Abstract
S-Adenosylhomocysteine hydrolase (EC 3.3.1.1) catalyzes the reversible hydrolysis of S-adenosylhomocysteine to adenosine and homocysteine. The enzyme from rat liver is a tetramer consisting of identical subunits, each of which contains 1 mol of tightly bound NAD^+. The bound coenzyme is involved in the oxidation-reduction of nucleoside substrates, and is not released during catalysis. Spectroscopic studies showed that the mode of binding of NAD^+ to the enzyme was similar to that seen in many NAD^+-dependent dehydrogenases. In the middle of the sequence, S-adenosylhomocysteine hydrolase has the nucleotide-binding motif. Mutagenesis of key residues in this motif abolished or weakened NAD^+ binding .Thus, despite having a high affinity for NAD^+, the structure of the NAD^+ side of S-adenosylhomocysteine hydrolase is similar to those of dehydrogenases. The reaction with an active site-directed reagent, 5'-p-fluorosulfonylbenzoyladenosine and mutational analyses showed the occurrence of Cys78 in the S-adenosylhomocysteine-binding site. This residue is the only cysteine residue that is conserved through evolution.
