Localization and mobility of coenzyme Q in lipid bilayers and membranes.

We have studied the mobility of coenzyme Q (CoQ) in lipid bilayers and mitochondrial membranes in relation to the control of electron transfer activities. A molecular dynamics computer simulation in the vacuum yielded a folded structure for CoQ10, with a length of only 21 A. Using this information we were able to calculate diffusion coefficients in the range of 10(-6) cm2/s in good agreement with those found experimentally by fluorescence quenching of pyrene derivatives. To investigate if CoQ diffusion may represent the rate-limiting step of electron transfer, we reconstituted complexes I and III and assayed the resulting NADH-cytochrome c reductase activity in presence of different CoQ10 levels and at different distances between complexes; the experimental turnovers were higher than the collision frequencies calculated using diffusion coefficients of 10(-9) cm2/s but compatible with values found by us by fluorescence quenching. Since the experimental turnovers are independent of the distance between complexes, we conclude that CoQ diffusion is not rate-limiting for electron transfer.     

An electron spin resonance study on pyrocatechase; the effects of substrate analogues and catechol

Analogues of catechol (protocatechuic acid and ethyl protocatechuate) as well as catechol itself, combine with the enzyme-bound iron of pyrocatechase and formed complexes with the enzyme. The complexes and the native enzyme showed ESR signals characteristic of a ferric iron complex at g=4.3, but the patterns of the signals were different from that of the native enzyme. After forming the complex, ethyl protocatechuate readily released the bound iron of the enzyme protein to give the apoenzyme. The addition of catechol to the enzyme in the absence of oxygen resulted in a decrease of the ESR signal at g=4.3, but which did not disappear. These results indicate that the bound iron of pyrocatechase in the ES complex is similar to ferric iron in a charge transfer complex.     

Rate-limiting steps in oxidations catalyzed by rabbit cytochrome P450 1A2

Several issues regarding the rate-limiting nature of individual reaction steps in catalysis by rabbit liver cytochrome P450 (P450) 1A2 were addressed using anisoles and other substrates. Substrate binding is very fast (k > 10(6) M(-1) s(-1)). Product release is not rate-limiting, as shown by the absence of bursts, placing rate-limiting steps at or before product formation. We had previously shown that the first 1-electron reduction step is fast (k > 700 min(-1)), even in the absence of ligand [Guengerich, F. P., and Johnson, W. W. (1997) Biochemistry 36, 14741-147500]. O(2) binding to ferrous P450 is fast (k >/= 10(6) M(-1) s(-1)). The decay of the P450 Fe(2+)-substrate-O(2) complex was slow in the absence of NADPH-P450 reductase, with a first-order rate constant of 14 min(-1) at 25 degrees C. During the decay, product was formed (from the substrate methacetin) in 61% theoretical yield, although this reaction requires electron transfer among P450 molecules and may not be related to normal turnover. Steady-state spectra suggest that one or more iron-oxygen complexes accumulate, representing entities between the Fe(2+)-O(2) complex and putative FeO(3+) entity. Kinetic isotope effect experiments were done with several substrates, mainly anisoles. Apparent intrinsic deuterium isotope effects as high as 15 were measured. In all cases, the C-H bond-breaking step is at least partially rate-limiting. The isotope effects were not strongly attenuated in noncompetitive or competitive experiments, consistent with relatively rapid P450-substrate exchange, except with the active enzyme Fe-O complex. Kinetic simulations with the available data (i) are consistent with the view that C-H bond breaking is a major rate-limiting step, (ii) demonstrate that increasing the rate of this step will affect k(cat), K(m), and kinetic hydrogen isotope effects but will only increase catalytic efficiency to a certain degree, (iii) indicate that increasing ground-state binding can increase catalytic efficiency but not k(cat), and (iv) suggest that nonproductive binding modes and abortive reduction of O(2) are factors that attenuate catalytic efficiency.     

Enzymatic release of halogens or methanol from some substituted protocatechuic acids.

