 Adenosylhomocysteinase

Adenosylhomocysteinase is an enzyme that reversibly hydrolyses S-adenosylhomocysteine to adenosine and homocysteine. Its current official classification is EC 3.13.2.1; the activity was transferred from EC 3.3.1.1 in 2022. The unchanged reaction and the revised classification number describe different kinds of information.^[ref]

Writing AdoHcy for S-adenosylhomocysteine, Ado for adenosine and Hcy for homocysteine, the net reaction isIn a constructed closed material balance, hydrolysis of 20 nanomoles of substrate forms 20 nanomoles of each organic product. If 0.020 nanomoles of active sites carry out that conversion, the mean completed turnover count is $20/0.020=1000$ per active site. Substrate conversion and enzyme abundance are different quantities.

The mechanism contains a further useful distinction. The official account describes one tightly bound NAD+ per subunit and a transient oxidation during the catalytic sequence. Yet NAD+ does not appear as a consumed reactant in the net equation. Participation of an oxidation step inside a catalytic cycle does not require a net stoichiometric production of reduced nucleotide for each substrate molecule hydrolysed.^[ref]

Because the stated reaction is reversible, the amounts of substrate and products also matter to its net direction. The enzyme provides a route toward chemical equilibrium; its presence alone does not prescribe complete conversion. Measuring one product, the other product and remaining substrate can test a balance that a single turnover signal leaves unresolved.