Okai_2008_Proteins_70_1646

Thermus thermophilus HB8, an extremely thermophilic bacterium. The open reading frame of TTHA1809 from Thermus thermophilus HB8 was annotated as a proline iminopeptidase-related protein. Proline iminopeptidase (PIP) is a serine peptidase that catalyzes the removal of N-terminal proline from peptides with high specificity. In this study, we report the crystal structure of the proline iminopeptidase-related protein TTHA1809 from Thermus thermophilus HB8, and compare the active site of the tricorn-interacting aminopeptidase F1, which is the best homolog found using the Dali program, with the corresponding region of TTHA1809. Comparison with the tricorn-interacting aminopeptidase F1: The tricorn-interacting aminopeptidase F120 from Thermoplasma acidophilum has a Ser-His-Asp catalytic triad in the active site. The superimposition between TTHA1809 and F1 revealed that the residue corresponding to the catalytic Ser105 in F1 is replaced by a Gly at TTHA1809. Asp229 and His255 of TTHA1809 were located at the same position as Asp244 and His271 of the catalytic triad of F1. His255 of TTHA1809 was located in a loop between the beta8-strand and the alpha10-helix. The ND-1 atom of His255 was hydrogen-bonded to Asp229 in a loop located between the beta7-strand and the alpha9-helix, whereas the NE-2 atom of His255 formed a hydrogen bond with a water molecule because of the lack of catalytic Ser residue. Thus, TTHA1809 and F1 would have different functions. A BLAST search using TTHA1809 revealed the possibility of the existence of enzymes lacking a catalytic Ser residue in several microorganisms (Thermus thermophilus HB27, Deinococcus geothermalis DSM 11300, Legionella pneumophila subsp. pneumophila str. Philadelphia 1, Dechloromonas aromatica RCB, and Hahella chejuensis KCTC 2396), but their functions have not yet been revealed.