Crucial effect of the first CXXC motif of human QSOX 1b on the activity to different substrates

Among all sulphhydryl oxidases involved in disulphide formation, quiescin-sulphhydryl oxidase (QSOX) is unique for its multidomain structure, protein thiol oxidation activity and highly efficient catalysis. In this study, site-directed mutagenesis and molecular modelling methods were integrated to investigate the structural and functional characteristics of QSOX, especially the importance of the three CXXC motifs. Site-directed mutagenesis suggested that the C449-C452 motif was essential for the activity of human QSOX 1b; the C70-C73 motif was fundamental in electron transfer from thiol-containing substrate including reduced proteins, DTT, GSH rather than the phosphine-based thiol reductant TCEP, to the C449-C452 motif; and the C509-C512 motif was not involved in electron transfer during disulphide formation. The different roles of the CXXC motifs indicated that there were discrepant electron transfer pathways for the oxidation of thiol-containing substrates and non-thiol disulphide reductants. Molecular modelling method was then used to draw a reasonable picture for the electron transfer process and to elucidate the mechanism of electron transfer when different substrates were oxidized, which will greatly enhance our understanding of the action mechanism of QSOX.