Recombinant protein detail — Compared with certain early bacterial preparations, recombinant human growth hormone (hGH) used clinically today typically lacks which terminal residue present in older methionyl-hGH formulations?

Introduction / Context:Human growth hormone (hGH) is a 191–amino acid polypeptide. Early recombinant products expressed in E. coli were sometimes produced as methionyl-hGH (somatrem), which contains an extra N-terminal methionine compared with pituitary-derived hormone. Modern recombinant hGH (somatropin) is engineered to match the native 191–amino acid sequence and therefore lacks the terminal methionine addition.

Given Data / Assumptions:

• Pituitary hGH begins with phenylalanine as the first residue in the mature 191–aa sequence.
• Some early bacterial expression systems initiated translation with an extra N-terminal methionine (192 aa total).
• Current therapeutics are designed to be structurally identical to endogenous hGH.

Concept / Approach:The question contrasts older methionyl-hGH with present-day recombinant hGH that is sequence-identical to human pituitary hormone. Therefore, the key residue “missing” in modern recombinant hGH (relative to methionyl-hGH) is the N-terminal methionine. Choosing “terminal methionine” captures this distinction.

Step-by-Step Solution:

Verification / Alternative check:Drug labels and protein databases list somatropin as identical to human GH without an N-terminal Met; somatrem historically contained Met and is less commonly used.

Why Other Options Are Wrong:

Common Pitfalls:Confusing pituitary GH with methionyl-hGH; always check whether a product is somatropin (191 aa) or somatrem (192 aa with Met).

Final Answer:Terminal methionine.