Four strains of gram-negative bacteria capable of growing at the expense of 5-chlorovanillate were isolated from soil, and the metabolism of one strain was studied in particular detail. In the presence of alpha, alpha'-bipyridyl, a suspension of 5-chlorovanillate-grown cells accumulated 5-chloroprotocatechuate from 5-chlorovanillate; in the absence of inhibitor these compounds, and various other 5-substituted protocatechuates and vanillates, were oxidized to completion. Cell suspensions of this strain grown on 5-chlorovanillate or vanillate released chloride quantitatively from 5-chlorovanillate and released methanol from syringate. Extracts of cells grown with 4-hydroxybenzoate, vanillate, or syringate possessed high levels of both protocatechuate 4,5-dioxygenase and 2-pyrone-4,6-dicarboxylate hydrolase; extracts from acetate-grown cells did not. Protocatechuate 4,5-dioxygenase, purified from strains that could grow with 5-chlorovanillate, oxidized 5-halogeno-protocatechuates and 3-O-methylgallate with the formation of 2-pyrone-4,6-dicarboxylate. A crude extract converted 5-chloroprotocatechuate into pyruvate plus oxaloacetate. On the basis of these observations, a meta-fission reaction sequence is proposed for the bacterial degradation of vanillate and protocatechuate substituted at C-5 of the benzene ring with halogen or methoxyl.     

Characterization of steady-state activities of cytochrome c oxidase at alkaline pH: mimicking the effect of K-channel mutations in the bovine enzyme

The cytochrome c oxidase activity of the bovine heart enzyme decreases substantially at alkaline pH, from 650 s(-1) at pH 7.0 to less than 10 s(-1) at pH 9.75. In contrast, the cytochrome c peroxidase activity of the enzyme shows little or no pH dependence (30-50 s(-1)) at pH values greater than 8.5. Under the conditions employed, it is demonstrated that the dramatic decrease in oxidase activity at pH 9.75 is fully reversible and not due to a major alkaline-induced conformational change in the enzyme. Furthermore, the Km values for cytochrome c interaction with the enzyme were also not significantly different at pH 7.8 and pH 9.75, suggesting that the pH dependence of the activity is not due to an altered interaction with cytochrome c at alkaline pH. However, at alkaline pH, the steady-state reduction level of the hemes increased, consistent with a slower rate of electron transfer from heme a to heme a3 at alkaline pH. Since it is well established that the rate of electron transfer from heme a to heme a3 is proton-coupled, it is reasonable to postulate that at alkaline pH, proton uptake becomes rate-limiting. The fact that this is not observed when hydrogen peroxide is used as a substrate in place of O2 suggests that the rate-limiting step is proton uptake via the K-channel associated with the reduction of the heme a3/CuB center prior to the reaction with O2. This step is not required for the reaction with H2O2, as shown previously in the examination of mutants of bacterial oxidases in which the K-channel was blocked. It is concluded that at pH values near 10, the delivery of protons via the K-channel becomes the rate-limiting step in the catalytic cycle with O2, so that the behavior of the bovine enzyme resembles that of the K-channel mutants in the bacterial enzymes.     

Reaction of N-acetylglucosamine oligosaccharides with lysozyme. Temperature, pH, and solvent deuterium isotope effects; equilbrium, steady state, and pre-steady state measurements*.

Temperature jump and stopped flow methods were used to study at pH 7.0 the temperature dependence of elementary steps of the reactions of lysozyme with the beta(1 yields 4)-linked trimer, tetramer, and hexamer of N-acetylglucosamine. The steady state rate of cleavage of the hexasaccharide was determined as a function of temperature (5 degrees-40 degrees) and pH(2 to 8) in H-2O solution and as a function of pD(2.5 to 9.5) at 40 degrees in D-2O solution. The apparent enthalpies of the two ionizations of apparent pK 3.8 and 6.7 observed in measurements of k are 0 to 2 kcal/mol. The energy of activation determined for the pH optimum is 21.5 kcal/mol. The solvent deuterium isotope effect measured for k at the pH (pD) optimum is 1.5 And reflects isotope effects on pre-equilibrium steps and on the rate-determining step. Transfer from H-2O to D-2O solution produces 0.2 to 0.4 kcal/mol more negative free energies of saccharide binding and no changes in the enthalpies of binding. Pre-steady state, steady state, and equilibrium measurements indicate a pathway for the reaction of lysozyme with hexasaccharide. The results define for this mechanism the complete free energy profile and an essentially complete enthalpy profile. Three of the five observable ES complexes are present at nearly equal concentrations. The free energies of the transition states are within a range of 3 kcal. The enthalpies of productive enzyme-substrate complexes are about 5 kcal/mol greater than the enthalpies of nonproductive complexes. Changes in tryptophan fluorescence were observed for each elementary step, and changes in pK of Glu-35 for the isomerizations of nonproductive and productive complexes. The signal changes during formation of nonproductive complexes are the same for the oligosaccharides (ClcNAc)3 to (GlcNAc)6. The changes for productive complexes are similar but not identical with saccharides (GlcNAc)4 to (GlcNAc)6. Correlations of the present data with previous crystallographic and solution measurements indicate the structures of productive and nonproductive ES complexes and suggest that full interaction of the substrate with the enzyme active site is established in the rate-determining step.     

