Electronic absorption and EPR spectroscopy of copper alcohol dehydrogenase: pink, violet and green forms of a type 1 copper center analog.

Blue and non-blue states of the copper center in copper-substituted alcohol dehydrogenase (EC 1.1.1.1) can be attained by coenzyme binding and/or ligand binding to the copper ion. Copper alcohol dehydrogenase has been studied by electronic absorption, CD and EPR spectroscopy in the presence and absence of coenzyme. On the basis of previous work on blue (Type 1) copper proteins with a CuSS*N2 chromophore the assignment of charge transfer transitions in copper alcohol dehydrogenase is discussed. The latter contains a CuS2N(OH2) unit in the ligand-free protein and a CuS2N2 unit in the ternary complex with NAD+ and pyrazole. It is proposed that the energy of the charge transfer transitions can be used as a structural marker in combination with EPR data. A comparison is made between the spectroscopic properties of the ternary complex of copper alcohol dehydrogenase and the copper centers in stellacyanin and cytochrome-c oxidase (CuA) in order to test the validity of recent structural models of the type CuS2N2, i.e., a cupric ion coordinated to two thiolate ligands. Finally, a close resemblance between the electronic absorption spectra of copper alcohol dehydrogenase and those of other variants of Type 1 copper centers such as the 'unusual' copper center of nitrous oxide reductase is noted as an indication of similar coordination environments.