Copper is an essential trace element to life and particularly plays a pivotal role in the physiology of aerobic organisms. The Cut protein family is associated with copper homeostasis and involved in uptake, storage, delivery, and efflux of copper. CutC is a member of the Cut family and is suggested to be involved in efflux trafficking of cuprous ion. We report here the biochemical and structural characterization of human CutC (hCutC). hCutC can bind Cu(I) with a stoichiometry of 1:1 and an apparent dissociation constant of 15.5 +/- 2.8 mu M. hCutC assumes a typical TIM-barrel fold and forms a tetramer in both crystal structure and solution which is different from the dimeric architecture of the bacterial CutC. Structure analysis and sequence comparison of CutC proteins from different species reveal two strictly conserved Cys residues on the inner surface of the C-terminal end of the TIM-barrel. Mutations of the two Cys residues can significantly impair the binding ability of hCutC with Cu(I). Our results suggest that hCutC functions as an enzyme with Cu(I) as a cofactor rather than a copper transporter and the potential Cu(I)-binding site consists of the two Cys residues and other conserved residues in the vicinity. (C) 2009 Elsevier Inc. All rights reserved.