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

Abstract

Abstract 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 → 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(H−1L)(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(H−2GlyGlyHis), 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.