The NIFS protein can function as a selenide delivery protein in the biosynthesis of selenophosphate.
The NIFS protein from Azobacter vinelandii is a pyridoxal phosphate-containing homodimer that catalyzes the formation of equimolar amounts of elemental sulfur and L-alanine from the substrate L-cysteine (Zheng, L., White, R. H., Cash, V. L., Jack, R. F., and Dean, D. R. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 2754-2758). A sulfur transfer role of NIFS in which the enzyme donates sulfur for iron sulfur center formation in nitrogenase was suggested. The fact that NIFS also can catalyze the decomposition of L-selenocysteine to elemental selenium and L-alanine suggested the possibility that this enzyme might serve as a selenide delivery protein for the in vitro biosynthesis of selenophosphate. In agreement with this hypothesis, we have shown that replacement of selenide with NIFS and L-selenocysteine in the in vitro selenophosphate synthetase assay results in an increased rate of formation of selenophosphate. These results thus support the view that a selenocysteine-specific enzyme similar to NIFS may be involved as an in vivo selenide delivery protein for selenophosphate biosynthesis. A kinetic characterization of the two NIFS catalyzed reactions carried out in the present study indicates that the enzyme favors L-cysteine as a substrate compared with its selenium analog. A specific activity for L-cysteine of 142 nmol/min/mg compared with 55 nmol/min/mg for L-selenocysteine was determined. This level of enzyme activity on the selenoamino acid substrate is adequate to deliver selenium to selenophosphate synthetase in the in vitro assay system described.
Lacourciere
G M
GM
Laboratory of Biochemistry, NHLBI, National Institutes of Health, Bethesda, Maryland 20892, USA.
Stadtman
T C
TC
D016948:Azotobacter vinelandii / Q000201:enzymology*; D001426:Bacterial Proteins / Q000037:antagonists & inhibitors / Q000378:metabolism*; D013437:Carbon-Sulfur Lyases; D029721:Drosophila Proteins; D008190:Lyases / Q000037:antagonists & inhibitors; D008956:Models, Chemical; D019906:Nuclear Magnetic Resonance, Biomolecular; D010710:Phosphates / Q000378:metabolism*; D010759:Phosphorus Isotopes; D010770:Phosphotransferases / Q000378:metabolism; D011732:Pyridoxal Phosphate; D018036:Selenium Compounds / Q000378:metabolism*; D017279:Selenocysteine / Q000378:metabolism*; D013379:Substrate Specificity
D001426:Bacterial Proteins; D029721:Drosophila Proteins; D010710:Phosphates; D010759:Phosphorus Isotopes; D018036:Selenium Compounds; C080001:nifS protein, Bacteria; D017279:Selenocysteine; C076818:selenophosphate; D011732:Pyridoxal Phosphate; D010770:Phosphotransferases; C062762:selenophosphate synthetase; D008190:Lyases; D013437:Carbon-Sulfur Lyases; C080546:cysteine desulfurase