D-amino acids are just one example of how NRPS assembly lines have evolved for the incorporation of nonproteinogenic amino acids. Thus far, the only D-amino acid activating enzyme that has been crystallized is DltA, which is responsible for the D-alanylation of cell wall components like lipoteichoic acid in Gram-positive bacteria. This activity is reminiscent of the glycylation of lipopolysaccharides described above, and in fact AlmE shows active site similarity to DltA. DltA has been crystallized several times from various organisms, and separate investigations of the active site binding pocket broadly corroborate each other. The first of these reports identified several residues important for coordination of both the α-amino group (D197, G295 and V301) and the D-methyl group (L198 and C269) in the Bacillus cereus-derived enzyme (Figure ). The most significant of these is C269, which would sterically clash with L-Ala if it were to bind in the active site. Indeed, these residues were almost identical when compared to those of the Staphylococcus aureus structure (Figure ), which recognizes the α-amino group with residues D197, T297 and V301 and the methyl side chain with residues L198, M200 and C268 (numbering from the B. cereus-derived structure).