Crystal structures of AEPs with APBS generated electrostatic potential maps contoured at ±10 kT/e and catalytic Cys and His shown in yellow and blue sticks, respectively. Overall topology of AEPs (left) appear similar with slight differences in surface electrostatic potential. Expanded views of catalytic region (βI sheet, β5-βIV loop and βI-βIII region) orientated over the catalytic His residue (right) illustrates variation around the active site, with residues that are proposed to alter substrate specificity shown with sticks. A model of an efficient macrocyclizer, C. ensiformis AEP (accession code: AIB06797), built on the HaAEP1 structure which displays 76.3% amino acid identity, is shown below the dotted line. C. ensiformis AEP appears to exhibit a more accessible active site, in comparison to other AEP structures, due to the presence of several short side-chain amino acids.