Serine is the correct answer (option c).

Serine stands out as the amino acid most commonly used in enzyme catalysis due to its hydroxyl group acting as a nucleophile in mechanisms like the catalytic triad of serine proteases.​

Option Analysis

Alanine: Lacks a reactive side chain (-CH₃), serving mainly in structural roles rather than direct catalysis. It appears in enzymes like alanine transaminase but not as a catalytic residue.​

Arginine: Its guanidinium group aids substrate binding, transition state stabilization, and occasionally acid-base catalysis in enzymes like arginase, but it ranks lower in frequency compared to nucleophilic residues.​

Serine: Frequently acts as a nucleophile in serine proteases (e.g., chymotrypsin), forming covalent intermediates via its -OH group in the triad with histidine and aspartate, dominating catalytic roles across enzyme classes.​

Methionine: Sulfur-containing side chain supports methionine synthase catalysis indirectly via cofactors like B12, but rarely participates directly in active site chemistry.​

In enzyme catalysis, specific amino acids drive chemical reactions by providing functional groups for nucleophilic attack, acid-base transfer, or stabilization. For CSIR NET Life Sciences aspirants, understanding which amino acid is commonly used in enzyme catalysis is crucial, as questions often test catalytic residues in biochemistry. Serine emerges as the top choice among options like Alanine, Arginine, and Methionine due to its prevalence in mechanisms across proteases and hydrolases.​

Catalytic Roles of Key Amino Acids

• Serine: Hydroxyl (-CH₂OH) enables nucleophilic catalysis in serine proteases, forming acyl-enzyme intermediates. Essential in the triad (Ser-His-Asp), lowering activation energy.​

• Arginine: Guanidinium aids phosphoryl transfer and binding, as in alkaline phosphatase, but less common for direct catalysis.​

• Alanine and Methionine: Primarily structural; Alanine in transaminases, Methionine in B12-dependent reactions, not core catalytic players.​

Exam Relevance for CSIR NET

Studies of enzyme active sites confirm seven residues (including Serine) dominate, with Serine/Threonine frequent in nucleophilic roles—vital for questions on protease mechanisms. Mastering this boosts scores in molecular biology units.​