d-3-Hydroxybutyrate dehydrogenase from Rhodopseudomonas spheroides. Kinetic mechanism from steady-state kinetics of the reaction catalysed by the enzyme in solution and covalently attached to diethylaminoethylcellulose

1. The reversible NAD(+)-linked oxidation of d-3-hydroxybutyrate to acetoacetate in 0.1m-sodium pyrophosphate buffer, pH8.5, at 25.0 degrees C, catalysed by d-3-hydroxybutyrate dehydrogenase (d-3-hydroxybutyrate-NAD(+) oxidoreductase, EC 1.1.1.30), was studied by initial-velocity, dead-end inhibition and product-inhibition analysis. 2. The reactions were carried out on (a) the soluble enzyme from Rhodopseudomonas spheroides and (b) an insoluble derivative of this enzyme prepared by its covalent attachment to DEAE-cellulose by using 2-amino-4,6-dichloro-s-triazine as coupling agent. 3. The insolubilized enzyme preparation contained 5mg of protein/g wet wt. of total material, and when freshly prepared its specific activity was 1.2mumol/min per mg of protein, which is 67% of that of the soluble dialysed enzyme. 4. The reactions catalysed by both the enzyme in solution and the insolubilized enzyme were shown to follow sequential pathways in which the nicotinamide nucleotides bind obligatorily first to the enzyme. Evidence is presented for kinetically significant ternary complexes and that the rate-limiting step(s) of both catalyses probably involves isomerization of the enzyme-nicotinamide nucleotide complexes and/or dissociation of the nicotinamide nucleotides from the enzyme. Both catalyses therefore are probably best described as ordered Bi Bi mechanisms, possibly with multiple enzyme-nicotinamide nucleotide complexes. 5. The kinetic parameters and the calculable rate constants for the catalysis by the soluble enzyme are similar to the corresponding parameters and rate constants for the catalysis by the insolubilized enzyme.     

Purification and properties of crystalline 3-hydroxybutyrate dehydrogenase from Rhodopseudomonas spheroides

1. The purification and crystallization of 3-hydroxybutyrate dehydrogenase from extracts of Rhodopseudomonas spheroides is described. 2. The molecular weight was calculated to be 85000 by sedimentation equilibrium. 3. Although the enzyme is stable at 0-4 degrees , dilute solutions are rapidly inactivated at 37 degrees ; NADH(2) or Ca(2+) ions prevent this inactivation. 4. The enzyme is extremely sensitive to mercurials, but can be protected by NADH(2) or Ca(2+) ions. 5. From studies on p-hydroxymercuribenzoate binding it is estimated that the enzyme contains 5-6 moles of rapidly reacting thiol groups/mole. 6. d-Lactate and dl-2-hydroxybutyrate are competitive inhibitors of d-3-hydroxybutyrate oxidation. 7. The properties of the crystalline enzyme are compared with those of 3-hydroxybutyrate dehydrogenase preparations from other sources.     

Kinetics of the reaction catalysed by rape alcohol dehydrogenase

The kinetics of the enzyme reaction of ethanol oxidation and acetaldehyde reduction catalysed by alcohol dehydrogenase (ADH) (EC 1.1.1.1) isolated from germinating rape seeds obeys the bi-bi ordered mechanism of Theorell and Chance. The enzyme reaction depends on the pH and temperature. The K_m values for the basic substrates have the lowest values around the pH optimum of the reaction. The enzyme is most stable at pH 6.5–7. The K_m values for ethanol and NAD increase with increasing temperature. The maximum rate of the ethanol oxidation satisfies the Arrhenius equation. The activation energy for the given temperature range is 40.11 kJ/mol. The rape ADH is denatured by heating above 60° but the enzyme-NAD complex is thermally more stable than the enzyme alone.     

Kinetics of p-nitrophenyl pivalate hydrolysis catalysed by cytoplasmic aldehyde dehydrogenase.

The effects of modifiers (NAD+, NADH, propionaldehyde, chloral hydrate, diethylstilboestrol and p-nitrobenzaldehyde) on the hydrolysis of p-nitrophenyl (PNP) pivalate (PNP trimethylacetate) catalysed by cytoplasmic aldehyde dehydrogenase are reported. In each case a different inhibition pattern is obtained to that observed when the substrate is PNP acetate; for example, propionaldehyde and chloral hydrate competitively inhibit the hydrolysis of PNP acetate, but are mixed inhibitors with PNP pivalate. The kinetic results can be rationalized in terms of different rate-determining steps: acylation of the enzyme in the case of the pivalate but acyl-enzyme hydrolysis for the acetate. This is confirmed by stopped-flow studies, in which a burst of p-nitrophenoxide is observed when the substrate is PNP acetate, but not when it is the pivalate. PNP pivalate inhibits the dehydrogenase activity of the enzyme competitively with the aldehyde substrate; this is most simply explained if the esterase and dehydrogenase reactions occur at a common enzymic site.     

On the nature of the activating enzyme of the inactive form of delta-aminolevulinate synthetase in Rhodopseudomonas spheroides.

The activating enzyme of the inactive form of Fraction I of delta-aminolevulinate (ALA) synthetase [EC 2.3.1.37] in Rhodopseudomonas (R.) spheroides was purified about 1,000-fold from an extract of R. spheroides cells grown anaerobically in the light. The purification of the activating enzyme was achieved by fractionating the 100,000 X g supernatant fraction of the crude extract with ammonium sulfate and acetone, followed by Sephadex G-200 chromatography, pyridoxamine phosphate-Sepharose 4B chromatography, and preparative gel electrophoresis. The final preparation of the activating enzyme still contained a minor contaminant (less than 20%) as judged by disc gel electrophoresis. The activating enzyme exhibited cystathionase [EC 4.4.1.1] activity throughout the purification. These two enzyme activities were not separated at all during any step of the purification. An apparently homogeneous preparation of cystathionase [EC 4.4.1.8] purified from rat liver also exhibited activating activity in the presence of L-cystine. It was concluded that the activating enzyme is a cystathionase.     

