New determinants in the catalytic mechanism of nucleoside hydrolases from the structures of two isozymes from Sulfolobus solfataricus

""The purine and pyrimidine-specific nucleoside hydrolases (NHs) from the archaeon Sulfolobus solfataricus participate in the fundamental pathway of nucleotide catabolism, and concur in maintaining adequate levels of free nitrogenous bases for cellular function. The two highly homologous isozymes display distinct specificities towards nucleoside substrates, and both lack the amino acids employed for activation of the leaving group in the hydrolytic reaction by the so far characterized NHs. We determined the high-resolution crystal structures of the purine and pyrimidine-specific NHs from S. solfataricus to reveal that both enzymes belong to the NH structural homology group I, despite the different substrate specificities. A Na+ ion is bound at the active site to the pyrimidine-specific NH instead of the prototypical Ca2+, delineating a role of the metals in the catalytic mechanism of NHs in the substrate binding rather than nucleophile activation. A conserved His residue, that regulates product release in other homologous NHs, provides crucial interactions for leaving group activation in the archaeal isozymes. Modeling of the enzyme-substrate interactions suggests that steric exclusion and catalytic selection underlie the orthogonal base specificity of the two isozymes.""