Evidence from electron-paramagnetic-resonance spectroscopy for a complex of sulphite ions with the molybdenum centre of sulphite oxidase.
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ABSTRACT:
SUBMITTER: Bray RC
PROVIDER: S-EPMC1163632 | biostudies-other | 1982 Jan
REPOSITORIES: biostudies-other
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Project description:Molybdenum(V) e.p.r. spectra from reduced forms of aldehyde oxidase were obtained and compared with those from xanthine oxidase. Inhibited and Desulpho Inhibited signals from aldehyde oxidase were fully characterized, and parameters were obtained with the help of computer simulations. These differ slightly but significantly from the corresponding parameters for the xanthine oxidase signals. Rapid type 1 and type 2 and Slow signals were obtained from aldehyde oxidase, but were not fully characterized. From the general similarities of the signals from the two enzymes, it is concluded that the ligands of molybdenum must be identical and that the overall co-ordination geometries must be closely similar in the enzymes. The striking differences in substrate specificity must relate primarily to structural differences in a part of the active centre concerned with substrate binding and not involving the catalytically important molybdenum site.
| S-EPMC1158218 | biostudies-other
Project description:A study has been made of e.p.r. signals due to Mo(V) in reduced sulphite oxidase (EC 1.8.3.1) from chicken liver. Reduction by SO3(2-), or photochemically in the presence of a deazaflavin derivative, produces spectra indistinguishable from one another. Three types of spectra from the enzyme were distingusihed and shown to correspond to single chemical species, since they could be simulated at both 9 and 35 GHz by using the same parameters. These were the low-pH form of the enzyme, with gav. 1.9805, the high-pH form, with gav. 1.9681 and a phosphate complex, with gav. 1.9741. The low-H form shows interaction with a single exchangeable proton, with A(1H)av. (hyperfine coupling constant) = 0.98 mT, probably in the form of an MoOH group. Parameters of the signals are compared with those for signals from xanthine oxidase and nitrate reductase. The signal from the phosphate complex of sulphite oxidase in unique among anion complexes of Mo-containing enzymes in showing no hyperfine coupling to protons. There is no evidence for additional weakly coupled protons or nitrogen nuclei in the sulphite oxidase signals. The possibility is considered that the enzymic mechanism involves abstraction of a proton and two electrons from HSO3- by a Mo = O group in the enzyme.
| S-EPMC1161366 | biostudies-other
Project description:Molybdenum enzymes contain at least one pyranopterin dithiolate (molybdopterin, MPT) moiety that coordinates Mo through two dithiolate (dithiolene) sulfur atoms. For sulfite oxidase (SO), hyperfine interactions (hfi) and nuclear quadrupole interactions (nqi) of magnetic nuclei (I ≠ 0) near the Mo(V) (d(1)) center have been measured using high-resolution pulsed electron paramagnetic resonance (EPR) methods and interpreted with the help of density functional theory (DFT) calculations. These have provided important insights about the active site structure and the reaction mechanism of the enzyme. However, it has not been possible to use EPR to probe the dithiolene sulfurs directly since naturally abundant (32)S has no nuclear spin (I = 0). Here we describe direct incorporation of (33)S (I = 3/2), the only stable magnetic sulfur isotope, into MPT using controlled in vitro synthesis with purified proteins. The electron spin echo envelope modulation (ESEEM) spectra from (33)S-labeled MPT in this catalytically active SO variant are dominated by the "interdoublet" transition arising from the strong nuclear quadrupole interaction, as also occurs for the (33)S-labeled exchangeable equatorial sulfite ligand [ Klein, E. L., et al. Inorg. Chem. 2012 , 51 , 1408 - 1418 ]. The estimated experimental hfi and nqi parameters for (33)S (aiso = 3 MHz and e(2)Qq/h = 25 MHz) are in good agreement with those predicted by DFT. In addition, the DFT calculations show that the two (33)S atoms are indistinguishable by EPR and reveal a strong intermixing between their out-of-plane pz orbitals and the dxy orbital of Mo(V).
