Effects of 2,4-substituents of deuteroheme upon peroxidase functions. II. Reactions of peroxidases and metmyoglobin with azide, cyanide and fluoride

Insights into porphyrin chemistry provided by the microperoxidases, the haempeptides derived from cytochrome c

The water-soluble haem-containing peptides obtained by proteolytic digestion of cytochrome c, the microperoxidases, have been used to explore aspects of the chemistry of iron porphyrins, and as mimics for some reactions catalysed by the haemoproteins, including the reactions catalysed by the peroxidases and the cytochromes P450. The preparation of the microperoxidases, their physical and chemical properties including their electronic structure, the kinetics and thermodynamics of their reactions with ligands, electrochemical studies and examples of their uses as haemoproteins mimics, is reviewed.

Effect of calcium, other ions, and pH on the reactions of barley peroxidase with hydrogen peroxide and fluoride. Control of activity through conformational change

Transient-state kinetic analysis of compound I formation for barley grain peroxidase (BP 1) has revealed properties that are highly unusual for a heme peroxidase but which may be relevant to its biological function. The enzyme shows very little reaction with H2O2 at pH > 5 and exhibited saturation kinetics at higher H2O2 concentrations (kcatapp increases from 1.1 s-1 at pH 4.5 to 4.5 s-1 at pH 3.1 with an enzyme-linked pKa < 3.7 (Rasmussen, C.B., Bakovic, M., Welinder, K. G., and Dunford, H. B. (1993) FEBS Lett. 321, 102-105)). In the present paper, it is shown that the presence of Ca2+ gives rise to biphasic kinetics for compound I formation, with a slow phase as described above and a fast phase that exhibits a second order rate constant more typical of a classical peroxidase (K1app = 1.5 x 10(7) M-1 S-1, which is pH-independent between 3.3 and 5.0). The amount of enzyme reacting in the fast phase increases with Ca2+ concentration (Kd = 4 +/- 1 mM at pH 4.0), although it is also moderately inhibited by Cl-. The absorption spectrum of BP 1, which appears to be a five-coordinate high spin ferric in the resting state changes insignificantly in the presence of Ca2+. In the presence of Cl-, it becomes six-coordinate high spin (Kd approximately 60 mM at pH 4.0) but only if Ca2+ is also present. Fluoride binds to BP 1 with monophasic kinetics in the presence of 0-5 mM Ca2+. The activating effect of Ca2+ can be mimicked only by replacing it with Sr2+ and Ba2+ ions. Comparing these data with the crystal structure of the inactive neutral form of BP 1 (Henriksen, A., Welinder, K. G., and Gajhede, M. (1997) J. Biol. Chem. 273, 2241-2248) and similar data for wild-type and mutant peroxidases of plant and fungal origin suggests (i) a proton-induced conformational change from an inactive BP 1 at neutral pH to a low activity BP 1 form with a functional distal histidine and (ii) a Ca(2+)-induced slow conformational change (at least compared with compound I formation) of this low activity form to a high activity BP 1 with a typical peroxidase reactivity. BP 1 is the first example of a plant peroxidase whose activity can be reversibly controlled at the enzyme level by pH- and Ca(2+)-induced conformational changes.

Structural and electronic characterization of heme moiety in oxygenated hemoproteins by using XANES spectroscopy

Iron K-edge X-ray absorption near edge structure (XANES) spectra were measured for oxy-forms of cytochrome P-450cam (P-450cam), horseradish peroxidase (HRP) and myoglobin (Mb) by using Synchrotoron Radiation of Photon Factory (Tsukuba). A pronounced 1s-4p transition and some fine structures were well-resolved in the spectra obtained. Comparing the spectra, the features at the fine structures termed P, C and D, were similar among the three hemoproteins, suggesting a similar site-symmetry around the heme iron and the same Fe-O-O bond angle (about 115 degrees). On the other hand, absorption features at the edge region (7115-7135 eV) were slightly but significantly different from one another; the absorption intensity at 7115-7125 eV region increased in the order of Mb, HRP and P-450cam, while that at 7125-7135 eV decreased in the same order. A similar absorption feature was also obtained with their deoxy (ferrous high spin) forms. We assumed that the absorption at the lower energy region (7115-7125 eV) reflects the pi-character in the Fe-ligand bond, whereas that at the higher energy region (7125-7135 eV) does the sigma-character, on the basis of the previous and comprehensive studies of the XANES spectroscopy of the adsorbed molecules on the metal surface (McGovern et al. (1989) Handbook on Synchrotoron Radiation, Vol. 2, pp. 467-539). According to our assumption, our XANES results indicated that the pi-character of the Fe-ligand bond increases in the order of Mb, HRP and P-450cam, and that the pi-electron of the thiolate S- in P-450cam is donated to the Fe-O-O moiety, most probably to the antibonding pi* orbital of O2. Such an interpretation is consistent with the experimental findings or data accumulated so far by other methods, such as the resonance Raman spectroscopy.

