pentaglycine

Pentaglycine is a pentapeptide consisting of five glycine residues linked by peptide bonds. In microbiology, the term refers to the glycine-rich cross-bridge that links stem peptides in the peptidoglycan of certain Gram-positive bacteria, most notably Staphylococcus aureus.

Most S. aureus peptidoglycan stems have a stem peptide sequence of L-Ala-D-Glu-L-Lys-D-Ala-D-Ala. A pentaglycine cross-bridge is appended to the lysine side chain and then connects to the D-Ala-D-Ala terminus of an adjacent stem, forming interpeptide cross-links. The cross-bridge length is five glycine units, although variations occur in related species.

Biosynthesis is mediated by the Fem family of non-ribosomal peptidyl transferases (for example FemX, FemA, and FemB). These enzymes catalyze the sequential transfer of glycine from glycyl-tRNA to the stem peptide, generating the pentaglycine bridge before incorporation into the mature peptidoglycan by transpeptidation.

Function and clinical relevance: The pentaglycine cross-bridge contributes to cell-wall rigidity and antibiotic susceptibility. Disruption of Fem enzymes reduces cross-bridge length, weakens the cell wall, and can alter sensitivity to beta-lactam antibiotics. Because pentaglycine cross-bridges are characteristic of certain pathogens, they are a point of interest for studies of bacterial physiology and antibiotic resistance.

Note: The term pentaglycine may also be encountered in discussions of model peptides and NMR studies of flexible glycine-rich sequences, though in those contexts pentaglycine denotes an isolated five-glycine chain rather than a cross-bridge in peptidoglycan.