Crystal structures of the wild type and the Glu376Gly/Thr255Glu mutant of human medium-chain acyl-CoA dehydrogenase: influence of the location of the catalytic base on substrate specificity

Crystal structures of the wild type human medium-chain acyl-CoA dehydrogenase (MCADH) and a double mutant in which its active center base-arrangement has been altered to that of long chain acyl-CoA dehydrogenase (LCADH), Glu376Gly/Thr255Glu, have been determined by X-ray crystallography at 2.75 and 2.4 A resolution, respectively. The catalytic base responsible for the alpha-proton abstraction from the thioester substrate is Glu376 in MCADH, while that in LCADH is Glu255 (MCADH numbering), located over 100 residues away in its primary amino acid sequence. The structures of the mutant complexed with C8-, C12, and C14-CoA have also been determined. The human enzyme structure is essentially the same as that of the pig enzyme. The structure of the mutant is unchanged upon ligand binding except for the conformations of a few side chains in the active site cavity. The substrate with chain length longer than C12 binds to the enzyme in multiple conformations at its omega-end. Glu255 has two conformations, "active" and "resting" forms, with the latter apparently stabilized by forming a hydrogen bond with Glu99. Both the direction in which Glu255 approaches the C alpha atom of the substrate and the distance between the Glu255 carboxylate and the C alpha atom are different from those of Glu376; these factors are responsible for the intrinsic differences in the kinetic properties as well as the substrate specificity. Solvent accessible space at the "midsection" of the active site cavity, where the C alpha-C beta bond of the thioester substrate and the isoalloxazine ring of the FAD are located, is larger in the mutant than in the wild type enzyme, implying greater O2 accessibility in the mutant which might account for the higher oxygen reactivity.

alpha-Oxidation of 3-methyl-substituted fatty acids in rat liver. Production of formic acid instead of CO2, cofactor requirements, subcellular localization and formation of a 2-hydroxy-3-methylacyl-CoA intermediate

alpha-Oxidation of 3-methyl-substituted fatty acids in rat liver was studied in intact and permeabilized rat hepatocytes, and in homogenates and subcellular fractions. The experiments revealed that the primary end product of alpha-oxidation is formic acid, which is then converted to CO2. Rates of alpha-oxidation identical to those observed in intact hepatocytes were obtained in the permeabilized hepatocytes and liver homogenates when ATP, Mg2+ and CoA, and Fe2+, 2-oxoglutarate and ascorbate were added, suggesting that alpha-oxidation involves a fatty acid activation reaction and a dioxygenase reaction. Subcellular fractionation by differential and density gradient centrifugation demonstrated that alpha-oxidation is confined to peroxisomes, which produce formic acid that is converted to CO2, mainly in the cytosol. alpha-Oxidation in broken cell systems went hand in hand with the formation of a 2-hydroxy-3-methylacyl-CoA ester. Formation of the metabolite was strictly dependent on the presence of the above-mentioned cofactors, was confined to peroxisomes and was inhibited by fenoprofen and propyl gallate, inhibitors of alpha-oxidation in intact cells, indicating that the 2-hydroxyacyl-CoA ester is a bona fide intermediate of alpha-oxidation. Selective omission of cofactors from the reaction mixture and analysis of the incubation mixtures for 3-methyl fatty acids, 3-methyl fatty acyl-CoAs and their respective 2-hydroxy derivatives revealed that the activation reaction precedes the dioxygenase (hydroxylase) reaction. Our experiments demonstrate that alpha-oxidation is a peroxisomal process that consists of at least three reactions: fatty acid activation, hydroxylation and the reaction(s) involved in the release of formic acid.

