GHK is a tripeptide (glycyl-L-histidyl-L-lysine) that forms a stable complex with Cu²⁺. The complex is naturally present in human plasma at concentrations that decline measurably with age. In dermal research, GHK-Cu has been documented to engage copper-dependent signaling in fibroblasts, with effects on collagen I and III synthesis, decorin expression, MMP regulation, and antioxidant gene transcription. The copper ion is integral to activity — uncomplexed GHK does not produce equivalent effects.