X-ray structure of pyrrolidone carboxyl peptidase from the hyperthermophilic archaeon Thermococcus litoralis

Background: Pyrrolidone carboxyl peptidases (pcps) are a group of exopeptid ases responsible for the hydrolysis of N-terminal pyroglutamate residues fr om peptides and proteins. The bacterial and archaeal pcps are members of a conserved family of cysteine proteases. The pcp from the hyperthermophilic archaeon Thermococcus litoralis is more thermostable than the bacterial enz ymes with which it has up to 40% sequence identity. The pcp activity in arc haea and eubacteria is proposed to be involved in detoxification processes and in nutrient metabolism; eukaryotic counterparts of the enzyme are invol ved in the processing of biologically active peptides. Results: The crystal structure of pop has been determined by multiple isomo rphous replacement techniques at 1.73 Angstrom resolution and refined to an R factor of 18.7% (R-free = 21.4%). The enzyme is a homotetramer of single open alp domain subunits, with a prominent hydrophobic core formed from lo ops coming together from each monomer. The active-site residues have been i dentified as a Cys143-His167-Glu80 catalytic triad. Structural homology to enzymes of different specificity and mechanism has been identified, Conclusions: The Thermococcus pcp has no sequence or structural homology wi th other members of the cysteine protease family. It does, however, show co nsiderable similarities to other hydrolytic enzymes of widely varying subst rate specificity and mechanism, suggesting that they are the products of di vergent evolution from a common ancestor. The enhanced thermostability of t he T. litoralis pcp may arise from hydrophobic interactions between the sub units and the presence of intersubunit disulphide bridges.