Structure of 4-chlorobenzoyl coenzyme A dehalogenase determined to 1.8 A resolution: an enzyme catalyst generated via adaptive mutation

Here we describe the three-dimensional structure of 4-chlorobenzoyl-CoA dehalogenase from Pseudomonas sp. strain CBS-3. This enzyme catalyzes the hydrolysis of 4-chlorobenzoyl-CoA to 4-hydroxybenzoyl-CoA. The molecular structure of the enzyme/4-hydroxybenzoyl-CoA complex was solved by the techniques of multiple isomorphous replacement, solvent flattening, and molecular averaging. Least-squares refinement of the protein model reduced the crystallographic R factor to 18.8% for all measured X-ray data from 30 to 1.8 A resolution. The crystallographic investigation of this dehalogenase revealed that the enzyme is a trimer. Each subunit of the trimer folds into two distinct motifs. The larger, N-terminal domain is characterized by 10 strands of beta-pleated sheet that form two distinct layers which lie nearly perpendicular to one another. These layers of beta-sheet are flanked on either side by alpha-helices. The C-terminal domain extends away from the body of the molecule and is composed of three amphiphilic alpha-helices. This smaller domain is primarily involved in trimerization. The two domains of the subunit are linked together by a cation, most likely a calcium ion. The 4-hydroxybenzoyl-CoA molecule adopts a curved conformation within the active site such that the 4-hydroxybenzoyl and the adenosine moieties are buried while the pantothenate and pyrophosphate groups of the coenzyme are more solvent exposed. From the three-dimensional structure it is clear that Asp 145 provides the side-chain carboxylate group that adds to form the Meisenheimer intermediate and His 90 serves as the general base in the subsequent hydrolysis step. Many of the structural principles derived from this investigation may be directly applicable to other related enzymes such as crotonase.

C-NMR study on the interaction of medium-chain acyl-CoA dehydrogenase with acetoacetyl-CoA

The change-transfer interaction in the complex of pig kidney medium-chain acyl-CoA dehydrogenase (MCAD) with acetoacetyl-CoA was investigated by 13C-NMR spectroscopy and molecular orbital treatment. The acyl carbons of acetoacetyl-CoA were separately 13C-labeled and 13C-NMR spectra of the complexes of MCAD with the 13C-labeled acetoacetyl-CoA were measured. Each 13C-carbon atom was observed as a distinct peak and easily distinguished from the protein background. The chemical shift values for free acetoacetyl-CoA were 198.5, 59.9, 208.8, and 32.8 ppm for C(1), C(2), C(3), and C(4), respectively, which shifted to 181.3, 103.4, 192.3, and 29.9 ppm, respectively, when acetoacetyl-CoA was complexed with MCAD. While C(4) underwent a small upfield shift, the other carbons complexed with MCAD. While C(4) underwent a small upfield shift, the other carbons experienced significant shifts; both the C(1) and C(3) carbonyl carbons shifted upfield by about 17 ppm, and the C(2) carbon was observed as a very broad peak at a position shifted downfield by more than 40 ppm. These results were compared with 13C-NMR spectra of the keto-, enol-, and enolate forms of ethyl acetoacetate labeled with 13C at the acyl carbons, and interpreted with reference to the charge-transfer model based on the optimum overlap between the lowest unoccupied molecular orbital (LUMO) of flavin and the highest occupied molecular orbital (HOMO) of the enolate state of the acetoacetyl moiety of acetoacetyl-CoA. The C(2) carbon of acetoacetyl-CoA takes on the sp2 configuration in the bound form, indicating that one of the protons at C(2) of acetoacetyl-CoA is abstracted when bound to MCAD. C(1) = O is substantially polarized in the bound form of acetoacetyl-CoA, implying the presence of a machinery that polarizes this carbonyl group at the binding site, which thereby lowers the pKa value of the alpha-proton at C(2). This machinery is of fundamental importance in the initial step of MCAD catalysis.

