Electron spin resonance studies of carbonic anhydrase: transition metal ions and spin-labeled sulfonamides

Electron spin resonance (esr) spectra of Cu(II) and Co(II) carbonic anhydrase, and a spin-labeled sulfonamide complex of the Zn(II) enzyme, are reported. The coordination geometry of Cu(II) bound in the enzyme appears to have approximately axial symmetry. Esr spectra of enzyme complexes with metal-binding anions also show axial symmetry and greater covalency, in the order ethoxzolamide < SH(-) < N(3) (-) </= CN(-). Well-resolved superhyperfine structure in the spectrum of the cyanide complex suggests the presence of two, and probably three, equivalent nitrogen ligands from the protein. Esr spectra of the Co(II) enzyme and its complexes show two types of Co(II) environment, one typical of the native enzyme and the 1:1 CN(-) complex, and one typical of a 2:1 CN(-) complex. Co(II) in the 2:1 complex appears to be low-spin and probably has a coordination number of 5. Binding of a spin-labeled sulfonamide to the active center immobilizes the free radical. The similarity of the esr spectra of spin-labeled Zn(II) and Co(II) carbonic anhydrases suggests that the conformation at the active center is similar in the two metal derivatives.

Magnetic resonance studies of spin-labeled creatine kinase system and interaction of two paramagnetic probes

Creatine kinase from rabbit muscle has been specifically labeled with a derivative of a nitroxide free radical on the two essential sulfhydryl groups, one per active site. The conformation in the environment of the enzyme-bound free radical and the change in conformation induced upon binding of the substrate, metal adenosine diphosphate, has been monitored by two parameters of the bound spin label, its electron paramagnetic resonance spectrum and its contribution to the proton relaxation rate of water. The specificity in activation of the enzymic reaction of the series of alkaline earth ions, Mg, Ca, Sr, Ba, was not reflected by differences in conformation of the ternary enzyme metal substrate complexes observable with the spin probe. However, with the inhibitory zinc ion, the conformation of ternary complex was distinctly different. The electron paramagnetic resonance spectra of complexes with two paramagnetic probes, namely, spin-labeled enzyme with paramagnetic metal adenosine diphosphate complexes (Mn(II), Ni(II), and Co(II)) revealed a spin-spin interaction between the protein-bound free radical and the paramagnetic metal ions. From the magnitude of the interaction, the distance between the manganous ion and the electron spin of the nitroxide moiety in the MnADP-spin-labeled creatine kinase was estimated to be in the range of 7-10 A.