Preparation and properties of cobalt(II) rubredoxin

Spectroscopic studies of cobalt and nickel substituted rubredoxin and desulforedoxin

The single iron site of rubredoxin was replaced by nickel and cobalt. The near-infrared/visible/UV spectra of these metal derivatives show ligand-field transitions and charge-transfer bands which closely resemble those of simple tetrathiolate complexes, indicating a tetrahedral arrangement of the sulfur cysteinyl ligands around the metal core. The 1H NMR spectra of the nickel and cobalt derivatives reveal extremely low-field contact shifted resonances of one proton intensity assigned to beta-CH2 and alpha-CH cysteinyl protons. Other well resolved resonances shifted out of the main protein spectral envelope are also observed and probably arise from contact plus pseudocontact shift mechanisms. Rubredoxins from different sulfate reducers were metal substituted and assignments of aliphatic protons are tentatively proposed, taking advantage of the amino acid sequence homologies. The present data is promising in terms of structural analysis of the coordination sphere of the metal core. It was also shown that replacement of the iron atom of desulforedoxin, a close analogue of rubredoxin, by cobalt and nickel was possible.

Cobalt(II)-substituted class III alcohol and sorbitol dehydrogenases from human liver

The catalytic zinc atoms in class III (chi) alcohol dehydrogenase (ADH) and sorbitol dehydrogenase (SDH) from human liver have been specifically removed and replaced by cobalt(II) with a new ultrafiltration technique. The electronic absorption spectrum of class III cobalt ADH (epsiolon 638 = 870 M-1 cm-1) is nearly identical with those of active site substituted horse EE and human class I (beta 1 beta 1) cobalt ADH. Thus, the coordination environment of the catalytic metal is strictly conserved in these enzymes. However, significant differences are noted when the spectra of class III ADH-coenzyme complexes are compared to the corresponding spectra of the horse enzyme. The spectrum of class III ADH.NADH is split into three bands, centered at 680, 638, and 562 nm. The class III ADH.NAD+ species resembles the alkaline form of the corresponding horse enzyme complex but without exhibiting the pH dependence of the latter. These spectral changes underscore the role of the coenzymes in differentially fine tuning the catalytic metal for its particular function in each ADH. The noncatalytic zinc of class III ADH exchanges with cobalt at pH 7.0. While 9 residues out of 15 in the loop surrounding the noncatalytic zinc of class III ADH differ from those of the class I ADH, the electronic absorption spectra of cobalt in the noncatalytic metal site of class III ADH establish that the coordination environment of this site is conserved as well. The spectrum of cobalt SDH differs significantly from those of cobalt ADHs.(ABSTRACT TRUNCATED AT 250 WORDS)

Spectroscopic studies of cobalt-substituted ferredoxin from Clostridium pasteurianum

Ferredoxin from Clostridium pasteurianum substituted with two Co atoms did not give any cobalt EPR signal at 8 K as isolated, but upon reduction with sodium dithionite, a broad signal appeared with g values that indicate highspin (S = 3/2) Co(II). These signals were distinct from Co(II)-dithiothreitol signals, and disappeared upon reoxidation with air. Under anaerobic incubation of apoferredoxin with Co(II), a green derivative showed a visible spectrum typical of tetrahedral Co(II)-thiolate coordination, which shifted dramatically upon exposure to air. The 1H-NMR spectrum of the aerobically isolated protein is reported at 300 MHz; magnetic susceptibility measurements were indicative of a diamagnetic species. These spectroscopic studies indicate that Co(II)-substituted ferredoxin is oxidized to low-spin Co(III)-ferredoxin in the presence of sulfide and oxygen. The diamagnetic Co(III) state could reversibly be reduced to highspin Co(II) by sodium dithionite.

