Abstract
Abstract Carbonic anhydrase, carboxypeptidase A, thermolysin, superoxide dismutase, and alkaline phosphatase in which Mn(II) has been substituted for the native Zn(II) ion show solid state ESR spectra typical of d5 ions in environments with large rhombic distortions from spherical symmetry. The ESR spectra show observable absorption from zero field up to 5600 G at Band X. All of the Mn(II) enzymes show a prominent ESR signal at geff = 4.3 typical of rhombic d5 systems. These spectra may reflect a distorted tetrahedral geometry around the Mn(II). Mn(II) carboxypeptidase A and alkaline phosphatase have prominent ESR absorption near zero field suggesting that the zero field splitting is ≅ hv, indicating the presence of large second rank distortions from spherical symmetry. Competitive inhibitors binding to the Mn(II) enzyme do not alter the basic coordination geometry of the Mn(II), but cause significant changes in transition probabilities and hyperfine structure of the ESR spectra. These changes must reflect small differences in the zero field splitting parameters and indicate alterations in the bond angles and bond lengths of the Mn(II) coordination complex. Mn(II) ESR can thus be used as a highly sensitive probe of conformation at the metal-binding sites of Zn(II) metalloproteins.
Highlights
Conversion to a solid state ESR spectrum is more compatible with binding sites, e.g. one site per mole for carboxypeptidase A, thermolysin, and the carbonic anhydrases, and two unique sites per mole for alkaline phosphatase (Fig. 2B)
The solid state Mn(I1) ESR spectra are, very useful in detecting distortions from regular geometry or alterations in the symmetry of the complex that may occur when the enzymes combine with inhibitors or substrates [3]
The unique features of the present spectra may reflect axial and rhombic distortions applied to a 4-coordinate geometry
Summary
Conversion to a solid state ESR spectrum is more compatible with binding sites, e.g. one site per mole for carboxypeptidase A, thermolysin, and the carbonic anhydrases, and two unique sites per mole for alkaline phosphatase (Fig. 2B). If the ZFS is relatively small and the rotational correlation time of the molecule is short, the ZFSs are effectively averaged to zero and the spectrum appears as a single isotropic line (split by the nuclear spin of 3-i in the case of Mn(II) into six hyperfine lines) containing all five allowed ESR transitions (Am = f 1, AN = 0) as in Mn(H,O)c (Fig. 2A).
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