CK is a _____ (protein unit).

• A. Dipeptide
• B. Monopeptide
• C. Tripeptide
• D. Tetrapeptide

Answer: (A)

Proteins are made from amino acids linked together by peptide bonds. A single amino acid isn’t a peptide, so it isn’t a peptide unit. When two amino acids join, they form a dipeptide, the smallest meaningful peptide unit listed here. Three and four amino acids form tri- and tetrapeptides, which are longer chains. Among the given options, the dipeptide is the smallest peptide unit described, so it’s the best match for a “protein unit” in this context. Keep in mind that real proteins are much larger polypeptides, but this question focuses on the smallest peptide fragment.