Oxidative inactivation of a charge transfer complex in the medium-chain acyl-CoA dehydrogenase

The intense charge transfer complex between the enolate of 3-thia-octanoyl-CoA and the oxidized flavin of the medium-chain acyl-CoA dehydrogenase is discharged by the ferricenium ion with irreversible inactivation of the enzyme. Charge transfer complex formation is a necessary, but insufficient, condition for oxidative inactivation: the 3-oxa-octanoyl-CoA complex is also inactivated, whereas the comparable trans-3-octenoyl-CoA species is not. Complete inactivation of the dehydrogenase with 3-thia-octanoyl-CoA requires 1 molecule of thioester and apparently 3 molecules of ferricenium hexafluorophosphate. Experiments with 8-Cl-FAD substituted enzyme and the crystal structure of enzyme.ligand complexes argue that ferricenium ion-mediated oxidation proceeds through the flavin prosthetic group. Synthesis of [2-14C]-3-thia-octanoyl-CoA, followed by isolation of radiolabeled peptide from the modified medium-chain dehydrogenase, showed that inactivation results in labeling the catalytic base, GLU376. Oxidative modification is accompanied by the release of CoASH. A mechanism for inactivation is proposed involving generation of a sulfonium salt which efficiently captures the carboxylate nucleophile.

Evidence for electrophilic catalysis in the 4-chlorobenzoyl-CoA dehalogenase reaction: UV, Raman, and 13C-NMR spectral studies of dehalogenase complexes of benzoyl-CoA adducts

This paper reports on the mechanism of substrate activation by the enzyme 4-chlorobenzoyl coenzyme A dehalogenase. This enzyme catalyzes the hydrolytic dehalogenation of 4-chlorobenzoyl coenzyme A (4-CBA-CoA) to form 4-hydroxybenzoyl coenzyme A (4-HBA-CoA). The mechanism of this reaction is known to involve attack of an active site carboxylate (Asp or Glu side chain) at C(4) of the substrate benzoyl ring to form a Meisenheimer complex. Loss of chloride ion from this intermediate results in the formation of an arylated enzyme intermediate. The arylated enzyme is hydrolyzed to free enzyme plus 4-HBA-CoA by the addition of water at the acyl carbon [Yang, G., Liang, P.-H., & Dunaway-Mariano, D. (1994) Biochemistry 33, 8527]. The present studies have focused on the activation of the 4-CBA-CoA for nucleophilic attack by the active site carboxylate group. UV-visible, 13C-NMR, and Raman spectroscopic techniques were used to monitor changes in the distribution of the pi electrons of the benzoyl moiety of benzoyl-CoA adducts [substituted at C(4) with methyl (4-MeBA-CoA), methoxy (4-MeOBA-CoA), or hydroxyl (4-HBA-CoA) groups or at C(2) or C(3) with a hydroxyl group (2-HBA-CoA and 3-HBA-CoA)] resulting from the binding of these ligands to the dehalogenase active site. The UV-visible spectra measured for 4-HBA-CoA in aqueous buffer at pH 7.5 and in the dehalogenase active site revealed that a large red shift (from 292 to 373 nm) in the lambda max of the benzoyl moiety occurs upon binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate specificity of microsomal 1-acyl-sn-glycero-3-phosphoinositol acyltransferase in rat submandibular gland for polyunsaturated long-chain acyl-CoAs

Microsomal 1-acyl-sn-glycero-3-phosphoinositol (1-acyl-GPI) acyltransferase in the rat submandibular gland showed the highest specific activities for eicosanoid-related polyunsaturated acyl-CoAs, such as arachidonoyl-, bishomo-gamma-linolenoyl- and 5,8,11,14,17-eicosapentaenoyl-CoAs, with low Km values. High activities were also obtained with acyl-CoAs having long (more than 14 carbon atoms) and n - 6 unsaturated (more than 3 double bonds) acyl chains. This enzyme also utilized acyl-CoAs having trans-unsaturated or branched chains, but not short-chains, as substrates, although the activity levels for trans-unsaturated acyl-CoAs were lower than those for cis-unsaturated acyl-CoAs. Chronic administration of isoproterenol induced decreases of this enzyme activity and the content of arachidonic, bishomo-gamma-linolenic and 5,8,11,14,17-eicosapentaenoic acids at the sn-2 position of phosphatidylinositol. These results suggest that enrichment of arachidonic acid in the sn-2 position of phosphatidylinositol is established by the high specificity and affinity of 1-acyl-GPI acyltransferase for arachidonoyl-CoA. On the other hand, the low level of bishomo-gamma-linolenic and 5,8,11,14,17-eicosapentaenoic acids in the sn-2 position of phosphatidylinositol may be explained by their limited availability.

