Chirality in proteins: What is the stereochemical configuration of the alpha-amino acids found in naturally occurring proteins (ignoring achiral glycine)?

Introduction / Context:
Biological homochirality is a defining feature of life. Proteins overwhelmingly use the L-enantiomers of amino acids, while sugars in nucleic acids are D-configured. Recognizing this pattern helps in understanding enzyme specificity and peptide synthesis.

Given Data / Assumptions:

• We consider alpha-amino acids encoded by the genetic code.
• Glycine is achiral and is excluded from the chirality rule.

Concept / Approach:
Ribosomal protein synthesis selects L-amino acids via aminoacyl-tRNA synthetases and the ribosome. D-amino acids may occur in bacterial cell walls (e.g., D-Ala) but not in ribosomally synthesized proteins.

Step-by-Step Solution:
Identify convention: proteinogenic amino acids are L-form.Note exception: glycine lacks chirality (R = H).Select ‘‘L’’ as the correct configuration for chiral residues in proteins.

Verification / Alternative check:
Crystal structures and biochemical analyses confirm L-stereochemistry at alpha carbon for natural proteins; synthetic peptides using D-residues alter folding and protease susceptibility.

Why Other Options Are Wrong:

• D: not used in ribosomal protein synthesis.
• L and D: mixed chirality does not reflect natural ribosomal products.
• None: contradicted by extensive structural biology data.

Common Pitfalls:
Confusing D-amino acids in peptidoglycan or non-ribosomal peptides with ribosomal proteins; mixing up L/D with R/S notation.

Final Answer:
L