Reactions of beta-(2-furyl)propionyl coenzyme A with "general" fatty acyl-CpA dehydrogenase

The acyl-CoA oxidases from the yeast Yarrowia lipolytica: characterization of Aox2p

One of the acyl-CoA oxidases from the yeast Yarrowia lipolytica, acyl-CoA oxidase 2 (Aox2p), has been expressed in Escherichia coli as an active, N-terminally tagged (His)(6) fusion protein. The specific activity of the purified enzyme, containing FAD, was 19.7 micromolmin(-1)mg(-1) using myristoyl-CoA as substrate. Using substrates with different chain lengths and different substituents, its kinetic properties were further analyzed. Straight-chain acyl-CoAs, with a chain length of 10-14C, are well oxidized, reflecting the properties of Aox2p as deduced from in vivo studies. Acyl-CoAs containing more than 14C were also desaturated, if their concentration was below 25 microM or if proteins capable of binding these CoA-esters, such as albumin or beta-casein, were added to the assay. These long-chain acyl-CoAs, although poor substrates, acted as competitors for the short- and medium-chain substrates. Compared to palmitoyl-CoA, activity toward hexadecadioyl-CoA, containing a omega-carboxy group, was similar. Taken together, these data suggest that micelles of long-chain acyl-CoAs are able to bind and inhibit Aox2p. The enzyme was also active toward acyl-CoA-esters containing a 2-methyl group, but only the 2S isomer was recognized.

Activation of substrate/product couples by medium-chain acyl-CoA dehydrogenase

Natural substrate/product binding activates medium-chain acyl-CoA dehydrogenase (MCAD) to accept electrons from its substrate by inducing a positive flavin midpoint potential shift. The energy source for this activation has never been fully elucidated. If ground-state alterations of the ligand, such as polarization, are entirely responsible for enzyme activation, the ligand potential should shift equally to that of the flavin but in the opposite direction. Ligand polarization is likely responsible for only a small portion of this activation. Here, thiophenepropionoyl- and furylpropionoyl-CoA analogs were used to directly measure the redox modulations of several ligand couples upon binding to MCAD. These measurements identified the thermodynamic contribution of ligand polarization to enzyme activation. Because the ligand potential alterations are significantly smaller than modulations in the flavin potential due to binding, other phenomena such as pK(a) changes, desolvation, and charge alterations are likely responsible for the thermodynamic modulations required for MCAD's activity.

Influence of Glu-376 --> Gln mutation on enthalpy and heat capacity changes for the binding of slightly altered ligands to medium chain acyl-CoA dehydrogenase

We showed that the alpha-CH(2) --> NH substitution in octanoyl-CoA alters the ground and transition state energies for the binding of the CoA ligands to medium-chain acyl-CoA dehydrogenase (MCAD), and such an effect is caused by a small electrostatic difference between the ligands. To ascertain the extent that the electrostatic contribution of the ligand structure and/or the enzyme site environment modulates the thermodynamics of the enzyme-ligand interaction, we undertook comparative microcalorimetric studies for the binding of 2-azaoctanoyl-CoA (alpha-CH(2) --> NH substituted octanoyl-CoA) and octenoyl-CoA to the wild-type and Glu-376 --> Gln mutant enzymes. The experimental data revealed that both enthalpy (DeltaH degrees ) and heat capacity changes (DeltaC(p) degrees ) for the binding of 2-azaoctanoyl-CoA (DeltaH degrees (298) = -21.7 +/- 0.8 kcal/mole, DeltaC(p) degrees = -0.627 +/- 0.04 kcal/mole/K) to the wild-type MCAD were more negative than those obtained for the binding of octenoyl-CoA (DeltaH degrees (298) = -17.2 +/- 1.6 kcal/mole, DeltaC(p) degrees = -0.526 +/- 0.03 kcal/mole/K). Of these, the decrease in the magnitude of DeltaC(p) degrees for the binding of 2-azaoctanoyl-CoA (vis-à-vis octenoyl-CoA) to the enzyme was unexpected, because the former ligand could be envisaged to be more polar than the latter. To our further surprise, the ligand-dependent discrimination in the above parameters was completely abolished on Glu-376 --> Gln mutation of the enzyme. Both DeltaH degrees and DeltaC(p) degrees values for the binding of 2-azaoctanoyl-CoA (DeltaH degrees (298) = -13.3 +/- 0.6 kcal/mole, DeltaC(p) degrees = -0.511 +/- 0.03 kcal/mole/K) to the E376Q mutant enzyme were found to be correspondingly identical to those obtained for the binding of octenoyl-CoA (DeltaH degrees (298) = -13.2 +/- 0.6 kcal/mole, DeltaC(p) degrees = -0.520 +/- 0.02 kcal/mole/K). However, in neither case could the experimentally determined DeltaC(p) degrees values be predicted on the basis of the changes in the water accessible surface areas of the enzyme and ligand species. Arguments are presented that the origin of the above thermodynamic differences lies in solvent reorganization and water-mediated electrostatic interaction between ligands and enzyme site groups, and such interactions are intrinsic to the molecular basis of the enzyme-ligand complementarity.