Mitochondrial beta-oxidation of 2-methyl fatty acids in rat liver

The mitochondrial beta-oxidation of 2-methyl fatty acids was studied with coupled rat liver mitochondria and purified enzymes. Measurements of mitochondrial respiration supported by 2-methyl fatty acids, straight chain fatty acids, or their coenzyme A (CoA) thioesters revealed that free short-chain and medium-chain 2-methyl fatty acids are oxidized nearly or as efficiently as are their straight chain analogs. Long-chain 2-methyl hexadecanoyl-CoA is also oxidized, although more slowly than its unbranched counterpart. However, medium-chain 2-methyldecanoyl-CoA, in contrast to its unbranched analog, is not oxidized at all. Of all acyl-CoA dehydrogenases only long-chain acyl-CoA dehydrogenase acts on medium-chain and long-chain 2-methylacyl-CoA thioesters. The resultant 2-methyl-2-enoyl-CoA thioesters are substrates of the mitochondrial trifunctional beta-oxidation complex which catalyzes the sequential hydration, dehydrogenation, and thiolytic cleavage of 2-methyl-substituted substrates to yield chain-shortened acyl-CoA thioesters and propionyl-CoA. The matrix enzymes L-3-hydroxyacyl-CoA dehydrogenase and 3-ketoacyl-CoA thiolase, in contrast to enoyl-CoA hydratase, are inactive with medium-chain and long-chain 2-methyl-substituted chain substrates. The specificity of the beta-oxidation enzymes toward 2-methyl-branched substrates forms the basis for assays of long-chain acyl-CoA dehydrogenase and the trifunctional beta-oxidation complex in the presence of their mitochondrial isozymes. It is concluded that rat liver mitochondria can oxidize 2-methyl fatty acids, but does so most effectively with medium-chain and short-chain ones that can enter mitochondria directly in a carnitine-independent manner.

Long-chain acyl-CoA profiles in cultured fibroblasts from patients with defects in fatty acid oxidation

Negative chemical ionization (NCI) mass spectrometry was used to quantify the acyl-CoA intermediates present in human fibroblasts growing in media containing the long-chain fatty acid, palmitate. The acyl-CoA intermediates were detected as the N-acyl pentafluorobenzyl glycinates. In fibroblasts from normal individuals only saturated acyl-CoA esters were detected, supporting the concept that the acyl-CoA dehydrogenase reaction is the rate-limiting step of intramitochondrial fatty acid oxidation. In patients with inherited enzymatic defects of intramitochondrial long-chain fatty acid oxidation, there was not a significant increase in the amount of long-chain acyl-CoA compounds, with palmitoyl-CoA amounts similar to those found in controls. However, there was a sharp decrease in the relative amount of lauroyl-CoA and a resultant sixfold elevation in the palmitoyl-CoA:lauroyl-CoA ratio. In contrast, fibroblasts with a defect involving the transport of fatty acids across the mitochondrial membrane, carnitine palmitoyl transferase 1 deficiency, had a fourfold increase in palmitoyl-CoA. Our results suggest that acyl-CoA esters in biological tissues are readily detectable using NCI mass spectrometry. This approach is significantly more sensitive than previous methods for the detection of these important metabolic intermediates, and may prove useful in the study of fatty acid oxidation in both normal and enzyme-deficient tissues.

