Cobalt bovine carbonic anhydrase and a number of inhibitor derivatives have been investigated by electronic spectroscopy, in the range 8000-25 000 cm-1, and by *H longitudinal relaxation studies of water protons. The electronic spectra have distinguished between fourand five-coordinate derivatives. The two forms of the enzyme are essentially pseudotetrahedral.T\ values have been interpreted as being due to the presence of exchangeable protons of water bound to the metal ion at every pH. Some inhibitors replace water, whereas others bind to a fifth donor position.X-ray data at 2.0 Á of resolution have shown that the zinc(II) ion in carbonic anhydrase is bound to three histidyl nitrogens in an arrangement which has been considered typical of a distorted geometry.1 Although the problem is still quite controversial,2™5 a fourth group has been guessed to be a water molecule.6™8The cobalt analogue, which substantially retains the enzymatic activity of the native enzyme,9•10 shows an electronic spectrum in the visible region which is strongly pH dependent.3 The equilibrium between an acidic and a basic form of the enzyme can be represented as:with a pXa of 6.6.3The electronic spectra of both the acidic and basic forms are typical of a low symmetry chromophore, and susceptibility measurements indicate that the cobalt(II) ion is high spin.11 These data could be consistent with either a tetracoordinate pseudotetrahedral or five-coordinate structure, since planar complexes are expected to be low spin and six-coordinate complexes are expected to have quite a lower molar absorbance

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The compound obtained from zinc-free therrnolysin and copper(//) sulphate, which is biologically inactive, has been characterized together with some inhibitor derivatives of the native enzyme through electronic, e.s.r., and n.m.r. spectroscopies. The co-ordination at the metal is suggested to be pseudo-tetrahedral. A comparison with the similar copper( 11) -substituted metalloenzyme carboxypeptidase is presented.