, a S-ribosylhomocysteine lyase ( EC 4.4.1.21 ) is an enzyme that catalyzes the chemical reaction S-(5-deoxy-D-ribos-5-yl)-L-homocysteine L-homocysteine + (4S)-4,5-dihydroxypentan-2,3-dione Hence, this enzyme has one substrate , S-(5-deoxy-D-ribos-5-yl)-L-homocysteine , and two products , L-homocysteine and (4S)-4,5-dihydroxypentan-2,3-dione . This enzyme belongs to the family of lyases , specifically the class of carbon-sulfur