Fatty acyl-CoA esters inhibit glucose-6-phosphatase in rat liver microsomes

In native rat liver microsomes glucose 6-phosphatase activity is dependent not only on the activity of the glucose-6-phosphatase enzyme (which is lumenal) but also on the transport of glucose-6-phosphate, phosphate and glucose through the respective translocases T1, T2 and T3. By using enzymic assay techniques, palmitoyl-CoA or CoA was found to inhibit glucose-6-phosphatase activity in intact microsomes. The effect of CoA required ATP and fatty acids to form fatty acyl esters. Increasing concentrations (2-50 microM) of CoA (plus ATP and 20 microM added palmitic acid) or of palmitoyl-CoA progressively decreased glucose-6-phosphatase activity to 50% of the control value. The inhibition lowered the Vmax without significantly changing the Km. A non-hydrolysable analogue of palmitoyl-CoA also inhibited, demonstrating that binding of palmitoyl-CoA rather than hydrolysis produces the inhibition. Light-scattering measurements of osmotically induced changes in the size of rat liver microsomal vesicles pre-equilibrated in a low-osmolality buffer demonstrated that palmitoyl-CoA alone or CoA plus ATP and palmitic acid altered the microsomal permeability to glucose 6-phosphate, but not to glucose or phosphate, indicating that T1 is the site of palmitoyl-CoA binding and inhibition of glucose-6-phosphatase activity in native microsomes. The type of inhibition found suggests that liver microsomes may comprise vesicles heterogeneous with respect to glucose-6-phosphate translocase(s), i.e. sensitive or insensitive to fatty acid ester inhibition.

Structural modulation of 2-enoyl-CoA bound to reduced acyl-CoA dehydrogenases: a resonance Raman study of a catalytic intermediate

A catalytic intermediate, the so-called "purple complex," of acyl-CoA dehydrogenase is produced on its reaction with the substrate, acyl-CoA. The purple complex is a charge-transfer complex between the reduced enzyme and the product, enoyl-CoA. Resonance Raman spectra of the purple complexes of three acyl-CoA dehydrogenases [short-chain acyl-CoA (SCAD), medium-chain acyl-CoA (MCAD), and isovaleryl-CoA (IVD) dehydrogenases] were measured with excitation at 632.8 nm within charge-transfer absorption bands. The 1,577 cm-1 band of the SCAD purple complex formed in the reaction with butyryl-CoA is mainly associated with the C(1) = O stretching of crotonyl-CoA, judging from the isotopic frequency shifts upon 13C or 18O substitution of butyryl-CoA. The 1,627 cm-1 band of the C(1) = O moiety of crotonyl-CoA in solution shifted downward by 50 cm-1 on complexation with reduced SCAD. This large frequency shift indicates a substantial interaction between C(1) = O and the enzyme, and is further evidence for an appreciable contribution of a polarized form of the C(1) = O moiety in the enzyme-bound enoyl-CoA. This frequency shift can be explained by the hydrogen bond of C(1) = O. The 1,577 cm-1 band of the MCAD purple complex remained constant, regardless of the acyl carbon-chain length (from C4 to C16 of the substrate, acyl-CoA); the alky chain scarcely affected the interaction of the C(1) = O moiety in the active site.(ABSTRACT TRUNCATED AT 250 WORDS)

Direct effects of fatty acids and other charged lipids on ion channel activity in smooth muscle cells

A variety of fatty acids increase the activity of certain types of K+ channels. This effect is not dependent on the three enzymatic pathways that convert arachidonic acid to various bioactive oxygenated metabolites. One type of K+ channel in toad stomach smooth muscle cell membranes in activated by fatty acids and other single chain lipids which possess both a negatively charged head group and a sufficiently hydrophobic acyl chain. Neutral lipids have no effect on K+ channel activity, while positively charged lipids with a sufficiently hydrophobic acyl chain suppress channel activity. Acyl Coenzyme A's, which do not flip across the bilayer, act only from the cytosolic surface of the membrane, suggesting that the binding site for channel activation is also located there. This fatty acid-activated channel is also activated by membrane stretch. Moreover, this mechanical response is either mediated or modulated by fatty acids. Thus, fatty acids and other charged single chain lipids may comprise another class of first or second messenger molecules that target ion channels.