Purification and characterization of the recombinant form of Acyl CoA oxidase 3 from the yeast Yarrowia lipolytica

The Acyl CoA dependent oxidase 3 (Aox3p) from the yeast Yarrowia lipolytica, expressed in Escherichia coli, as an active protein with a 6 His tag at its N-terminal region has been purified to electrophoretic homogeneity. The purified enzyme exhibits a specific activity of 1.95 microM/min/mg using hexanoyl-CoA as substrate, and it remains active for at least 1 month upon storage at -30 degrees C in the presence of 35% (V/V) glycerol. The pH and temperature optima of the enzyme are 7.4 and 28-38 degrees C, respectively. Aox3p catalyzes the oxidation of both aliphatic acyl-CoA substrates of different chain lengths (e.g., hexanoyl-CoA, decanoyl-CoA, myristyl-CoA) as well as of the aromatic/heterocyclic ring-substituted chromogenic substrates, such as furylpropionyl-CoA. Of the above substrates, the efficiency of the enzyme, as judged by its kcat to Km ratio, exhibits the following order: decanoyl CoA > myristyl CoA > hexanoyl CoA > furyl-propionyl-CoA (FPCoA). Phenol, which is normally used in the coupled assay system for monitoring the H2O2 formation, functions as both an activator (at low concentrations) and a competitive inhibitor (at high concentrations) with respect to acyl-CoA substrates. The magnitude of activation and inhibition of the enzyme is dependent on the nature of the acyl-CoA substrates.

Inhibition of acyl-CoA oxidase by phenol and its implication in measurement of the enzyme activity via the peroxidase-coupled assay system

Yeast (Candida tropicalis) acyl-CoA oxidase catalyzes the oxidation of a variety of acyl-CoA substrates to their corresponding alpha-beta enoyl-CoA products, with concomitant reduction of the buffer-dissolved O2 to H2O2. By utilizing indolepropionyl-CoA as a chromogenic substrate, we could measure the enzyme activity either directly by monitoring formation of the reaction product indoleacryloyl-CoA (lambda(max) = 367 nm) or indirectly by measuring the formation of H2O2 via the oxidative-coupled assay system, involving 4-aminoantipyrine, phenol, and horseradish peroxidase. We compared the rates of the enzyme catalysis by the above two methods. The experimental data revealed that the rate measured via the direct method was about twofold higher than that measured by the coupled-assay system. The above difference was found to be due to the inhibition of the enzyme by phenol, one of the reagents of the coupled assay system. The inhibitory role of phenol is not unique for indolepropionyl-CoA as substrate, but is also evident with aliphatic acyl-CoA substrates of varied chain lengths. Since the magnitude of inhibition is dependent on the nature of the acyl-CoA substrate, it is suggested that the coupled-reaction conditions must be carefully standardized with individual substrates. Some tips on standardizing the reaction conditions for quantitative measurement of the acyl-CoA oxidase-catalyzed reaction are offered.