Transient kinetic studies of substrate inhibition in the horse liver alcohol dehydrogenase reaction

The rate-limiting step of ethanol oxidation by alcohol dehydrogenase (E) at substrate inhibitory conditions (greater than 500 mM ethanol) is shown to be the dissociation rate of NADH from the abortive E-ethanol-NADH complex. The dissociation rate constant of NADH decreased hyperbolically from 5.2 to 1.4 s-1 in the presence of ethanol causing a decrease in the Kd of NADH binding from 0.3 microM for the binary complex to 0.1 microM for the abortive complex. Correspondingly, ethanol binding to E-NADH (Kd = 37 mM) was tighter than to enzyme (Kd = 109 mM). The binding rate of NAD+ (7 X 10(5) M-1s-1) to enzyme was not affected by the presence of ethanol, further substantiating that substrate inhibition is totally due to a decrease in the dissociation rate constant of NADH from the abortive complex. Substrate inhibition was also observed with the coenzyme analog, APAD+, but a single transient was not found to be rate limiting. Nevertheless, the presence of substrate inhibition with APAD+ is ascribed to a decrease in the dissociation rate of APADH from 120 to 22 s-1 for the abortive complex. Studies to discern the additional limiting transient(s) in turnover with APAD+ and NAD+ were unsuccessful but showed that any isomerization of the enzyme-reduced coenzyme-aldehyde complex is not rate limiting. Chloride increases the rate of ethanol oxidation by hyperbolically increasing the dissociation rate constant of NADH from enzyme and the abortive complex to 12 and 2.8 s-1, respectively. The chloride effect is attributed to the binding of chloride to these complexes, destabilizing the binding of NADH while not affecting the binding of ethanol.     

Novel irreversible butyrylcholinesterase inhibitors: 2-chloro-1-(substituted-phenyl)ethylphosphonic acids

2-Chloroethylphosphonic acid (ethephon) as the dianion phosphorylates butyrylcholinesterase (BChE) at its active site. In contrast, the classical organophosphorus esterase inhibitors include substituted-phenyl dialkylphosphates (e.g., paraoxon) with electron-withdrawing aryl substituents. The chloroethyl and substituted-phenyl moieties are combined in this study as 2-chloro-1-(substituted-phenyl)ethylphosphonic acids (1) to define the structure--activity relationships and mechanism of BChE inhibition by ethephon and its analogues. Phenyl substituents considered are 3- and 4-nitro, 3- and 4-dimethylamino, and 3- and 4-trimethylammonium. Phosphonic acids were synthesized via the corresponding O,O-diethyl phosphonate precursors followed by deprotection with trimethylsilyl bromide. They decompose under basic conditions about 100-fold faster than ethephon to yield the corresponding styrene derivatives. Electron-withdrawing substituents on the phenyl ring decrease the hydrolysis rate while electron-donating substituents increase the rate. The 4-trimethylammonium analogue has the highest affinity (K(i)=180 microM) and potency (IC(50)=19 microM) in first binding reversibly at the substrate site (possibly with stabilization in a dianion--monoanion environment) and then progressively and irreversibly inhibiting the enzyme activity. These observations suggest dissociation of chloride as the first and rate-limiting step both in the hydrolysis and by analogy in phosphorylation of BChE by bound at the active site.     