Analysis of acyl coenzyme A binding to the transcription factor FadR and identification of amino acid residues in the carboxyl terminus required for ligand binding

The Escherichia coli FadR protein regulates the transcription of many unlinked genes and operons encoding proteins required for fatty acid synthesis and degradation. Previously, we demonstrated that the ability of purified FadR to bind DNA in vitro is inhibited by long chain acyl coenzyme A esters (DiRusso, D. D., Heimert, T. L., and Metzger, A. K. (1992) J. Biol. Chem. 267, 8685-8691). In the present work, we show that FadR binds acyl-CoA directly. Ligand binding resulted in a shift in the apparent pI of FadR from 6.9 to 6.2 and in a marked decrease in intrinsic fluorescence. The Km for FadR binding of oleoyl coenzyme A was determined to be 12.1 nM using the fluorescence quenching assay. The binding site for acyl-CoA was identified by selection of non-inducible mutations in the FadR gene. One altered protein carrying the change Ser219 to Asn (S219N) was purified and shown to have a reduced affinity for oleoyl coenzyme A as evidenced by a Km of 257 nM. S219N retained the ability to bind DNA and to repress or activate transcription. Alanine substitution of amino acid residues 215 through 230 identified Gly216 and Trp223 as also required specifically for induction. This region of FadR shares amino acid identities and similarities with the coenzyme A-binding site of Clostridium thermoaceticum CO dehydrogenase/acetyl-coenzyme A synthase. Due to the alteration in binding affinity of the purified S219N protein, the non-inducible phenotype of several proteins carrying alanine substitutions and similarities to CO dehydrogenase/acetyl-coenzyme A synthase we propose this region of FadR forms part of the acyl-CoA-binding domain.

Isomerization of trans-2,delta 5-dienoyl-CoA's to delta 3,delta 5-dienoyl-CoA's in the beta-oxidation of delta 5-unsaturated fatty acids

The NADPH-dependent reduction pathway for the metabolism of delta 5-unsaturated fatty acids involves the isomerization of trans-2,delta 5-dienoyl-CoA, initially formed from the dehydrogenation of delta 5-enoyl-CoA, to isomeric delta 3,delta 5-dienoyl-CoA. The latter intermediates were then isomerized to trans-2,trans-4-dienoyl-CoA, which then follows the NADPH-dependent pathway mediated by 2,4-dienoyl-CoA reductase. The isomerization from trans-2,delta 5-dienoyl-CoA to delta 3,delta 5-dienoyl-CoA is catalyzed by delta 3,delta 2-enoyl-CoA isomerase. In this investigation, we identified the stereoisomers of delta 3,delta 5-dienoates that were formed in the reaction. Starting from trans-2,cis-5-decadienoyl-CoA, the isomerization produced cis-3,cis-5- and trans-3,cis-5-decadienoates. On the other hand, trans-2,trans-5-decadienoyl-CoA yielded cis-3,trans-5- and trans-3,trans-5-decadienoates. In addition to purified rat liver delta 3,delta 2-enoyl-CoA isomerase, acyl-CoA oxidase from Arthrobacter also catalyzed the isomerization from trans-2,cis-5-dienoyl-CoA. However, this acyl-CoA oxidase could not catalyze the similar isomerization of trans-2,trans-5-dienoyl-CoA. delta 3,delta 5-t-2,t-4-Dienoyl-CoA isomerase used cis-3,cis-5-, trans-3,cis-5-, and cis-3,trans-5-dienoyl-CoA's as substrates and converted them to trans-2,trans-4-dienoyl-CoA. In contrast, trans-3,trans-5-dienoyl-CoA was not a substrate for this isomerization. Extensive purification of acyl-CoA oxidase through column chromatography could not remove or diminish the isomerization activity associated with acyl-CoA oxidase. Acyl-CoA oxidases derived from Candida and rat liver also possess isomerization activity.(ABSTRACT TRUNCATED AT 250 WORDS)

