Another type of nonproteinogenic amino acid that is occasionally used by NRPSs is βamino acids. The first structure of a β-amino acid activating A domain was observed in VinN, which binds (2S,3S)-3-methylaspartate (3-MeAsp) as a β-amino acid. There were two key structural differences that allowed VinN to specifically activate its substrate: the β13β14 loop is one residue shorter than other L-α-amino acid A domains, and two amino acids opposite the loop are replaced by residues with bulkier side chains. The β13β14 loop contains the key residues K330 and R331 (I330 and C331 in GrsA_A), and its shortening shifts the backbone atoms to provide space for the C1-C2 bond in 3-MeAsp. The two residues opposite this loop in VinN are F231 and S299 (Figure ), in contrast to A236 and A301 in GrsA_A, and it is thought that these larger residues might push the β-amino acid substrate closer to the β13β14 loop. F231, in particular, was assessed by mutagenesis experiments and confirmed to be important for binding of both 3-MeAsp and L-Asp in VinN. The β-amino group was found to be coordinated by the conserved D230 present in many adenylating enzymes (D235 in GrsA_A). Despite this report noting that another β-amino acid activating A domain, SlgN1, binds 3-MeAsp in the opposite orientation to VinN, structural characterization of other β-amino acid A domains has supported the findings of VinN. Specifically, IdnL1 and CmiS6 seem to recognize their substrates via the same mechanism as VinN. The only new insight gained from this follow-up report is that in IdnL1 and CmiS6, position 220 (IdnL1 numbering) seems to be important in dictating the size of substrate that they can accommodate. L220 of IdnL1 helps recognize (S)-3-aminobutanoic acid ((S)-3-Aba), while G220 of CmiS6 expands the binding pocket to accommodate 3-aminononanoic acid (3-Ana).