GrsA_A, also known as PheA, was the first crystallized A domain, identified in the bacterium Brevibacillus brevis. As the first A domain in the gramicidin S biosynthetic pathway, GrsA_A selectively activates and incorporates L-Phe, and its structural report postulated the first set of 10 amino acid residues that lined the binding pocket (the 10AA code). Two of these residues were considered to be invariant and essential for α-amino acid binding and orientation (D235 and K517), but the other eight residues were thought to discriminate between different substrates based on their side chain chemistries (A236, W239, T278, I299, A301, A322, I330 and C331) (Figure ). Another L-Phe activating A domain, McyG from microcystin biosynthesis, was crystallized in 2015. This study found that V227 (corresponding to D235 in GrsA_A) was essential in the selection of hydrophobic substrates. Further, the benzene ring of Phe was stabilized in the pocket by the side chain of W272 and the backbone atoms of A333, G335 and S341 (McyG numbering). Several A domain structures have been described that activate glycine or other small L-αamino acids. AlmE from Vibrio cholerae O1 (El Tor biotype) activates Gly to modify lipopolysaccharides (LPS) as a mechanism of resistance to cationic antimicrobial peptides. Several residues mediate this adenylation reaction: the conserved D247 and K452 coordinate with the glycyl amine and carbonyl groups, respectively (Figure ). The carbonyl oxygens of G341 and I347 also interact with the amine, and L248 and C316 (AlmE numbering) are close to the C α , which might help discourage D-amino acids and/or L-α-amino acids larger than glycine, respectively, from binding through steric interactions. This proposed role was verified by mutagenesis of L248 and C316. Tarry et al. were able to crystallize a Gly-incorporating A domain as part of a larger cross-module complex. They noted that in DhbF, the conserved D656 (corresponding to D235 of GrsA_A) binds to the amino group of Gly, but otherwise, the substrate binding pocket seemed to be occluded by W755, which also hydrogen bonds with Q699 (DhbF numbering). A third protein structure that can activate Gly has also been analyzed, but unlike previous examples, IdnL7 shows relaxed substrate selectivity towards small L-α-amino acids (Gly, L-Ala, or L-Ser) with a marked intolerance for D-Ala. Beyond the conserved first and last positions of its selectivity code (D216 and K500), three additional residues appear to be responsible for this activation pattern: C217, A285 and T318 (IdnL7 numbering). C217 and A285 directly interact with the methyl group of L-Ala, with C217 thought to enforce the stereochemical recognition of Lamino acids over their D-counterparts, while T318 is adjacent to the methyl group and positioned in such a way that accommodation of L-Ser can be rationalized. As it happens, these three residues are highly conserved in VinM-type enzymes.