A carbon-skeleton walk: a novel double rearrangement of glutaryl-CoA catalyzed by the human methylmalonyl-CoA mutase

Methylmalonyl-CoA mutase is a member of the coenzyme B12-dependent family of isomerases and interconverts methylmalonyl-CoA and succinyl-CoA. We have examined the ability of the enzyme to effect a double rearrangement reaction when presented with glutaryl-CoA, a substrate analog with a three-carbon template on which two successive 1,2 migrations can occur. Our results demonstrate that the enzyme converts glutaryl-CoA to both methylsuccinyl-CoA and ethylmalonyl-CoA. To our knowledge, this is the first example of a double rearrangement reaction catalyzed by a coenzyme B12-dependent enzyme.

Inhibition of the human methylmalonyl-CoA mutase by various CoA-esters

Human methylmalonyl-CoA mutase is inhibited by ethylmalonyl-CoA, cyclopropylcarbonyl-CoA carboxylate, and methylenecyclopropylacetyl-CoA, which are substrate, intermediate, and product analogs, respectively. The mode of inhibition by each analog is reversible and mixed with respect to the substrate, methylmalonyl-CoA. This implies that the inhibitors are able to bind to both free enzyme and to the enzyme-substrate complex, although with affinities that are 4.5- to 10-fold different for the two species. The Ki1 for the cyclopropylcarbonyl-CoA carboxylate (0.26 +/- 0.07 mM), is 4-fold greater than the Km(app) measured for the substrate, methylmalonyl-CoA. Additionally, ethylmalonyl-CoA functions as an alternate substrate and is metabolized to methylsuccinyl-CoA. The human mutase is a homodimer that binds 1 mol of cobalamin per subunit. So, the observed mixed inhibition kinetics by substrate analogs is curious. Our finding that methylenecyclopropylacetyl-CoA, the causative agent of Jamaican "vomiting sickness," inhibits methylmalonyl-CoA mutase, while interesting, is probably not physiologically important because of the relatively high inhibition constants (Ki1 = 0.47 +/- 0.12 mM and Ki2 = 2 +/- 0.34 mM) observed with this compound.

Cysteine-containing peptide sequences exhibit facile uncatalyzed transacylation and acyl-CoA-dependent acylation at the lipid bilayer interface

A variety of simple cysteine-containing lipopeptides, with sequences modeled on those found in naturally occurring S-acylated proteins, undergo spontaneous S-acylation in phospholipid vesicles at physiological pH when either long-chain acyl-CoAs or other S-acylated peptides are added as acyl donors. Fluorescent or radiolabeled lipopeptides with the sequence myristoyl-GCX- (X = G, L, R, T, or V), a motif found to undergo S-acylation in several intracellular regulatory proteins, and the prenylated peptide -SCRC(farnesyl)-OMe, modeled on the carboxyl terminus of p21H-ras, were all found to be suitable acyl acceptors for such uncatalyzed S-acyl transfer reactions at physiological pH. Acylation of these cysteinyl-containing lipopeptides to high stoichiometry was observed, on time scales ranging from a few hours to a few tens of minutes, in vesicles containing relatively low concentrations (< or = mol %) and only a modest molar excess (2.5:1) of the acyl donor species. No evidence was obtained for acyl transfer to peptide serine or threonine hydroxyl groups under the same conditions. These observations may have significant implications both for the design of in vitro studies of the S-acylation of membrane-associated proteins and for our understanding of the mechanisms of S-acylation of these species in vivo.