Zinc metalloproteins involved in replication and transcription

RNA polymerase (RPase) from E. coli contains two tightly incorporated Zn(II) ions, while the monomeric RPase from bacteriophage T7 does not contain zinc and does not require Zn(II) in the assay. One of the two Zn(II) ions can be differentially removed from E. coli RPase with p-hydroxymercuriphenylsulfonate (PMPS) combined with EDTA and thiol. The resultant Znl or ZnA RPase shows no alteration in transcription initiation and elongation rate from sigma-specific promoters. Biosynthesis of a Co2 RPase and formation of CoA RPase by similar treatment shows the tetrahedral-type Co(II) d-d absorption bands to be associated only with the Co(II) at the A site with maxima at 760 (epsilon = 800), 710 (epsilon = 900), 602 (epsilon = 1500), and 484 (epsilon = 4000) nm. Sulfur to Co(II) charge transfer bands are present at 350 (epsilon = 9600) and 370 (epsilon = 9500) nm. The absorption characteristics strongly suggest that the A site is a tetrathiolate site. While DNA polymerases do not in general appear to contain zinc, gene 32 protein (g32P) from bacteriophage T4, an accessory protein essential for DNA replication and recombination and translational control in the T4 life cycle, is a Zn(II) metalloprotein and contains 1 gram atom of tightly incorporated Zn(II). PMPS displaces the zinc by reacting with three SH groups. Apo-g32P shows markedly altered DNA binding properties. Co(II) substitution gives a protein with intense d-d transitions typical of a tetrahedral Co(II) complex with absorption maxima at 680 (epsilon = 480), 645 (epsilon = 660), 605 (epsilon = 430), 355 (epsilon = 2250), and 320 (epsilon = 3175) nm. The data support a 3 Cys, 1 His coordination site located in the middle of the DNA binding domain of g32P. Data thus far suggest that the Zn(II) binding sites in multisubunit RNA polymerases and in accessory proteins involved in polynucleotide biosynthesis are more likely to play structural or allosteric (regulatory) roles rather than directly participating in catalysis.

Structural and functional differences between the two intrinsic zinc ions of Escherichia coli RNA polymerase

DNA-dependent RNA polymerase (RPase) from Escherichia coli contains 2 mol of intrinsic Zn(II)/mol of core enzyme (alpha 2 beta beta'). In techniques analogous to those employed with the Zn(II) metalloenzyme aspartate transcarbamoylase [Hunt, J. B., Neece, S. H., Schachman, H. K., & Ginsberg, A. (1984) J. Biol. Chem. 259, 14793-14803], we show that titration of core or holoRPase with 10 or 16 equiv, respectively, of the sulfhydryl reagent p-(hydroxymercuri)benzenesulfonate (PMPS) results in the facile release of 1 mol of Zn(II) [B-site Zn(II)] in a reaction totally reversible with the addition of excess thiol provided no metal chelator is present. If ethylenediaminetetraacetic acid (EDTA) is present, reversal of the PMPS-enzyme complex results in formation of a Zn1 RPase [A-site Zn(II)]. This enzyme retains full transcriptional activity relative to Zn2 RPase on both calf thymus (nonspecific) and T7 (sigma-dependent, specific) DNA templates. If the core enzyme-PMPS complex is incubated with a large excess of another metal such as Cd(II) followed by thiol treatment, a hybrid ZnACdB RPase is formed. Direct treatment of the enzyme with excess Cd(II) also gives rise to a hybrid ZnACdB RPase. Transcription by these enzymes is also comparable to that of the starting Zn2 enzyme. Isolation of in vivo synthesized Co2 RPase and Cd2 RPase and treatment of either enzyme with PMPS/EDTA results in formation of a CoA and CdA enzyme, respectively. Co(II)A and Cd(II)A enzymes show 123 and 76%, respectively, of the elongation rates on T7 DNA observed for the Zn(II) enzyme. Visible absorption spectroscopy of the Co2 enzyme exhibits four d-d transition bands positioned at 760 (epsilon = 800), 710 (epsilon = 900), 602 (epsilon = 1500), and 484 (epsilon = 4000) nm. In addition, two charge-transfer bands are found at 350 (epsilon = 9600) and 370 (epsilon = 9500) nm. Only the Co(II) ion bound at site A is associated with this unique set of intense d-d transitions. The positions and intensities of both the visible and charge-transfer bands of Co(II)A RPase approximate those shown by Co(II)-substituted metalloenzyme sites where the ligands are four S rather than mixed S,N or S,O sites.

Cobalt-(cysteinyl)4 tetrahedra in yeast cobalt(II)-thionein

The conversion of yeast Cu(I)-thionein into the Co(II) derivative was successful. 2.6 Co atoms were incorporated per mole of protein yielding a Co : S ratio of 1 : 3. The electronic absorption of this highly air sensitive Co(II)-thionein is virtually identical to those of the Co(II) derivatives of other metallothioneins originating from vertebrates and N. crassa. Weaker Cotton extrema are noticed and the two doublet splittings of Cu-thionein disappeared. Throughout the molar ellipticities of the cobalt protein were markedly lower compared to those of the Cu-thionein. Owing to the characteristic charge transfer bands and d-d transitions a tetrahedral Co-thiolate coordination was deduced. The best fit proposal maintaining the above Co : S ratio of 1 : 3 was a six-membered ring with three bridging cysteine sulphurs.

