Role of conservative residue Cys158 in the formation of an active photoprotein complex of obelin

Using site directed mutagenesis, the conservative residue Cys158 of recombi nant apoobelin was substituted for sera ine (C158S, S-mutant) or alanine (C 158A, A-mutant). These point mutations resulted in significant changes in t he apoobelin structure accompanied by slowing of photoprotein complex forma tion, decrease of its stability, and changing of its bioluminescence charac teristics. The enzymatic properties of the photoprotein decreased in the se ries: wild-type protein > S-mutant > A-mutant. This is consistent with rank of nucleophilicity SH > OH > CH3 of cysteine, serine, and alanine side cha in functional groups, respectively. Possible mechanisms of the involvement of the apoobelin Cys158 SH-group in the formation of the enzyme-substrate c omplex are considered.