The correct answer is (1) A and B.

Introduction

Enzyme kinetics provide critical insights into how substrate structure influences catalytic efficiency and binding. Serine proteases, important for protein digestion, exhibit unique preferences for peptide length and sequence near their hydrolytic cleavage sites. By comparing kinetic parameters (KcatKcat, KmKm) for different peptide substrates, we can dissect the role of peptide size and residue identity in enzyme function.

Interpreting the Data

Statement Analysis

A. Catalytic efficiency (Kcat/KmKcat/Km) increases with peptide size

• Compare peptide size (number of amino acids) and efficiency.

• The peptide Ac-X-Ala-CO-$\downarrow$-NH22 (smallest) has the lowest efficiency.

• Efficiency increases as peptide size grows, peaking at Ac-Z-Y-X-Ala-CO-$\downarrow$-NH22 (three amino acids added). The fourth peptide, with Val at the cleavage position, shows lowered efficiency, highlighting the effect of the cleavage site residue.

B. Amino acid at hydrolytic cleavage position is critical for binding

• Dramatic difference in KmKm and efficiency when cleavage site residue changes, e.g., Ac-Y-X-Val-CO-$\downarrow$-NH22 vs. Ac-Z-Y-X-Ala-CO-$\downarrow$-NH22.

• Indicates that the identity of the amino acid at the cleavage position strongly influences binding and recognition.

C. Catalytic efficiency decreases from three amino acid peptide to four amino acid peptide

• Efficiency increases with peptide length up to three residues, then drops for the four-residue peptide (with Val).

• However, this statement ignores the role of the cleavage site residue; the decrease is likely due more to residue identity than peptide length.

Justification for the Correct Choice

• A and B are consistent with the kinetic data and biochemical principles: efficiency rises with peptide size (up to a point), and the hydrolytic site amino acid, influencing KmKm, is key for substrate binding.

• C is less universally true, relying on only two data points and confounded by side-chain effects.

Summary Table

Conclusion

The correct interpretation of the serine protease data is that catalytic efficiency increases with peptide size, and the nature of the amino acid at the cleavage position importantly affects binding, confirming statements A and B. Understanding these relationships guides enzyme engineering and enhances competence in enzymology for exams and research.