| S-EPMC3927148 | biostudies-literature
Project description:The phosphate complex of sulphite oxidase in the Mo(V) oxidation state was investigated by e.p.r. spectroscopy. Third-derivative spectra reveal a wealth of structural detail previously unobserved in this spectrum. Most notable is the presence of hyperfine coupling from two inequivalent I = 1/2 nuclei, which we tentatively attribute to two 31P nuclei. Unresolved hyperfine interactions from at least one exchangeable 1H nucleus are also present.
| S-EPMC1135405 | biostudies-other
Project description:The molybdenum centre of spinach (Spinacia oleracea) nitrate reductase has been investigated by e.p.r. spectroscopy of molybdenum(V) in reduced forms of the enzyme. The resting enzyme gives no signals attributable to Mo(V). However, on reduction with NADH, Mo(V) signals appeared at relatively short reaction times but decreased again on prolonged exposure to excess of the substrate as the enzyme was further reduced. On brief treatment of such samples with nitrate, Mo(V) signals reappeared but disappeared again on longer exposure to excess nitrate as the enzyme became fully reoxidized. Detailed investigation of the signals carried out in both 1H2O and 2H2O revealed the presence of two signal-giving species, referred to as 'signal A' and 'signal B', analogous to corresponding signals from nitrate reductase from Escherichia coli and from liver sulphite oxidase. Signal A has gav. 1.9767 and shows coupling to a single proton, exchangeable with the solvent, with A(1H)av. 1.3mT, whereas signal B shows no more than weak coupling to protons. Investigation of interconversion between the two species indicated that decreasing the pH from 8.0 to 6.7 had little effect, but that signal A was favoured by the presence of Cl-. This suggests, by analogy with recent work on sulphite oxidase by Bray, Gutteridge, Lamy & Wilkinson [Biochem. J. (1983) 211, 227-236] that Cl- is a ligand of molybdenum in the species giving signal A.
| S-EPMC1152100 | biostudies-other
Project description:The molybdenum centre of respiratory nitrate reductase from Paracoccus denitrificans has been investigated by e.p.r. spectroscopy of Mo(V). In common with the centres of the analogous enzymes from Escherichia coli and Pseudomonas aeruginosa, it undergoes a pH- and anion-dependent transition between two different e.p.r. signal-giving species. Comparison of the relevant e.p.r. parameters extracted with the help of computer simulations reveals ligation of the metal in the active centres of the three enzymes to be identical.
| S-EPMC1149239 | biostudies-other
Project description:Studies were carried out on the inhibitory complex of alloxanthine (1H-pyrazolo[3,4-d]pyrimidine-4,5-diol) with xanthine oxidase, in extension of the work of Williams & Bray [Biochem. J. (1981) 195, 753-760]. By suitable regulation of the reaction conditions, up to 10% of the functional enzyme could be converted into the complex in the Mo(V) oxidation state. The e.p.r. spectrum of the complex was investigated in detail with the help of computer simulation and substitution with stable isotopes. Close structural analogy of the signal-giving species to that of the Very Rapid intermediate in enzyme turnover is shown by g-values (2.0279, 1.9593 and 1.9442) and by coupling to 33S in the cyanide-labile site of the enzyme [A(33S) 0.30, 3.10 and 0.70mT]. However, whereas in the Very Rapid signal there is strong coupling to 17O [Gutteridge & Bray, Biochem. J. (1980) 189, 615-623], instead, in the Alloxanthine signal there is strong coupling to a single nitrogen atom [A(14N) 0.35, 0.35, 0.32 mT]. This is presumed to originate from the 2-position of the heterocyclic ring system. From this work and from earlier kinetic studies it is concluded that alloxanthine, after being bound reversibly at the active centre, reacts slowly with it, in a specific manner, distinct from that in the normal catalytic reaction with substrates. This reaction involves elimination of an oxygen ligand of molybdenum and co-ordination, in this site, of alloxanthine via the N-2 nitrogen atom, to give a complex that is structurally but not chemically closely analogous to that of the Very Rapid species.
