Figure 4.

Sortase structure overview. (A) The conserved sortase-fold was first seen in S. aureus SrtA. Some essential features are color coded. The active site is displayed in yellow, Ca²⁺-binding residues are shown in green, and the β6/β7 loop is in red. (B) S. aureus SrtB contains the same overall sortase fold and arrangement of catalytic residues, with additional helices. (C) S. pneumoniae SrtC-1 also contains the sortase fold, but has an additional loop that lies across the substrate binding site (in orange).