**Question:** Trypsin cleaves carboxy terminal of:

A. Arginine
B. Tyrosine
C. Serine
D. Asparagine

**Core Concept:**
Trypsin is an enzyme that belongs to the family of proteolytic enzymes, which are involved in the degradation of proteins and peptides. Proteolytic enzymes act by cleaving peptide bonds between amino acid residues within proteins. Trypsin specifically cleaves carboxy-terminal amino acids.

**Why the Correct Answer is Right:**
Trypsin cleaves carboxy terminal of Serine. In the context of peptide bond cleavage, this means that trypsin acts by breaking peptide bonds at the carboxy-terminal side of serine residues. This specificity is due to the active site of trypsin, which is composed of amino acids and hydrogen bonding interactions that enable it to recognize and cleave specifically at the carboxy-terminal side of serine residues.

**Why Each Wrong Option is Incorrect:**

A. Arginine: Trypsin does not cleave carboxy-terminal of arginine. Arginine is an amino acid with a positively charged side chain, and trypsin's active site does not form hydrogen bonds with the positively charged side chain of arginine, leading to the enzyme's inability to cleave carboxy-terminal arginine.

B. Tyrosine: Similar to arginine, tyrosine has a side chain that is not recognized by trypsin's active site. Tyrosine's side chain is not involved in hydrogen bonding with the enzyme, making carboxy-terminal tyrosine cleavage not specific for trypsin.

C. Asparagine: Asparagine shares the same side chain structure as aspartic acid, which is not recognized by trypsin's active site. Trypsin does not cleave carboxy-terminal asparagine due to the lack of hydrogen bonding interactions with the side chain.

D. D-amino acids are not recognized by trypsin due to its specificity for L-amino acids. Trypsin's active site is designed to interact with L-amino acids, making it unable to cleave carboxy-terminal D-amino acids.

**Clinical Pearl:**
Understanding the specificity of enzymes like trypsin is crucial for understanding protein digestion and the functioning of the digestive system. The correct answer, carboxy-terminal serine, is essential for understanding the role of trypsin in the digestion of proteins in the small intestine, where it acts in combination with other enzymes and hormones to break down proteins into smaller peptides and amino acids, which can then be absorbed by the body for cellular utilization.