| S-EPMC1153429 | biostudies-other
Project description:Further electron-paramagnetic-resonance studies relating to the role of molybdenum in the enzymic mechanisms of xanthine oxidase were carried out. The classification of the various molybdenum signals obtained on reducing the enzyme is briefly discussed. The group of ;Rapidly appearing' signals, which are obtained with all substrates within the turnover time and which show interaction with exchangeable protons, were studied in detail. Signals with salicylaldehyde, purine and xanthine in H(2)O and in 95% D(2)O were examined at 9 and 35GHz and interpreted with the help of computer simulation. Molybdenum atoms in a number of different chemical environments are involved, each substrate giving rise to two superimposed spectra with slightly different parameters; g values and proton splittings were determined. The spectrum with salicylaldehyde is believed to represent the reduced enzyme alone not in the form of a complex with substrate and its two constituents are believed to represent the two molybdenum atoms bonded slightly differently within the enzyme molecule. With purine and xanthine the spectra are thought to represent complexes of reduced enzyme with substrate molecules. With xanthine one signal-giving species shows coupling to two equivalent protons, whereas in all the other species observed two non-equivalent protons are involved. The origin of the protons is discussed in the light of the direct hydrogen-transfer mechanism implicated earlier for the enzyme. It is concluded that the proton derived from the substrate is located at least 3å from the molybdenum atom with which it interacts.
| S-EPMC1184959 | biostudies-other
Project description:Studies of the effect of substitution with 17O on the e.p.r. spectra at 9 and 35 GHz of Mo(V) in the phosphate complex of sulphite oxidase are reported. Substitution of 17O-enriched water for normal water, for samples of the enzymes reduced by sulphite in the presence of normal phosphate, produced no detectable effect on the e.p.r. signal. If phosphate substituted with 17O was used, coupling due to 17O, producing large anisotropic splittings in the spectrum, was clearly detectable. It is concluded that phosphate is co-ordinated directly to molybdenum in the active site of the enzyme, in an equatorial type of ligand position. An oxygen ligand must be displaced from the molybdenum in the process of binding the phosphate. Implications concerning the mechanism of the enzyme reactions are discussed.
| S-EPMC1162213 | biostudies-other
Project description:Calmodulin (CaM) is a highly conserved calcium-binding protein consisting of two homologous domains, each of which contains two EF-hands, that is known to bind well over 300 proteins and peptides. In most cases the (Ca(2+))(4-)form of CaM leads to the activation of a key regulatory enzyme or protein in a myriad of biological processes. Using the nitroxide spin-labeling reagent, 3-(2-iodoacetamido)-2,2,5,5-tetramethyl-1-pyrrolidinyl oxyl, bovine brain CaM was modified at 2-3 methionines with retention of activity as judged by the activation of cyclic nucleotide phosphodiesterase. X-band electron paramagnetic resonance (EPR) spectroscopy was used to measure the spectral changes upon addition of Ca(2+) to the apo-form of spin-labeled protein. A significant loss of spectral intensity, arising primarily from reductions in the heights of the low, intermediate, and high field peaks, accompanied Ca(2+) binding. The midpoint of the Ca(2+)-mediated transition determined by EPR occurred at a higher Ca(2+) concentration than that measured with circular dichroic spectroscopy and enzyme activation. Recent data have indicated that the transition from the apo-state of CaM to the fully saturated form, [(Ca(2+))(4-)CaM], contains a compact intermediate corresponding to [(Ca(2+))(2-)CaM], and the present results suggest that the spin probes are reporting on Ca(2+) binding to the last two sites in the N-terminal domain, i.e. for the [(Ca(2+))(2)-CaM] ? [(Ca(2+))(4-)CaM] transition in which the compact structure becomes more extended. EPR of CaM, spin-labeled at methionines, offers a different approach for studying Ca(2+)-mediated conformational changes and may emerge as a useful technique for monitoring interactions with target proteins.
| S-EPMC4096354 | biostudies-literature