Spectral studies of cobalt (II)- and Nickel (II)-metallothionein

The zinc and cadmium of native rabbit metallothionein-1 were replaced stoichiometrically with either cobalt (II) or nickel (II). The electronic, magnetic circular dichroic (MCD), and electron spin resonance spectra of Co (II)-metallothionein reflect distorted tetrahedral coordination of the cobalt atoms. Both the d-d and charge-transfer spectral regions closely resemble those of simple cobalt-tetrathiolate complexes, implying that their coordination chemistry is analogous. Ni (II) complex ions and Ni (II)-metallothionein similarly exhibit analogous MCD bands in the d-d region. The circular dichroic bands associated with ligand-metal charge-transfer transitions in the non-d-d region of Co (II)- and Ni (II)-metallothionein afford additional evidence for the similarity in tetrahedral microsymmetry of the two metal derivatives. The known ratio of 20 thiolate ligands to 7 metal ions, in conjunction with the spectral evidence for tetrathiolate coordination in metallothionein, represents good evidence that these metal thiolates are organized in clusters.

A spectroscopic study of metal ion and ligand binding to beta-lactamase II

beta-Lactamase II has two metal-binding sites. The electronic spectra of Cd(II)- and Co(II)-substituted beta-lactamase II have been investigated. It is suggested that a thiol ligand is involved in metal binding at the first site. The stoichiometric dissociation constants for Co(II) binding to beta-lactamase II were estimated to be 0.13 and 2.66 mM (pH 6.0, 4 degrees C, 1 M NaCl) by equilibrium dialysis. Competition between Zn(II) and Co(II) for the first metal binding site suggests a value of 0.7 microM (pH 6.0, 30 degrees C, 1 M NaCl) for the dissociation constant of Zn(II). The electronic spectra of the Co(II) enzyme lead to the suggestion that the coordination geometries around the metal ions in the first and second sites are related to those of a distorted tetrahedron and octahedron, respectively.

Molar absorptivity and A1%1 cm values for proteins at selected wavelengths of the ultraviolet and visible regions--XVIII

A table of molar absorptivity and A1cm1% values for more than 180 proteins is given. The conditions used to obtain these values and references to the original literature are also'given.

Site-specific substituted cobalt(II) horse liver alcohol dehydrogenases. Preparation and characterization in solution, crystalline and immobilized state

The specific substitution, using highly selective techniques, of catalytic and/or noncatalytic zinc ions by cobaltous ions in horse liver alcohol dehydrogenase (EC 1.1.1.1) has been studied with dissolved, crystalline and agarose-immobilised enzyme, in order to examine the effect of protein structure on the specificity of the metal exchange. The different binding sites can be clearly distinguished by the absorption spectra of their cobalt derivatives. In solution an anaerobic column chromatographic method made it possible to exchange half of the zinc in the enzyme by cobalt ions in a much shorter time than previous procedures. By raising the temperature in the exchange step, even the slowly exchanging zinc ions were substituted by cobalt, yielding products similar to cobalt alcohol dehydrogenases described earlier. Treatment of crystal suspensions of the enzyme with chelating agents (preferentially dipicolinic acid) gave an inactive protein with two zinc ions remaining bound. The enzyme could be reactivated by treatment of the crystalline protein with 5 mM zinc or cobaltous ions or by dialysis of dissolved inactive protein against 20 microM zinc or 1 mM cobaltous ions. Higher metal concentrations led to denaturation but the inactive protein could be crystallized from solution and then reactivated completely at higher metal concentrations. The preparation and absorption spectrum show that cobalt is bound specifically at the catalytic sites. Since metal substitution at these sites critically depends on the maintenance of the correct tertiary and quaternary structure, these must be preserved in the crystal lattice and partially altered in solution when the catalytic zinc ions are removed (or when excess of metal ions is applied), thus demonstrating the structure-stabilizing role of the catalytic metal ions. The enzyme immobilised on agarose, with unchanged content of active sites [Schneider-Bernlöhr et al. (1978) Eur. J. Biochem. 41, 475--484], was treated like the crystal suspensions. Although half of the zinc was removed, some activity remained. After reactivation with cobaltous ions, a loss of about 30% active sites was measured. Thus the apparently homogenous bound enzyme was rather heterogeneous in the properties of its catalytic metal binding sites. These results are taken as further proof for the dependence of the metal substitution on the proper tertiary and quaternary structure which is strained by multiple interactions in the covalently immobilised enzyme.