SCHEME 1.

Possible reaction path for AF and SmTGR. One molecule of AF reacts with wild type reduced SmTGR (species 1) to yield a transient intermediate bearing an Au(I) coordinated with Sec⁵⁹⁷ and triethylphosphine (species 2); we hypothesize that acetoxythioglucose is released first (see text). The intermediate species releases triethylphosphine to form a Sec-gold-Cys complex with any of the available cysteinyl residues; three reaction products are possible (species 3, 4, and 6). The chemical species in which the C-terminal arm is cross-linked via the gold ion to a distant Cys residue (species 4 and 6) may rearrange to yield the more stable short distance Cys-gold-Cys complexes (species 5 and 7). When equilibrium is reached, a mixture of species 3, 5, and 7 is expected to be present, dynamically interchanging via the less stable intermediates 4 and 6. Additional molecules of AF may be introduced, leading to SmTGR molecules bearing two or three gold ions. The gold-binding site 3, in which the metal ion is not coordinated by Cys or Sec, is not considered in the scheme.