Transport of the Cu(II) Bound with Histidine-Containing Tripeptides to Cysteine. Coordination Mode and Exchangeability of Cu(II) in the Complexes

The transport of Cu(II) complexed with histidine-containing tripeptides (Cu(H-iL), L = HisGlyGly (i = 2), GlyHisGly (i = 1), and GlyGlyHis (i = 2)) to cysteine was examined by a stopped-flow spectrophotometric method. The S (right arrow) Cu(II) charge transfer (LMCT) bands at 335 nm and 390 nm were used as probes for tracing the reaction. Primarily formed was the ternary Cu(H-1L)(Cys^-) complex. The rate of the Cu(H-1L)(Cys^-) formation depended on the affinity of Cu(II) for the donor atoms at the fourth binding site of Cu(H-2L). Cu(H-1L)(Cys^-) subsequently reacted with free Cys^- to yield a binary complex, Cu(Cys^-)2. The rate of Cu(H1L)(Cys^-) formation was generally faster than that of conversion from Cu(H-1L)(Cys^-) to Cu(Cys^-)2. An exception was found in the reaction with Cu(H2GlyGlyHis), where the relation k1+ < k2+ existed. The ternary complex, Cu(H-1HisGlyGly)(Cys^-), was too labile to be detect by the conventional stopped-flow methods. Probably, Cu(H-1HisGlyGly)(Cys^-) upon forming changed spontaneously to Cu(HisGlyGly)(Cys^-), in which the N-terminal His residue coordinated to the Cu(II) via the amino and imidazole nitrogens, and rapidly changed to Cu(Cys^-)2.