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

Thioester enolate stabilization in the acyl-CoA dehydrogenases: the effect of 5-deaza-flavin substitution

The redox-inactive thioester analog 3-thia-octanoyl-CoA blocks transfer of a hydride equivalent to the flavin prosthetic group of the medium-chain acyl-CoA dehydrogenase with the accumulation of a stable enolate intermediate not encountered with normal substrates. Substitution of the normal flavin with 5-deaza-FAD would thus be expected to lead to enolate formation with both normal and 3-thia-substrate analogs, because reduction of the 5-deaza-enzyme is thermodynamically highly unfavorable. However, spectrophotometric titrations show that neither ligand forms significant enolate species with the 5-deaza-FAD enzyme. Similarly, the substituted dehydrogenase catalyzes undetectable alpha-proton exchange with octanoyl-CoA and ca. 1% of the corresponding rate with 3-thia-octanoyl-CoA when compared to the native enzyme. This inability to stabilize enolate species is not simply due to impaired binding of CoA-thioester analogs, because binding of a range of ligands is weakened by only 2- to 10-fold with the 5-deaza-enzyme. 4-Thia-trans-2-enoyl-CoA product is polarized normally on binding to the substituted protein, showing that this critical aspect of catalysis is apparently normal. These data, together with studies with CoA-persulfide and acetoacetyl- and p-nitrophenylacetyl-CoA, suggest that 5-deaza-FAD substitution exerts subtle, unanticipated, effects on the reductive half-reaction of the medium-chain acyl-CoA dehydrogenase. The involvement of charge-transfer interactions in the acidification of weakly acidic acyl-CoA thioesters is discussed.

Functional role of a distal (3'-phosphate) group of CoA in the recombinant human liver medium-chain acyl-CoA dehydrogenase-catalysed reaction

The X-ray crystallographic structure of medium-chain acyl-CoA dehydrogenase (MCAD)-octenoyl-CoA complex reveals that the 3'-phosphate group of CoA is confined to the exterior of the protein structure [approx. 15 A (1.5 nm) away from the enzyme active site], and is fully exposed to the outside solvent environment. To ascertain whether such a distal (3'-phosphate) fragment of CoA plays any significant role in the enzyme catalysis, we investigated the recombinant human liver MCAD (HMCAD)-catalysed reaction by using normal (phospho) and 3'-phosphate-truncated (dephospho) forms of octanoyl-CoA and butyryl-CoA substrates. The steady-state kinetic data revealed that deletion of the 3'-phosphate group from octanoyl-CoA substrate increased the turnover rate of the enzyme to about one-quarter, whereas that from butyryl-CoA substrate decreased the turnover rate of the enzyme to about one-fifth; the Km values of both these substrates were increased by 5-10-fold on deletion of the 3'-phosphate group from the corresponding acyl-CoA substrates. The transient kinetics for the reductive half-reaction, oxidative half-reaction and the dissociation 'off-rate' (of the reaction product from the oxidized enzyme site) were all found to be affected by deletions of the 3'-phosphate group from octanoyl-CoA and butyryl-CoA substrates. A cumulative account of these results reveals that, although the 3'-phosphate group of acyl-CoA substrates might seem 'useless' on the basis of the structural data, it has an essential functional role during HMCAD catalysis.

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.

Facile and restricted pathways for the dissociation of octenoyl-CoA from the medium-chain fatty acyl-CoA dehydrogenase (MCAD)-FADH2-octenoyl-CoA charge-transfer complex: energetics and mechanism of suppression of the enzyme's oxidase activity

In a previous paper, we demonstrated that the reductive half-reaction of medium-chain fatty acyl-CoA dehydrogenase (MCAD), utilizing octanoyl-CoA as physiological substrate, generates two (kinetically distinct) forms of the reduced enzyme (MCAD-FADH2) - octenoyl-CoA charge-transfer complexes [Kumar, N.R., & Srivastava, D.K. (1994) Biochemistry 33, 8833-8841]. We present evidence that octenoyl-CoA dissociates from the second (most stable) charge-transfer complex (referred to as CT2) via two alternative ("facile" and "restricted") pathways. The dissociation of octenoyl-CoA via the facile pathway involves the reversal of the overall reductive half-reaction of the enzyme, generating MCAD-FAD - octanoyl-CoA as the Michaelis complex, followed by dissociation of the latter complex into MCAD-FAD + octanoyl-CoA. Hence, via this pathway, octenoyl-CoA is released from the enzyme site in the form of octanoyl-CoA. In contrast, the restricted pathway involves a direct (albeit slow) dissociation of octenoyl-CoA from CT2 to yield MCAD-FADH2 + octenoyl-CoA. The kinetic profile for the dissociation of octenoyl-CoA via the restricted pathway matches the rate of oxidation of the reduced flavin (within CT2) by O2. This suggests that the oxidase activity of the enzyme remains suppressed as long as the reduced enzyme predominates in the form of the charge-transfer complex(es). The oxidase activity of the enzyme emerges concomitantly with the conversion of CT2 to the MCAD-FADH2 - octenoyl-CoA Michaelis complex. The energetic basis for the dissociation of octenoyl-CoA via the facile and restricted pathways and the mechanism of suppression of the oxidase activity of the enzyme are discussed.

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)