The coordination geometry of the catalytic site in Cd-substituted horse liver alcohol dehydrogenase (LADH) has been investigated as a function of pH using the method of perturbed angular correlation of gamma-rays (PAC). LADH in solution fully loaded with cadmium, including radioactive 111mCd in the catalytic site [Cd2(111mCd)Cd2LADH], was studied over the pH range 7.9-11.5. Analysis of the PAC spectra showed the ionization of a group with pKa of 11. This pKa value is about 2 pH units higher than that of native zinc-containing LADH. A pKa of 9.6 was found for the binary complex of Cd2(111mCd)Cd2LADH with NAD+. This value is also about 2 pH units higher than that of the binary complex of native zinc-containing enzyme and NAD+. No pH dependency was detected for the binary complex of Cd2(111mCd)Cd2LADH with NADH within the pH range measured (pH 8.3-11.5). Assuming that metal-coordinated water is the ionizing group [Kvassman, J., & Pettersson, G. (1979) Eur. J. Biochem. 100, 115-123], we conclude that the larger ionic radius of Cd(II) relative to Zn(II) in the catalytic site causes the elevated pKa values of metal-bound water. Interpretation of nuclear quadrupole interaction (NQI) parameters derived from PAC spectra is based on the use of the angular overlap model, using the coordinates for the catalytic zinc site from the 1.8 A resolution crystal structure of the ternary complex between LADH, NADH, and dimethyl sulfoxide as a model.(ABSTRACT TRUNCATED AT 250 WORDS)

The ground and excited state properties of Co(II) substituted for Zn(II) at the catalytic (c) and the noncatalytic (n) sites of horse liver alcohol dehydrogenase EE isozyme have been investigated by parallel EPR and UV/ visible variable-temperature magnetic circular dichroism (VTMCD) spectroscopies. Samples were investigated as prepared and after formation of a ternary complex with NAD+ and the potent inhibitor pyrazole. In accord with the structural role proposed for the noncatalytic metal, the spectroscopic properties of Co(II) at the noncatalytic site were unperturbed by formation of the ternary complex. The EPR spectra were readily analyzed in terms of a S = 3/2 spin Hamiltonian using anisotropic intrinsic g-values in the range characteristic of tetrahedral Co(II), i.e. g = 2.1-2.4; E/D 0), and 0 (with D > 0) for Co(c)Zn(n)-HLADH, Co(c)Zn(n)-HLADH/NAD+/pyrazole, and Zn(c)Co(n)-HLADH, respectively. VTMCD studies facilitated resolution and assignment of S -Co(II) charge transfer bands (300-400 nm) and the components of the 4A2 -'Tl(P) tetrahedral d-d band (500-800 nm) that are split by spin-orbit coupling and low-symmetry distortions. The splittings of the highest energy d-d band are indicative of a much more distorted coordination environment for Co(I1) at the catalytic site than the noncatalytic site. This is also reflected in the magnitude of ground state zero-field splitting, A, determined by analysis of the temperature dependence of discrete MCD bands, IA( = 33, 56, and 7 cm-* for Co(c)Zn(n)-HLADH, Co(c)Zn(n)-HLADH/NAD+/pyrazole, and Zn(c)Co(n)-HLADH, respectively. MCD magnetization data are rationalized in terms of the EPR-determined ground state effective g-values, ground state zero-field splitting, and the polarization of the electronic transitions. The zero-field splittings for the samples with Co(I1) at the catalytic site determined by VTMCD are quite different from those determined by EPR from the temperature dependence of the spin relaxation (Makinen, M. W.; Yim, M. B. Proc. Nutl. Acad. Sci. U S A . 1981 78, 6221-6225), and the origin of this discrepancy is discussed. In accord with X-ray crystallographic studies, the EPR and VTMCD data are rationalized in terms of a highly distorted tetrahedral coordination environment for Co(I1) at the catalytic site (two cysteines, one histidine, and one H20 for Co(c)Zn(n)-HLADH and two cysteines, one histidine and one pyrazole for Co(c)Zn(n)-HLADWNAD+/pyrazole) and a more regular tetrahedral environment for Co(I1) at the noncatalytic site (four cysteines). 0.33, 0.05 (with D

