Protein-gold complexes, including serum albumin-gold (Au) complexes, exhibit strong, distinct red luminescence (λem = 660 nm) with an extremely large Stokes shift when excited using ultraviolet light (λex = 365 nm). Initial reports on red luminescent serum albumin (BSA)-Au suggested a wide array of potential applications in nanomedicine, imaging, and sensing. However, current knowledge about this complex is limited, including its structure and formation mechanism, and the commonly assumed single-site nucleation model doesn't align with experimental results. Our investigation into BSA-Au and four other protein-Au complexes revealed a similar and consistent formation mechanism, better explained by a multiple-site adsorption model rather than a single-site nucleation model. Molecular cloning of human serum albumin (HSA) indicated that the luminophore site requires Au to bind to only one intact cystine disulfide bond. Further experiments with the smaller red luminescent glutathione (GSSG)-Au complexes demonstrated that merely two neutral Au atoms are necessary for the formation of the red luminescent complex. Understanding the Au binding site in proteins, luminescence mechanism, and the structure could evolve this easily synthesizable red luminescent complex into a versatile, next generation, fluorescent protein.