Structures of the γ-class carbonic anhydrase homologue YrdA suggest a possible allosteric switch

The YrdA protein shows high sequence similarity to gamma-class carbonic anhydrase (gamma-CA) proteins and is classified as part of the gamma-CA protein family. However, its function has not been fully elucidated as it lacks several of the conserved residues that are considered to be necessary for gamma-CA catalysis. Interestingly, a homologue of gamma-CA from Methanosarcina thermophila and a beta-carboxysomal gamma-CA from a beta-cyanobacterium have shown that these catalytic residues are not always conserved in gamma-CAs. The crystal structure of YrdA from Escherichia coli (ecYrdA) is reported here in two crystallographic forms. The overall structure of ecYrdA is also similar to those of the gamma-CAs. One loop around the putative catalytic site shows a number of alternative conformations. A His residue (His70) on this loop coordinates with, or is reoriented from, the catalytic Zn2+ ion; this is similar to the conformations mediated by an Asp residue on the catalytic loops of beta-CA proteins. One Trp residue (Trp171) also adopts two alternative conformations that may be related to the spatial positions of the catalytic loop. Even though significant CA activity could not be detected using purified ecYrdA, these structural features have potential functional implications for gamma-CA-related proteins.