Kinetics of the coupled reaction catalysed by a fusion protein of beta-galactosidase and galactose dehydrogenase.

The mechanistic implications of the kinetic behaviour of a fusion protein of beta-galactosidase and galactose dehydrogenase have been analysed in view of predictions based on experimentally determined kinetic parameter values for the galactosidase and dehydrogenase activities of the protein. The results show that the time course of galactonolactone formation from lactose in the coupled reaction catalysed by the fusion protein can be most satisfactorily accounted for in terms of a free-diffusion mechanism when consideration is given to the mutarotation of the reaction intermediate galactose. It is concluded that no tenable kinetic evidence is available to support the proposal that the fusion protein catalyses galactonolactone formation from lactose by a mechanism involving channelling of galactose.     

Control of 5-aminolaevulinate synthetase activity in Rhodopseudomonas spheroides. Purification and properties of the high-activity form of the enzyme.

1. The high-activity form of aminolaevulinate synthetase has been prepared from extracts of semi-anaerobically grown cells of Rhodopseudomonas spheroides, which were allowed to become activated in air. Specific activity was 130 000--170 000 nmol of aminolaevulinate/h per mg of protein at 37 degree C. 2. Enzyme fraction Ia prepared on DEAE-Sephadex was a mixture of four active enzymes, pI5.55, 5.45, 5.35 and 5.2, when prepared in either Tris or phosphate buffers and when extracts were activated by air or by cystine trisulphide. 3. The enzyme was further purified by preparative polyacrylamide-gel electrophoresis in imidazole/veronal buffer, pH 7.6, followed by gel filtration on Sephadex G-100 and concentration with DEAE-Sephadex. 4. The most active enzyme, pI 5.55, ran as a single protein band, mol.wt. 49 000, in sodium dodecyl sulphate and 2-mercaptoethanol. The apparent molecular weight under non-denaturing conditions was 62 000--68 000 on Sephadex G-100 or G-200, pH 7.5, and on polyacrylamide-gel electrophoresis, pH 8.5, at enzyme concentrations below 10 000 units/ml, i.e. less than 60 microgram of protein/ml, and the enzyme was mainly monomeric. 5. The enzyme was homogeneous by gel disc electrophoresis at pH 8.9 and 7.6, but a slightly more diffuse band of protein was obtained during electrophoresis in glycine buffer, pH 7.4. 6. Enzyme samples possessed an intrinsic yellow fluorescence when viewed under u.v. light and this fluorescence coincided exactly with enzymic activity on gel electrophoresis. Fluorescence maxima were 420 nm (excitation) and 495 nm (emission). 7. Radioactive 35S-labelled enzyme had 14 atoms of sulphur/mol of protein (or/40 leucine residues) of which 5--6 residues were cyst(e)ine and 8--9 residues were methionine. 8. Mo carbohydrate was detected apart from glucose, which prevented accurate determination of tryptophan with methanesulphonic acid and tryptamine.     

Photochemical Electron Transport in Photosynthetic Reaction Centers from Rhodopseudomonas spheroides: I. Kinetics of the Oxidation and Reduction of P-870 As Affected by External Factors

Photosynthetic reaction centers from Rhodopseudomonas spheroides were prepared with the detergent lauryl dimethylamine oxide (LDAO). In contrast to reaction centers made with Triton X-100, these contained no cytochromes and little or no ubiquinone (UQ). The reduction of P-870, after its photochemical oxidation, was studied in these materials with the following results. In reaction centers made with Triton X-100, slow kinetic components (seconds to minutes) could be attributed to secondary electron acceptors or traps. In reaction centers made with LDAO the kinetics were predominantly fast (half-times, 100 msec or less); slower components could be introduced by adding UQ. Added UQ appeared to become bound to reaction centers made with LDAO, but the binding might have meant only that both components were trapped within detergent micelles. Ferricyanide could retard the reduction of oxidized P-870, apparently by capturing electrons from the reducing side of the photochemical system. Under conditions in which the participation of secondary electron acceptors seemed to have been eliminated, the recovery of P-870 was mainly by a first-order process with a half-time of about 60 msec at room temperature and 20-30 msec at about -80°C and below. The transition with decreasing temperature suggested the presence of a mixed population, exhibiting both the 60 and 20 msec components, but variations in the absorption spectra with temperature did not suggest the presence of a mixed population. Absorption difference spectra in the ultraviolet were compatible with the idea that UQ added to reaction centers became reduced in the light.     

Localization of ferrochelatase and of newly synthesized haem in membrane fractions from Rhodopseudomonas spheroides.

Cells of Rhodopseudomonas spheroides, strains R-26 or GVP, were grown photosynthetically, disrupted and two particulate fractions separated by sucrose-density-gradient centrifugation. The upper particulate fraction, enriched in bacteriochlorophyll, was identified as containing the chromatophores; the lower particulate fraction had the characteristics of the cell envelope. The two fractions differed in cytochrome content and cytochrome spectra. Ferrochelatase was found almost exclusively in the chromatophore fraction and was located on the outer face of the chromatophores, i.e. in contact with the cytosol in intact cells. The addition of 59FeCl3 to cells growing in low-iron media resulted in labelling of the protohaem fraction (probably arising from cytochrome b) of the membranes. The specific radioactivity of the haem of the chromatophores rose more rapidly than that of the envelope fraction and then after 2 h declined to approximately the same value, suggesting that haems of the chromatophore may act as precursors of haem of the envelope.