ATP SULFURYLASE FROM TROPHOSOME TISSUE OF RIFTIA-PACHYPTILA (HYDROTHERMAL VENT TUBE WORM)

ATP sulfurylase (ATP: sulfate adenylyltransferase, EC 2.7.7.4) was extensively purified from trophosome tissue of Riftia pachyptila, a tube worm that thrives in deep ocean hydrothermal vent communities. The enzyme is probably derived from the sulfide-oxidizing bacteria that densely colonize the tissue. Glycerol (20% v v) protected the enzyme against inactivation during purification and storage. The native enzyme appears to be a dimer (MW 90 kDa ± 10%) composed of identical size subunits (MW 48 kDa ± 5%). At pH 8.0, 30 †C, the specific activities (units × mg protein-1) of the most highly purified sample are as follows: ATP synthesis, 370; APS synthesis, 23; molybdolysis, 65; APSe synthesis or selenolysis, 1.9. The Km values for APS and PPi at 5 mm Mg2+ are 6.3 and 14 μm, respectively. In the APS synthesis direction, the Km values for MgATP and SO42- are 1.7 and 27 mm, respectively. The Km values for MgATP and MoO42- in the molybdolysis reaction are 80 and 150 μm, respectively. The Kia for MgATP is 0.65 mm. APS is a potent inhibitor of molybdolysis, competitive with both MgATP and MoO42- (Kiq = 2.2 μM). However, PPi (+ Mg2+) is virtually inactive as a molybdolysis inhibitor. Oxyanion dead end inhibitors competitive with SO42- include (in order of decreasing potency) ClO4- > FSO3- (Ki = 22 μM) > ClO3- > NO3- > S2O32- (Ki's = 5 and 43 M). FSO3- is uncompetitive with MgATP, but S2O32- is noncompetitive. Each subunit contains two free SH groups, at least one of which is functionally essential. ATP, MgATP, SO42-, MoO42-, and APS each protect against inactivation by excess 5,5′-dithiobis-(2-nitrobenzoate). FSO3- is ineffective as a protector unless MgATP is present. PPi (+Mg2+) does not protect against inactivation. Riftia trophosome contains little or no "ADP sulfurylase." The high trophosome level of ATP sulfurylase (67-176 ATP synthesis units × g fresh wt tissue -1 from four different specimens, corresponding to 4-10 μm enzyme sites), the high Kcat of the enzyme for ATP synthesis (296 s-1), and the high Km's for MgATP and SO42- are consistent with a role in ATP formation during sulfide oxidation, i.e., the physiological reaction is APS + MgPPi ⇄ SO42- + MgATP. © 1991.