Molecular basis of the medium-chain fatty acyl-CoA dehydrogenase-catalyzed "oxidase" reaction: pH-dependent distribution of intermediary enzyme species during catalysis

In a previous paper, we demonstrated that the medium-chain fatty acyl-CoA dehydrogenase-catalyzed (MCAD-catalyzed) reductive half-reaction of indolepropionyl-CoA proceeds via formation of a chromophoric intermediary species "X" (absorption maximum = 400 nm) and proposed that the decay of this species might limit the overall rate of the "oxidase" reaction [Johnson, J. K., & Srivastava, D. K. (1993) Biochemistry 32, 8004-8013]. During this latter reaction, the buffer-dissolved O2 served as an electron acceptor [Johnson, J. K., Wang, Z. X., & Srivastava, D. K. (1992) Biochemistry 31, 10564-10575]. To ascertain whether the intrinsic stability of X influences the oxidase activity, we undertook a detailed kinetic investigation of this enzyme at different pH values. The time-resolved spectra for the reductive half-reaction (obtained via the rapid-scanning stopped-flow method) at different pH values reveal that the amplitude of the intermediary (X) spectral band is more pronounced at a lower pH (pH 6.4) than at a higher pH (pH 9.0). Single-wavelength transient kinetic data for the reductive half-reaction (in both the forward and the reverse direction) at all pH values are consistent with fast (1/tau 1) and slow (1/tau 2) relaxation rate constants. Of these, whereas the fast relaxation rate constant for the reaction in the forward direction (1/tau 1f) decreases with an increase in pH, the corresponding slow relaxation rate constant (1/tau 2f) increases with an increase in pH. The pH-dependent steady-state kinetic data reveal that, like 1/tau 2f, kcat for the MCAD-catalyzed oxidase reaction increases with an increase in the pH of the buffer media.(ABSTRACT TRUNCATED AT 250 WORDS)

Microscopic pathway for the medium-chain fatty acyl CoA dehydrogenase catalyzed oxidative half-reaction: changes in the electronic structures of flavin and CoA derivatives during catalysis

In a previous communication, we demonstrated that the medium-chain fatty acyl CoA dehydrogenase (MCAD) catalyzed conversion of 3-indolepropionyl CoA (IPCoA) to trans-3-indoleacryloyl CoA (IACoA) proceeds via the formation of an intermediary species X that possesses the electronic properties of reduced flavin and highly conjugated CoA product. Since the steady-state turnover of the enzyme-catalyzed dehydrogenation reaction precisely matches with the rate of formation of X [Johnson, J. K., & Srivastava, D. K. (1993) Biochemistry 32, 8004-8013], the latter species appeared to be the likely site for the transfer of electrons to external electron acceptors (e.g., ferricenium hexafluorophosphate, FcPF6). To probe the microscopic pathway for the oxidative half-reaction, we employed a sequential mixing stopped-flow technique utilizing IPCoA as the enzyme substrate and FcPF6 as the electron acceptor. The time-dependent changes in absorption at 450, 415, and 367 nm were measured upon mixing FcPF6 with previously mixed and aged solutions of MCAD-FAD+IPCoA in the stopped-flow syringes. The kinetic traces show an increase (1/tau 1) followed by a decrease (1/tau 2) in absorption at 450 and 415 nm, and a lag (corresponding to the time regime of 1 u 1) followed by an increase in absorption (1/tau 2) at 367 nm. The relaxation rate constants (1/tau's) thus measured remain unaffected, with variations in the aging time; however, the amplitudes of these phases increase up to the aging time of 5 s, after which the amplitudes attain maxima. For an aging time of 5 s, 1/tau 1 and 1/tau 2 show a linear and a hyperbolic dependence on the FcPF6 concentration, respectively. These, coupled with the complementary studies involving butyryl CoA as a nonchromophoric substrate for this enzyme, lead us to propose the following sequence of events during the MCAD-catalyzed oxidative half-reaction: (1) The enzyme-catalyzed oxidative half-reaction proceeds via the formation of a collision complex between X and FcPF6 during the fast (1/tau 1) relaxation phase. (2) The reduced flavin moiety of X is oxidized via (rapid) transfer of electrons to FcPF6 within the collision complex, without formation of a detectable (metastable) flavin semiquinone intermediate. (3) The transfer of electrons is accompanied by changes in the electronic structures of both the flavin and IACoA moieties within the enzyme-IACoA complex. The electronic structure of this newly formed complex is exactly the same as that formed upon isomerization of the MCAD-FAD-IACoA complex [Johnson, J. K., Wang, Z. X., & Srivastava, D. K. (1992) Biochemistry 31, 10564-10575].(ABSTRACT TRUNCATED AT 400 WORDS)

