W0466

Structural and Functional Evolution Within a Family of Nucleoside Cleaving Enzymes. Yan Zhang, Steven E. Ealick, Dept of Chemistry and Chemical Biology, Cornell Univ., Ithaca, NY.

Glycosidic bond cleavage of nucleosides is a fundamental reaction in the purine and pyrimidine salvage pathways. In most organisms, nucleoside phosphorylases (NPs) catalyze glycosidic bond cleavage. According to structural studies and sequence alignments, these enzymes can be divided into three groups. (1) Trimeric NPs have subunits of about 30 kDa, (2) hexameric NPs have subunits of about 25 kDa and (3) dimeric NPs have subuits about 50 kDa. Two groups of trimeric NPs have been identified: one is specific for 6-oxopurine nucleosides and the other is specific for methylthioadenosine. At least three groups of hexameric NPs are known: one cleaves both 6-oxo- and 6-aminopurine nucleosides, a second cleaves methylthioadenosine as well as 6-oxo- and 6-aminopurine nucleosides and a third cleaves uridine. Two groups of dimeric NPs are known: one is specific for thymidine and the other cleaves uridine as well as thymidine. The trimeric and hexameric enzymes have similar folds, while the dimeric enzymes have a different fold. Despite the differences in structures, sequences and substrate specificities, the NPs are thought to use similar catalytic mechanisms. It has been shown that methylthioadenosine/ S-adenosylhomocysteine nucleosidase has the same fold as the trimeric and hexameric NPs and sequence comparisons predict that AMP nucleosidase also shares this fold. These enzymes utilize water rather than phosphate as the nucleophile for glycosidic bond cleavage. An analysis of the known structures and sequences was carried out to understand the structural basis for substrate specificity. The analysis also suggests evolutionary relationships among the enzymes within this family.