Synthesis and characterization of selenolipoylated H-protein of the glycine cleavage system

H-protein of the glycine cleavage system has a lipoic acid prosthetic group. Selenolipoic acid is a lipoic acid analog ire which both sulfur atoms sire replaced by selenium atoms. Two isoforms of bovine lipoyltransferase that are responsible for the attachment of lipoic acid to H-protein had an affinity for selenolipoyl-AMP and transferred the selenolipoyl moiety to bovine apoH-protein comparable to lipoyl-AMP. Selenolipoylated H-protein was overexpressed in Escherichia coli and purified. Selenolipoylated H-protein was 26% as effective as lipoylated H-protein in the glycine decarboxylation reaction, in which reduction of the diselenide bond of selenolipoylated H-protein is catalyzed by P-protein. The diselenide form of selenolipoylated H-protein was a poor substrate for L-protein, and the rate of reduction was 0.5% of that of lipoylated H-protein. The rate of the overall glycine cleavage reaction with selenolipoylated H-protein was <1% of that with lipoylated H-protein. These results are consistent with the difference in the redox potential between the diselenide and disulfide bonds, In contrast, selenolipoylated H-protein showed three times as high glycine-(CO2)-C-14 exchange activity as lipoylated H-protein, presumably because the rate of reoxidation of reduced selenolipoylated H-protein is much higher than that of lipoylated H-protein.