Ligand binding properties of cytochromes c'

The cytochromes c' bind CO, alkylisocyanides and CN- with rate and equilibrium constants which are 10(2)- to 10(6)-fold smaller than other high-spin hemoproteins. The decreased affinity for exogenous ligands is largely associated with steric interactions at the heme coordination site. While CO and alkylisocyanides bind noncooperatively to the dimeric Rhodospirillum molischianum cytochrome c', CO, alkylisocyanides and CN- appear to bind cooperatively to the dimeric Chromatium vinosum cytochrome c' due to a ligand-linked dimer-monomer dissociation equilibrium. The differences between the cytochromes c' are thought to be due to differences in amino acid residues near the heme coordination site and subunit interface.

Cyanide-linked dimer-monomer equilibrium of Chromatium vinosum ferric cytochrome c'

Cyanide binding to Chromatium vinosum ferricytochrome c' has been studied to further investigate possible allosteric interactions between the subunits of this dimeric protein. Cyanide binding to C. vinosum cytochrome c' appears to be cooperative. However, the cyanide binding reaction is unusual in that the overall affinity of cyanide increases as the concentration of cytochrome c' decreases and that cyanide binding causes the ligated dimer to dissociate to monomers as shown by gel-filtration chromatography. Therefore, the cyanide binding properties of C. vinosum ferricytochrome c' are complicated by a cyanide-linked dimer to monomer dissociation equilibrium of the complexed protein. The dimer to monomer dissociation constant is 20-fold smaller than that for CO linked dissociation constant of ferrocytochrome c'. Furthermore, the pH dependence of both the intrinsic equilibrium binding constant and the dimer to monomer equilibrium dissociation constant was investigated over the pH range of 7.0 to 9.2 to examine the effect of any ionizable groups. The equilibrium constants did not exhibit a significant pH dependence over this pH range.

Preparation and spectral characterization of the heme d1.apomyoglobin complex: an unusual protein environment for the substrate-binding heme of Pseudomonas cytochrome oxidase

The heme d1 prosthetic group isolated from Pseudomonas cytochrome oxidase combines with apomyoglobin to form a stable, optically well-defined complex. Addition of ferric heme d1 quenches apomyoglobin tryptophan fluorescence suggesting association in a 1:1 molar ratio. Optical absorption maxima for heme d1.apomyoglobin are at 629 and 429 nm before, and 632 and 458 nm after dithionite reduction; they are distinct from those of heme d1 in aqueous solution but more similar to those unobscured by heme c in Pseudomonas cytochrome oxidase. Cyanide, carbon monoxide and imidazole alter the spectrum of heme d1.apomyoglobin demonstrating axial coordination to heme d1 by exogeneous ligands. The cyanide-induced optical difference spectra exhibit isosbestic points, and a Scatchard-like analysis yields a linear plot with an apparent dissociation constant of 4.2 X 10(-5) M. However, carbon monoxide induces two absorption spectra with Soret maxima at 454 or 467 nm, and this duplicity, along with a shoulder that correlates with the latter before binding, suggests multiple carbon monoxide and possibly heme d1 orientations within the globin. The 50-fold reduction in cyanide affinity over myoglobin is more consistent with altered heme pocket interactions than the intrinsic electronic differences between the two hemes. However, stability of the heme d1.apomyoglobin complex is verified further by the inability to separate heme d1 from globin during dialysis and column chromatography in excess cyanide or imidazole. This stability, together with a comparison between spectra of ligand-free and -bound derivatives of heme d1-apomyoglobin and heme d1 in solution, implies that the prosthetic group is coordinated in the heme pocket through a protein-donated, strong-field ligand. Furthermore, the visible spectrum of heme d1.apomyoglobin varies minimally with ligand exchange, in contrast to the Soret, which suggests that much spectral information concerning heme d1 coordination in the oxidase is lost by interference from heme c absorption bands. A comparison of the absorption spectra of heme d1.apomyoglobin and Pseudomonas cytochrome oxidase, together with a critical examination of the previous axial ligand assignments from magnetic resonance techniques in the latter, implies that it is premature to accept the assignment of bishistidine heme d1 coordination in oxidized, ligand-free oxidase and other iron-isobacteriochlorin-containing enzymes.

