Crystal structure of the conserved protein TTHA0727 from Thermus thermophilus HB8 at 1.9 Å resolution: A CMD family member distinct from carboxymuconolactone decarboxylase (CMD) and AhpD

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Abstract
TTHA0727 is a conserved hypothetical protein from Thermus thermophilus HB8, with a molecular mass of 12.6 kDa. TTHA0727 belongs to the carboxymuconolactone decarboxylase (CMD) family (Pfam 02627). A sequence comparison with its homologs suggested that TTHA0727 is a distinct protein from alkylhydroperoxidase AhpD and γ‐carboxymuconolactone decarboxylase in the CMD family. Here we report the 1.9 Å crystal structure of TTHA0727 (PDB ID: 2CWQ) determined by the multiwavelength anomalous dispersion method. The TTHA0727 monomer structure consists of seven α‐helices (α1–α7) and one short 310‐helix. The crystal structure and the analytical ultracentrifugation revealed that TTHA0727 forms a hexameric ring structure in solution. The electrostatic potential distribution on the solvent‐accessible surface of the TTHA0727 hexamer showed that positively charged regions exist on the side of the ring structure, suggesting that TTHA0727 interacts with some negatively charged molecules. A structural homology search revealed that the structure of three α‐helices (α4–α6) is remarkably conserved, suggesting that it is the common structural motif for the CMD family proteins. In addition, the nine residues of the N‐terminal tag bound to the cleft region between α1 and α3 in chains A and B of TTHA0727, implying that this region is the putative binding/active site for some small molecules.