17O-water and cyanide ligation by the active site iron of protocatechuate 3,4-dioxygenase. Evidence for displaceable ligands in the native enzyme and in complexes with inhibitors or transition state analogs

Hyperfine broadening is observable in the EPR spectrum of Brevibacterium fuscum protocatechuate 3,4-dioxygenase after lyophilization and rehydration in 17O-enriched water, demonstrating H2O ligation to the active site iron. Lack of detectable broadening in the sharp features of the spectra of three substrate complexes suggests that H2O is displaced by substrate. Water is bound in the monodentate complex with the competitive inhibitor 3-hydroxybenzoate which binds directly to the iron showing that two iron ligation sites can be occupied by nonprotein ligands. Ketonized substrate analogs which mimic a proposed transition state of the reaction cycle, 2-hydroxyisonicotinic acid N-oxide (2-OHINO) and 6-hydroxynicotinic acid N-oxide (6-OH NNO), have H2O bound in their final, bleached enzyme complexes, suggesting that these complexes are also monodentate. In contrast, a transient, initial complex of 6-OH NNO which is spectrally similar to the substrate complex, apparently does not have H2O bound. Cyanide binding occurs in two steps. The active site Fe3+ of the initial, rapidly formed, violet complex is high spin while that of the second, slowly formed, green complex is low spin; a unique state for mononuclear non-heme iron enzymes. The data suggest that the Fe-CN- and Fe-(CN-)2 complexes form sequentially. CN- binds to enzyme complexes with 2-OH INO and 6-OH NNO in one step to yield high spin Fe3+ species. In contrast, preformed substrate complexes prevent CN- binding. CN- binding eliminates the broadening due to 17O-water in the EPR spectra of both native enzyme and the enzyme-ketonized analog complexes. A model is proposed in which H2O is displaced by bidentate binding of the substrate but can potentially rebind after a subsequent substrate ketonization. The proximity of the vacatable H2O-binding site of the iron to the site of oxygen insertion suggests, however, that this site may serve to stabilize an oxygenated intermediate during the reaction cycle.     

EPR and M?ssbauer studies of protocatechuate 4,5-dioxygenase. Characterization of a new Fe2+ environment

Protocatechuate 4,5-dioxygenase from Pseudomonas testosteroni has been purified to homogeneity and crystallized. The iron containing, extradiol dioxygenase is shown to be composed of two subunit types (alpha, Mr = 17,700 and beta, Mr = 33,800) in a 1:1 ratio; such a composition has not been observed for other extradiol dioxygenases. The 4.2 K M?ssbauer spectrum of native protocatechuate 4,5-dioxygenase prepared from cells grown in 57Fe-enriched media consists of a doublet with quadrupole splitting, delta EQ = 2.22 mm/s, and isomer shift delta Fe = 1.28 mm/s, demonstrating a high spin Fe2+ site. These parameters, and the temperature dependence of delta EQ, are unique among enzymes but are strikingly similar to those reported for the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26, suggesting very similar ligand environments. The Fe2+ of protocatechuate 4,5-dioxygenase can be oxidized, for instance by H2O2, to yield high spin Fe3+ with EPR g values around g = 6 (and g = 4.3). In the oxidized state, protocatechuate 4,5-dioxygenase is inactive; the iron, however, can be rereduced by ascorbate to yield active enzyme. Our data suggest that protocatechuate binds to Fe2+; the spectra indicate that the ligand binding is heterogenous. The M?ssbauer spectra observed here are fundamentally different from those reported earlier (Zabinski, R., Münck, E., Champion, P., and Wood, J. M. (1972) Biochemistry 11, 3212-3219). The spectra of the earlier (reconstituted) preparations, which had substantially lower specific activities, probably reflect adventitiously bound Fe3+. We discuss here how adventitiously bound iron can be identified and removed. The Fe2+ which is present in native protocatechuate 4,5-dioxygenase and its complexes with substrates and inhibitors reacts quantitatively with nitric oxide to produce a species with electronic spin S = 3/2. The EPR and M?ssbauer spectra of these complexes compare favorably with EDTA . Fe(II) . NO. We have studied the latter complex extensively and have analyzed the M?ssbauer spectra with an S = 3/2 spin Hamiltonian. EPR spectra show that protocatechuate 4,5-dioxygenase-NO complexes with substrates or inhibitors are heterogeneous and consist of several well defined subspecies. The data show that NO, and presumably also O2, has access to the active site Fe2+ in the enzyme-substrate complex. The use of EPR-detectable NO complexes as a rapid and sensitive tool for the study of the EPR silent active site iron of extradiol dioxygenases is discussed.     