Alternate electron acceptors for medium-chain acyl-CoA dehydrogenase: use of ferricenium salts

Medium-chain acyl-CoA dehydrogenase reduced with octanoyl-CoA is reoxidized in two one-electron steps by two molecules of the physiological oxidant, electron transferring flavoprotein (ETF). The organometallic oxidant ferricenium hexafluorophosphate (Fc+PF6-) is an excellent alternative oxidant of the dehydrogenase and mimics a number of the features shown by ETF. Reoxidation of octanoyl-CoA-reduced enzyme (200 microM Fc+PF6- in 100 mM Hepes buffer, pH 7.6, 1 degree C) occurs in two one-electron steps with pseudo-first-order rate constants of 40 s-1 and about 200 s-1 for k1 and k2, respectively. The reaction is comparatively insensitive to ionic strength, and evidence of rate saturation is encountered at high ferricenium ion concentration. As observed with ETF, the free two-electron-reduced dehydrogenase is a much poorer kinetic reductant of Fc+PF6-, with rate constants of 3 s-1 and 0.3 s-1 (for k1 and k2, respectively) using 200 microM Fc+PF6-. In addition to the enoyl-CoA product formed during the dehydrogenation of octanoyl-CoA, binding a number of redox-inert acyl-CoA analogues (notably 3-thia- and 3-oxaoctanoyl-CoA) significantly accelerates electron transfer from the dehydrogenase to Fc+PF6-. Those ligands most effective at accelerating electron transfer favor deprotonation of reduced flavin species in the acyl-CoA dehydrogenase. Thus this rate enhancement may reflect the anticipated kinetic superiority of anionic flavin forms as reductants in outer-sphere electron-transfer processes. Evidence consistent with the presence of two distinct loci for redox communication with the bound flavin in the acyl-CoA dehydrogenase is presented.

4-Thia-trans-2-alkenoyl-CoA derivatives: properties and enzymatic reactions

4-Thiaacyl-CoA analogues, in which the 4-methylene group is replaced by a thioether sulfur atom, represent new chromophoric substrates of acyl-CoA dehydrogenases and oxidase. The corresponding 4-thia-trans-2-enoyl-CoA products exhibit a strong new absorption band (extinction coefficient 22 mM-1 cm-1) that is red shifted from 312 to 338 nm upon binding to the medium-chain acyl-CoA dehydrogenase. 4-Thiaoctanoyl-CoA reduces the dehydrogenase several-fold slower than octanoyl-CoA, although in turnover it is dehydrogenated 1.5-fold faster. The redox potential of 4-thia analogues is some 30 mV more negative than that of their unsubstituted counterparts. 4-Thia-trans-2-enoyl-CoA derivatives are slowly hydrated by enoyl-CoA hydratase (EC 4.2.1.17) to the corresponding thiohemiacetal which fragments nonenzymatically to 1 equiv each of malonylsemialdehyde-CoA and alkanethiol. This fragmentation reaction might explain the release of methanethiol during the transamination pathway of methionine degradation. 4-Oxaoctanoyl-CoA is a much poorer substrate and kinetic reductant of acyl-CoA dehydrogenase and oxidase than the 4-thia analogue. The corresponding enoyl-CoA product is also fragmented by the hydratase, yielding butanol and malonylsemialdehyde-CoA. Thus, 4-heterosubstituted acyl-CoA derivatives provide new tools for the study of beta-oxidation enzymes.