Kinetic analysis of the acid-alkaline conversion of horseradish peroxidases

The nature of the acid-alkaline conversion of horseradish peroxidases was studied by measuring four rate constants in reactions, E + H+ (k1) in equilibrium (k2) EH+ and E + H2O (k3) in equilibrium (k4) EH+ + OH-, where EH+ and E denote the acid and alkaline forms of the enzymes. The values of k1, (k2 + k3), and k4 were obtained by measuring the relaxation rates of the acid leads to alkaline and alkaline leads to acid conversions by means of th pH jump method with a stopped-flow apparatus. The value of k3 could also be obtained by measuring the rate of reactions between hydrogen peroxide and peroxidases at alkaline pH. The measurements were conducted with four peroxidases having different pKa values: peroxidase A )pKa = 9.3), peroxidase C (pKa = 11.1), diacetyldeuteroperoxidase A (pKa = 7.7), and diacetyldeuteroperoxidase C (pKa = 9.1). The value of k1 was about 10(10) M-1 s-1 in the reaction of the four enzymes while k4 was quite different between the enzymes. The pKa was determined by k3 and k4 for the natural peroxidases and by k1 and k2 for the diacetyldeuteroperoxidases. The mechanism of the acid-alkaline conversion was discussed in comparison with that of metmyoglobin.

Oxygen binding to cobalt(II) proto-, deutero- and meso-porphyrin IX dimethyl ester complexes in organic solvents

The binding of oxygen to cobalt(II) meso, deutero- and proto-porphyrin IX dimethyl esters complexed with pyridine or 2-methylimidazole was investigated at -10 degress - -60 degrees C in toluene or DMF solution, and the thermodynamic data related with the binding were presented. The oxygen affinity of cobalt meso-porphyrin complex was larger by the factor of 2.0-1.4 than those of the other complexes where oxygen affinities were not explained by a simple electron-withdrawing capability of 2,4-substituents of the porphyrin ring. The oxygen binding property was, generally, dependent on the solvent, suggesting that the solvation affects appreciably the oxygen binding to the complexes. The oxygen affinities of cobalt porphyrin complexes in various organic solvents were compared with those of their apomyoglobin complexes. The differences of oxygen affinities between both systems decreased with increasing the size of 2,4-substituents, and it was in the following order on 2,4-substituted porphyrins: Deutero greater than Proto greater than Meso. It was suggested that the 2,4-substitutent effect on the oxygen affinity of cobalt myoglobin complexes was not only caused by the direct electronic effect on the central cobalt atom, but also controlled by the stereochemical interaction between apomyoglobin and the porphyrin.

Comparison of function of the distal base between myoglobin and peroxidase

The heme-linked protonation of a ferrous horseradish peroxidase is assigned to a distal amino acid residue. The conclusion is drawn from analyses of reactions that involve the protonation. Unfortunately it is difficult to apply it to the myoglobin case because no reaction has been found which is coupled with the protonation of the distal histidine. It is therefore of special interest to note that the pK value of 5.7 has been assigned to the distal histidine of metmyoglobin from binding kinetics with ligands20). It is well known that the reaction with hydrogen peroxide is quite different for the two types of hemoproteins. The distal base may be associated with the stabilization of the primary compound with hydrogen peroxide and also another amino acid residue may serve as a nucleophile for the stabilization of the pi-cation radical of porphyrin in the case if peroxidases. Numerous papers have dealt with heme substitution, heme linked protonation and reactions of hemoproteins related to the present subject. In this short paper, however, the discussion has mostly centered around the data obtained recently in our laboratory.

The steady-state kinetics of peroxidase with 2,2'-azino-di-(3-ethyl-benzthiazoline-6-sulphonic acid) as chromogen

The chemical nature of the important new chromogen ABTS [2,2'-azino-di-(3-ethyl-benzthiazoline-6-sulphonic acid)] is described together with an account of the redox and spectroscopic properties of the system ABTS--H2O2--peroxidase. Keq. is calculated and a study of the steady-state kinetics over a whole range of substrate concentrations is reported. By using novel methods of kinetic analysis, an interpretation of the results is given which requires some extension of the classical peroxidase mechanism.