Elucidation of the chemical nature of the steady-state intermediates in the mechanism of carboxypeptidase A

Cryospectrokinetic studies of zinc and cobalt carboxypeptidase A disclosed two intermediates in the hydrolysis of both peptides and depsipeptides and furnished all the rate and equilibrium constants for the reaction scheme E + S in equilibrium ES1 in equilibrium ES2---E + P [Auld, D. S., Galdes, A., Geoghegan, K. F., Holmquist, B., Martinelli, R. A., & Vallee, B. L. (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 5041-5045]. Since the ES2 intermediate is the predominate enzyme species present at steady state, its chemical nature is deducible from subzero chemical quench studies done after steady state is established. Extrapolation of the product concentration to zero time, [P0], measures the concentration of the enzyme species in which bond cleavage has occurred. For peptides, the [P0]values are zero, indicating that no product is generated prior to turnover and therefore the ES2 intermediate involves a complex between enzyme and intact peptide substrate. For depsipeptides, [P0] values are 1 mol of produce per mole of enzyme over the entire temperature range -20 to -50 degrees C, indicating cleavage of the ester bond occurs prior to the rate-limiting step so that ES2 is more properly denoted by EP1P2, where P1 and P2 are the substrates for the reverse reaction. The rate-limiting step for depsipeptides thus involves release of the products which may occur directly or through a mandatory conformational change followed by rapid product release.     

Transition state analogs for protocatechuate 3,4-dioxygenase. Spectroscopic and kinetic studies of the binding reactions of ketonized substrate analogs

The binding reactions of two heterocyclic analogs of protocatechuate (PCA), 2-hydroxyisonicotinic acid N-oxide and 6-hydroxynicotinic acid N-oxide, to Brevibacterium fuscum protocatechuate 3,4-dioxygenase have been characterized. These analogs were synthesized as models for the ketonized tautomer of PCA which we have previously proposed as the form which reacts with O2 in the enzyme complex (Que, L., Jr., Lipscomb, J.D., Munck, E., and Wood, J.M. (1977) Biochim. Biophys. Acta 485, 60-74). Both analogs have much higher affinity for the enzyme than PCA. Repetitive scan optical spectra of each binding reaction show that at least one intermediate is formed. The spectra of the intermediates are red-shifted (lambda max = 500 nm) relative to that of native enzyme (lambda max = 435 nm) but are similar to that of the anaerobic enzyme-PCA complex. In contrast, the spectrum of the final, deadend complex formed by each analog is significantly blue-shifted (lambda max less than 340 nm) resulting in an apparent bleaching of the chromophore of the enzyme. A transient intermediate exhibiting a similar bleached spectrum has been detected in the enzyme reaction cycle immediately after O2 is added to the enzyme-PCA complex (Bull C., Ballou D.P., and Otsuka, S. (1981) J. Biol. Chem. 256, 12681-12686). Stopped flow measurements of the analog binding reactions show that a relatively weak enzyme complex is initially formed followed by at least two isomerizations leading to the bleached, high affinity complexes. EPR spectra of both the early and final complexes reveal only high spin Fe3+ with negative zero field splitting, showing that the optical bleaching is not due to Fe reduction. The studies show that the ketonized analogs are poor models for the enzyme-substrate complex but do successfully mimic many features of the first oxy complex of the reaction cycle. We propose that substrate ketonization occurs coincident with or after O2 binding and may be involved directly in the O2 insertion reaction.     

Binding of 17O-labeled substrate and inhibitors to protocatechuate 4,5-dioxygenase-nitrosyl complex. Evidence for direct substrate binding to the active site Fe2+ of extradiol dioxygenases