Oxidation-reduction of general acyl-CoA dehydrogenase by the butyryl-CoA/crotonyl-CoA couple. A new investigation of the rapid reaction kinetics

Pig kidney general acyl-CoA dehydrogenase (GAD) can be reduced by butyryl-CoA to form reduced enzyme and crotonyl-CoA. This reaction is reversible. Stopped-flow, kinetic investigations on GAD have been made, using the following reaction pairs: oxidized GAD/butyryl-CoA, oxidized GAD/crotonyl-CoA, oxidized GAD/alpha,beta-dideuteriobutyryl-CoA, reduced GAD/butyryl-CoA, and reduced GAD/crotonyl-CoA (in 50 mM potassium phosphate buffer, pH 7.6 at 4 degrees C). Reduction of GAD by butyryl-CoA is triphasic. The slowest phase is 100-fold slower than the preceding phase and appears to represent a secondary process not directly related to the primary reduction events. The first two fast phases are responsible for reduction of GAD. Reduction proceeds via a reduced enzyme/crotonyl-CoA charge-transfer complex. alpha, beta-Dideuteriobutyryl-CoA elicits a major deuterium isotope effect (15-fold) on the reduction reaction. Oxidation of GAD by crotonyl-CoA is biphasic. Oxidation proceeds via the same reduced enzyme/crotonyl-CoA charge-transfer complex seen during reduction. The oxidation reaction ends in a mixture composed largely of oxidized GAD species. From the data, we constructed a mechanism for the reduction/oxidation of GAD by butyryl-CoA/crotonyl-CoA. This mechanism was then used to simulate all of the observed kinetic time course data, using a single set of kinetic parameters. A close correspondence between the observed and simulated data was obtained.

The nature of enzyme-substrate complexes in acyl-coenzyme A dehydrogenases

The nature of the purple complex formed upon the addition of octanoyl-CoA to the medium chain acyl-CoA dehydrogenase from pig kidney has been addressed by chemical quenching studies. Previous work, using quenching in 0.1 M KOH, suggested that the dehydrogenation product, trans-2-octenoyl-CoA, was not a participant in reduced rat liver enzyme complexes because no octenoic acid was detected after denaturation (Y. Ikeda, D. G. Hine, K. Okamura-Ikeda and K. Tanaka (1985) J. Biol. Chem. 260, 1326-1337). However, when the octanoyl-CoA-reduced pig kidney enzyme is quenched rapidly in 2 M HCl, the ratio of trans-2-octenoyl-CoA/octanoyl-CoA released is 9/1. A milder acid denaturation procedure yields the corresponding ratio of 0.4/1, i.e., now with an excess of the saturated substrate. Similarly, quenching the pig kidney dehydrogenase in 0.1 M KOH reveals only minor levels of octenoyl chains released into the supernatant. When quenching is insufficiently rapid compared to the internal equilibration of oxidized enzyme.octanoyl-CoA and reduced enzyme.octenoyl-CoA forms, the outcome is decided by the greater kinetic lability of the oxidized enzyme species. These data are fully consistent with the original ascription that the purple species observed upon reduction of the acyl-CoA dehydrogenases with substrate represents a charge transfer complex between reduced flavin as the donor and trans-2-octenoyl-CoA as the acceptor.

Studies on the reaction mechanism of general acyl-CoA dehydrogenase. Determination of selective isotope effects in the dehydrogenation of butyryl-CoA

The kinetic properties of general acyl-CoA dehydrogenase from pig kidney have been investigated using normal butyryl-CoA as well as an alpha-deutero, beta-deutero- and perdeutero-butyryl-CoA. In turnover catalysis, isotope effects of 2, 3.6, and 9 were found respectively. In the reductive half reaction the isotope effects were 2.5, 14, and 28 for the same substrates, and 21 for (2R,3R)-(2,3-D2)butyryl-CoA. No intermediates are apparent during the reduction of oxidized enzyme to the presumed complex of reduced enzyme and crotonyl-CoA. The results are interpreted as indicating a high degree of concertedness during the rupture of the alpha and beta C-H bonds. They are compatible with a mechanism in which simultaneously the alpha-hydrogen is abstracted as a proton, while the beta-hydrogen is transferred to the oxidized flavin as a hydride.

