3] Cobalt as probe and label of proteins

Abstract

Cobalt ions exhibit characteristic spectra. When cobalt is introduced into proteins, the spectral properties become a desirable attribute to probe the environment of active enzymatic sites. Moreover, cobalt can be substituted for spectroscopically silent zinc when this is the native, catalytically active metal atom of enzymes. Such a metal exchange has been particularly fruitful in the study of these systems because the majority of cobalt substituted zinc enzymes retain their enzymatic activity. Hence, cobalt substitution for zinc has become an essential technique to address the structural basis of catalytic properties in zinc enzymes, as well as the coordination environment of structural zinc sites in proteins. This chapter discusses the preparation and handling of cobalt enzymes and the nature of the information that can be obtained from spectroscopic measurements regarding the role of the metal center. It evaluates the circumstances under which cobalt becomes a structural and functional substitute for zinc atoms. The existence of structural data for cobalt enzymes now makes possible such comparisons between cobalt and zinc in proteins.