Pseudomonas testosteroni protocatechuate 4,5-dioxygenase catalyzes extradiol-type oxygenolytic cleavage of the aromatic ring of its substrate. The essential active site Fe2+ binds nitric oxide (NO) to produce an EPR active complex with an electronic spin of S = 3/2. Hyperfine broadening of the EPR resonances of the nitrosyl complex of the enzyme by protocatechuate (3,4-(OH)2-benzoate, PCA) enriched specifically with 17O (I = 5/2) in either the 3 or the 4 hydroxyl group shows that both groups can bind directly to the Fe2+ in the ternary complex. Analogous results are obtained for PCA binding to catechol 2,3-dioxygenase-NO complex suggesting that substrate binding by the Fe2+ may be a general property of extradiol dioxygenases. The protocatechuate 4,5-dioxygenase inhibitor, 4-17OH-benzoate binds directly to the Fe of the nitrosyl adduct of the enzyme through the OH group. Since previous studies have shown that water also is bound to the Fe in this ternary complex, but not in the ternary complex with PCA, the data strongly imply that there are 3 sites in the Fe coordination which can be occupied by exogenous ligands. 3-17OH-benzoate is an inhibitor of the enzyme but does not elicit detectable hyperfine broadening in the EPR spectrum of the nitrosyl adduct suggesting that it binds to the enzyme, but not to the Fe. The EPR spectra of ternary enzyme-NO complexes with PCA or 4-OH-benzoate labeled with 17O exclusively in the carboxylate substituent are not broadened, suggesting that this moiety does not bind to the Fe.     

Investigation of the pre-steady-state kinetics of fructose bisphosphatase by employment of an indicator method.

The pre-steady-state kinetics for the hydrolysis of fructose 1,6-bisphosphate by rabbit liver fructose bis-phosphatase have been investigated by stopped-flow kinetics utilizing an acid-base indicator method that permits the continuous monitoring of the inorganic phosphate product. The reaction sequence is characterized by two successive first-order steps followed by establishment of the steady-state rate. The first exponential process results from a conformational change in the protein that is dye sensitive owing to a perturbation of an acidic residue on the protein. A second process reflects the rapid initial turnover of all four subunits of the enzyme with the concomitant release of inorganic phosphate followed by the rate-limiting step of the catalytic cycle. This latter step may involve a product release (fructose 6-phosphate) or a second conformational change. The catalytic cycle ends with decay of the enzyme to its initial unreactive resting state.     

17O]Water and nitric oxide binding by protocatechuate 4,5-dioxygenase and catechol 2,3-dioxygenase. Evidence for binding of exogenous ligands to the active site Fe2+ of extradiol dioxygenases

Pseudomonas testosteroni protocatechuate 4,5-dioxygenase and Pseudomonas putida catechol 2,3-dioxygenase (metapyrocatechase) catalyze extradiol-type oxygenolytic cleavage of the aromatic ring of their substrates. The essential active site Fe2+ of each enzyme binds nitric oxide (NO) to produce an EPR active complex with an electronic spin of S = 3/2. Hyperfine broadening of the EPR resonances of the nitrosyl complexes by 17O-enriched H2O shows that water is bound directly to the Fe2+ in the native enzymes, but is apparently displaced in substrate complexes. NO is not displaced by either substrates or inhibitors. The EPR spectra of several enzyme-inhibitor-NO complexes are different from those of enzyme-NO or enzyme-substrate-NO complexes and are found to be broadened by 17O-enriched water. The data show that at least 2 and perhaps 3 sites in the Fe ligation can be occupied by exogenous ligands. Furthermore, it is likely that substrates and inhibitors displace water by binding either at or near to the Fe in the nitrosyl complex. Nitric oxide binding is found to be substrate-dependent for each enzyme. Native catechol 2,3-dioxygenase exhibits KD values of 190 microM and 2.0 mM for NO binding in two types of independent sites. Only one type of site is observed in the catechol complex which exhibits a KD for NO of 3.4 microM. One type of NO binding site is observed for both the native and substrate complexed protocatechuate 4,5-dioxygenase with KD values of 360 and 3 microM, respectively. The presence of a specific site in the Fe coordination for NO which is modified in the substrate complex, suggests that O2 binding by the extradiol dioxygenases may also occur at the Fe.     

Detection of peroxisomal fatty acyl-coenzyme A oxidase activity.