Active site substituted Cd(II) horse liver alcohol dehydrogenase has been studied by Perturbed Angular Correlation of Gamma rays Spectroscopy during turnover conditions for benzaldehyde and 4-trans-(N,N-dimethylamino)cinnamaldehyde. The ternary complex between alcohol dehydrogenase NAD+ and Cl-, and the binary complex between alcohol dehydrogenase and orthophenanthroline have also been studied. The Nuclear Quadrupole Interaction parameters have been interpreted in terms of different coordination geometries for Cd(II) in the catalytic zinc site of the enzyme. Calculation of the nuclear quadrupole interaction for cadmium in the catalytic site of the enzyme with and without coenzyme, based upon the four coordinated geometries determined from X-ray diffraction, agrees with the experimentally determined values. The ternary complexes between enzyme, NAD+ and either Cl- or trifluoroethanol and the binary complex between enzyme and orthophenanthroline have almost identical spectral parameters which are not consistent with a four coordinated geometry, but are consistent with a five coordinated geometry. The non-protein ligands for the ternary complex with trifluoroethanol are suggested to be an alkoxide group and a water molecule. The Nuclear Quadrupole Interaction parameters for the productive ternary complex between enzyme, NADH and an aldehyde is consistent with the four coordinated geometry predicted from X-ray diffraction data having the carbonyl group of the aldehyde substituting the water molecule as ligand to the metal.

Replacement of the catalytic Zn(II) in horse liver alcohol dehydrogenase (HLADH) with copper produces a mononuclear Cu(II) chromophore with a ligand set consisting of two cysteine sulphurs, one histidine nitrogen plus one further atom. The fourth ligand to the metal ion and the conformation of the protein may be altered by addition of exogenous ligands and/or the cofactor NADH. Absorbance, CD, low-temperature magnetic CD (MCD) and EPR spectra are presented of copper-substituted HLADH samples in both 'open' and 'closed' conformations and in the presence and absence of the exogenous ligands pyrazole and DMSO. The EPR spectra indicate a strong, predominantly axial field about the copper(II) ion with high copper-thiol (cysteine) covalence. The optical and MCD spectra are interpreted in terms of four d-d transitions to low energy, also reflecting the axial ligand field, and four charge-transfer transitions to copper(II) between 30000 and 16000 cm-1 arising from the two cysteine sulphur atoms which give two pairs of oppositely signed MCD C-terms. These transitions are polarized mainly in the axial plane defined by Cys-46, Cys-174 and His-67. The binary complex formed with pyrazole displays quite different EPR and optical spectra which can be understood in terms of a rotation of the copper hole-orbital away from the axial plane thus decreasing sharply the copper-thiol covalence. The magneto-optical spectra in the presence and absence of DMSO are indistinguishable.

Oxyllen treatment of horse liver alcohol dchydro=cnase EE isoxymc sub.geStured with Ca(Ill at the catalytic site l.¢ads to bleaching with concomitant reduction to Call) of',.90~ of total Ca(ill, The Ca(Ill of the remaininll'minor species' cannot bc reduced nor does it intcraca with =oilrno~ liipmds. e,ll. 2-ntrrcaplorthanol, imida~ole, pyntzole, or ttxid¢ ions, The EPR spectrum is axial with a super.hyl~tl"ln¢ splitting of i S,6 (3 indicating binding of one nitrol~n atom to Cu(ll). There data as well as th¢ cncrl;ies and intensiti~ of the ~:bsorption and CD spa:tea SUli~t the Ca(H) ion of the minor species to tm located in the cat.dytic slt¢ of HLADH in a position and geometey different from that or the. major glories,