Incorporation of L-Thr has been described with the free-standing enzyme Thr1 en route to 4-chloro-Thr biosynthesis. Briefly, Thr1 activates L-Thr which is loaded on to the free-standing Thr2 carrier protein, halogenated by Thr3, and offloaded by the thioesterase Thr4. Upon crystallization, it was noted that D212 and K515 interact with the amine and carbonyl, respectively, and the methyl group of Thr is within van der Waals distance of F213 and G284 (Thr1 numbering) (Figure ). Hydrogen bond interactions are formed between the hydroxyl group and the side chain of H119 and the backbone carbonyls of M310 and V318. But upon adenylation and formation of Thr-AMP, the positioning shifts so that the hydroxyl group is stabilized instead by the backbone amine of G285 and carbonyl of G312 (Thr1 numbering). An isomer of L-Thr, αme-L-Ser is activated by the A domain of FmoA3 in the biosynthesis of the radical scavengers JBIR-34 and JBIR-35. It was proposed that residues A688, N730 and S760 (Figure ), which correlate to V649, H691 and S722 of the L-Ser activating EntF, were important for recognizing the side chain of α-me-L-Ser . In pyochelin biosynthesis, PchE activates L-Cys. The predicted specificity code was DLFNLSLIWK, but the identified binding pocket residues were F741, D742, L743, S813, G814, A841, T842, I846, W847 and K948 (PchE numbering) (Figure ). Out of these residues, two pairs of hydrophobic interaction partners (I846/W847 and F741/L743) narrow the pocket to provide Cys-binding specificity, similar to AB3403. The incorporation of L-Lys into ε-poly-Llysine is performed by Pls_A. Outside of D213 and K495, which interact with the amine and carbonyl moieties, respectively, E217 locks the ε-amino group at the base of the pocket via a salt bridge, along with T301 and S265 via hydrogen bonding (Pls_A numbering) (Figure ). And, from the anabaenopeptin pathway, ApnA_A1 is the only crystallized A domain to date that displays dual specificity towards both L-Arg and L-Tyr. Substrate orientation is maintained by E204 and S243, and A307 (corresponding to C331 in GrsA_A) interacts with the aliphatic chain of Arg and the phenyl ring of Tyr. E204 is conserved in naturally-occurring homologues of ApnA_A1, and the mutation S243H switched the specificity 100-fold to 4-azidophenylalanine (ApnA_A1 numbering).