It has been postulated that the peroxisomal fatty acid-oxidizing system [Lazarow & de Duve (1976) Proc. Natl. Acad. Sci. U.S.A. 73, 2043--2046; Lazarow (1978) J. Biol. Chem. 253, 1522--1528] resembles that of mitochondria, except for the first oxidative reaction. In this step, O2 would be directly reduced to H2O2 by an oxidase. Two specific procedures developed to detect the activity of the characteristic enzyme fatty acyl-CoA oxidase are presented, namely polarographic detection of palmitoyl-CoA-dependent cyanide-insensitive O2 consumption and palmitoyl-CoA-dependent H2O2 generation coupled to the peroxidation of methanol in an antimycin A-insensitive reaction. Fatty acyl-CoA oxidase activity is stimulated by FAD, which supports the flavoprotein nature postulated for this enzyme. Its activity increases 7-fold per g wet wt. of liver in rats treated with nafenopin, a hypolipidaemic drug. Subcellular fractionation of livers from normal and nafenopin-treated animals provides evidence for its peroxisomal localization. The stoicheiometry for palmitoyl-CoA-dependent O2 consumption, H2O2 generation and NAD+ reduction is 1 : 1 : 1. This suggests that fatty acyl-CoA oxidase is the rate-limiting enzyme of the peroxisomal fatty acid-oxidizing system.     

On the mechanism of NADP+-linked isocitrate dehydrogenase from heart mitochondria. II. The transient-state kinetics of the oxidative decarboxylation of isocitrate

The kinetics of a single turnover of enzyme-catalysed oxidative decarboxylation have been studied by mixing a stoichiometric complex of enzyme, isocitrate and Mg2+ with large concentrations of NADP+ in a stopped-flow apparatus, and monitoring the formation of NADPH by fluorescence measurements. A transient is revealed that exhibits enhanced nucleotide fluorescence and is not detectable by light absorption measurements. The results obtained with the largest NADP+ concentrations, such that the product NADPH is largely displaced from its enzyme complex, show that a step that precedes the release of free NADPH is rate-limiting in the oxidative decarboxylation reaction under conditions of catalytic cycling. The rate constants for this step, tentatively identified as the formation of the complex of enzyme, Mg2+ and NADPH from a precursor NADPH-containing complex, and for the dissociation of NADPH from this complex have been estimated from the integrated rate equation for a simple model for the product phase of the reaction, by methods of nonlinear regression analysis. In line with the conclusions from the preceding paper, it is suggested that formation of an abortive complex of enzyme, Mg2+, isocitrate and NADPH under catalytic cycling conditions serves to by-pass the potentially rate-limiting dissociation of NADPH from the enzyme-Mg2+-NADPH complex.     

A study of the kinetics and mechanism of yeast alcohol dehydrogenase with a variety of substrates

1. The kinetics of oxidation of ethanol, propan-1-ol, butan-1-ol and propan-2-ol by NAD(+) and of reduction of acetaldehyde and butyraldehyde by NADH catalysed by yeast alcohol dehydrogenase were studied. 2. Results for the aldehyde-NADH reactions are consistent with a compulsory-order mechanism with the rate-limiting step being the dissociation of the product enzyme-NAD(+) complex. In contrast the results for the alcohol-NAD(+) reactions indicate that some dissociation of coenzyme from the active enzyme-NAD(+)-alcohol ternary complexes must occur and that the mechanism is not strictly compulsory-order. The rate-limiting step in ethanol oxidation is the dissociation of the product enzyme-NADH complex but with the other alcohols it is probably the catalytic interconversion of ternary complexes. 3. The rate constants describing the combination of NAD(+) and NADH with the enzyme and the dissociations of these coenzymes from binary complexes with the enzyme were measured.     

Kinetic analysis of the mechanism of Escherichia coli dihydrofolate reductase

A kinetic mechanism is presented for Escherichia coli dihydrofolate reductase which describes the full time course of the enzymatic reaction over a wide range of substrate and enzyme concentrations at pH 7.2 and 20 degrees C. Specific rate constants were estimated by computer simulation of the full time course of single turnover, burst, and steady-state experiments using both nondeuterated and deuterated NADPH. The mechanism involves the random addition of substrates, but the substrates and enzyme are not at equilibrium prior to the chemical transformation step. The rate-limiting step follows the chemical transformation, and the maximum velocity of the reaction is limited by the release of the product tetrahydrofolate. The full time course of the reaction is markedly affected by the formation of the enzyme-NADPH-tetrahydrofolate abortive complex, but not by the enzyme-NADP-dihydrofolate abortive complex.