enzyme activity only by changing the substrate concentration, not by altering enzyme structure or active-site chemistry.
• B. Changing pH alters the ionization states of amino acid side chains (including those in the active site), which can change substrate binding and catalytic activity, producing an optimal pH with decreased activity at more acidic or basic values.
• C. Lowering pH always increases enzyme activity by increasing molecular collisions between enzyme and substrate.
• D. Enzyme activity is independent of pH because the primary amino acid sequence does not change with pH.

Answer: (B)

Verified step by step guidance

1. Understand that enzyme activity depends on the enzyme's three-dimensional structure and the chemical environment of its active site.
2. Recognize that pH can influence the ionization state of amino acid side chains, especially those in the active site, which are critical for substrate binding and catalysis.
3. Recall that changes in ionization can alter the shape or charge properties of the active site, affecting how well the substrate fits and how efficiently the enzyme catalyzes the reaction.
4. Note that each enzyme has an optimal pH at which its activity is maximal; deviations from this pH (either more acidic or more basic) can reduce activity due to altered ionization and enzyme conformation.
5. Conclude that pH affects enzyme activity primarily by changing the ionization states of amino acid side chains, not by changing substrate concentration or the primary amino acid sequence.