Studies with general acyl-CoA dehydrogenase from pig kidney. Inactivation by a novel type of "suicide" inhibitor, 3,4-pentadienoyl-CoA

3,4-Pentadienoyl-CoA, an allenic substrate analog, is a potent inhibitor of the flavoprotein pig-kidney general acyl-CoA dehydrogenase. The analog reacts very rapidly (k = 2.4 X 10(3) min-1) with the native oxidized enzyme to form a covalent flavin adduct probably involving the isoalloxazine position N-5. This species is inactive, but activity may be regained by two pathways. The allenic thioester can be displaced (k = 0.3 min-1) by a large excess of octanoyl-CoA substrate upon reversal of covalent adduct formation. Alternatively, the enzyme inactivator adduct slowly decomposes (t1/2 = 75 min) to form the strongly thermodynamically favoured 2,4-diene and catalytically active, oxidized enzyme. During this latter process 15-20% of the activity is irreversibly lost probably due to covalent modification of the protein. These data suggest that 3,4-pentadienoyl-CoA should be considered a suicide substrate of the acyl-CoA dehydrogenase. The mechanism of the reactions, and in particular the 3,4----2,4 tautomerization, are consistent with a catalytic sequence initiated by abstraction of an alpha-hydrogen as a proton.

Kinetic methods for the study of the enzyme systems of beta-oxidation

Kinetic methods for studying the reactions of the "general" fatty acyl CoA dehydrogenase under three sets of substrate and enzyme concentration conditions have been developed. The reaction of butyryl-CoA and electron transfer flavoprotein (ETF) can be studied either under steady-state conditions with enzyme at catalytic concentration or under single-turnover conditions with enzyme in excess. Under the latter conditions, acyl-CoA dehydrogenase acts both as a catalyst and an ultimate electron-transfer acceptor. The reductive half-reaction of butyryl-CoA and enzyme can also be studied in a separate kinetic experiment. Comparison of the pH dependences of the rate constants and isotope effects of the steady-state reaction of butyryl-CoA and ETF with the same parameters for the reductive half-reaction is consistent with a mechanism involving transfer of electrons from butyryl-CoA to ETF within a ternary complex. An alternative mechanism in which the reductive half-reaction takes place prior to the binding and reaction of ETF seems unlikely because the pH 8.5 isotope effect on the reductive half-reaction is much larger than that on the complete reaction in spite of the fact that the rates of the reactions are comparable. The pH dependence of the Km for substrate and KI for inhibitor is consistent with a mechanism for transfer of electrons within the ternary complex which involves protonation of the C = O group of substrates. The protonation labilizes the C-2 proton and base catalysis of the removal of the C-2 proton results in the production of the active enzyme-substrate species, namely the C-2 anion of substrate.

Chemical and catalytic properties of the peroxisomal acyl-coenzyme A oxidase from Candida tropicalis

The peroxisomal acyl-CoA oxidase has been purified from extracts of the yeast Candida tropicalis grown with alkanes as the principal energy source. The enzyme has a molecular weight of 552,000 and a subunit molecular weight of 72,100. Using an experimentally determined molar extinction coefficient for the enzyme-bound flavin, a minimum molecular weight of 146,700 was determined. Based on these data, the oxidase contains eight perhaps identical subunits and four equivalents of FAD. No other beta-oxidation enzyme activities are detected in purified preparations of the oxidase. The oxidase flavin does not react with sulfite to form an N(5) flavin-sulfite complex. Photochemical reduction of the oxidase flavin yields a red semiquinone; however, the yield of semiquinone is strongly pH dependent. The yield of semiquinone is significantly reduced below pH 7.5. The flavin semiquinone can be further reduced to the hydroquinone. The behavior of the oxidase flavin during photoreduction and its reactivity toward sulfite are interpreted to reflect the interaction in the N(1)-C(2)O region of the flavin with a group on the protein which acts as a hydrogen-bond acceptor. Like the acyl-CoA dehydrogenases which catalyze the same transformation of acyl-CoA substrates, the oxidase is inactivated by the acetylenic substrate analog, 3-octynoyl-CoA, which acts as an active site-directed inhibitor.