Hepatic beta-oxidation of 3-phenylpropionic acid and the stereospecific dehydration of (R)- and (S)-3-hydroxy-3-phenylpropionyl-CoA by different enoyl-CoA hydratases

The hepatic beta-oxidation of 3-phenylpropionic acid (PPA) was studied by the use of subcellular fractions and purified enzymes with the aim of characterizing intermediates and the subcellular location of this pathway. Respiration measurements with coupled rat liver mitochondria indicate that PPA is efficiently metabolized by mitochondrial beta-oxidation. In contrast, the peroxisomal beta-oxidation of this compound is at best a very slow process, as evidenced by the low activity of peroxisomal acyl-CoA oxidase toward 3-phenylpropionyl-CoA. In mitochondria, 3-phenylpropionyl-CoA is effectively dehydrogenated to cinnamoyl-CoA, which is only slowly converted to benzoylacetyl-CoA due to the unfavorable equilibrium of the hydration of cinnamoyl-CoA to 3-hydroxy-3-phenylpropionyl-CoA. Benzoylacetyl-CoA is a substrate of 3-ketoacyl-CoA thiolase. The dehydration of 3-hydroxy-3-phenylpropionyl-CoA to cinnamoyl-CoA forms the basis for a sensitive and stereospecific assay of enoyl-CoA hydratases. The progress of this reaction, which proceeds to near completion, can be measured spectrophotometrically at 308 nm. Soluble mitochondrial and peroxisomal enoyl-CoA hydratases only act on the (R,L) isomer, whereas the peroxisomal D-3-hydroxyacyl-CoA dehydratase is specific for the (S,D) isomer. Both substrates can be easily prepared from the commercially available enantiomeric acids. It is concluded that PPA, a key compound in Knopp's classical study that led him to formulate the principle of beta-oxidation, is overwhelmingly, if not completely, degraded by mitochondrial beta-oxidation.

Photolabeling of soybean microsomal membrane proteins with photoreactive acyl-CoA analogs

Synthesis of 32P-labeled 12-azidooleoyl-CoA and 125I-labeled 12-[(azidosalicyl)amino]dodecanoyl-CoA (ASD-CoA) was achieved. The synthesized radioactive, photoreactive reagents were tested as photoaffinity labels for acyl-CoA:lysophosphatidylcholine acyltransferase from the microsomal membranes of developing soybean cotyledons. When a mixture of microsomal membranes and the azidooleoyl-CoA or ASD-CoA were incubated in the dark, the analogs were recognized as substrate and competitive inhibitor, respectively. The enzyme preferentially utilizes unsaturated acyl-CoAs rather than saturated acyl-CoAs. Incubation of microsomal membranes with acyl-CoA analogs and immediately followed by photolysis resulted in an irreversible inhibition of lysophosphatidylcholine acyltransferase activity. Analysis of photolyzed microsomal membranes by SDS/PAGE and autoradiography revealed that azidooleoyl-CoA preferentially labeled eight acyl-CoA binding proteins, but ASD-CoA labeled only three polypeptides with molecular masses of 110, 90 and 32 kDa that are commonly labeled by both the analogs. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme against photoinactivation by azidooleoyl-CoA and ASD-CoA, respectively. The protection was profound in 110-kDa polypeptide indicating that this protein could be lysophosphatidylcholine acyltransferase. These results demonstrate that the photoaffinity of acyl-CoA analogs makes them potential probes to identify and characterize lipid biosynthetic enzymes.

Photoaffinity labeling of acyl-CoA oxidase with 12-azidooleoyl-CoA and 12-[(4-azidosalicyl)amino]dodecanoyl-CoA

Synthesis of 32P-labeled CoA of high specific activity was achieved using partially purified dephospho-CoA kinase (EC 2.7.1.24) from pig liver with [gamma-32P]ATP as donor and dephospho-CoA as acceptor. A photoaffinity dodecanoic acid analog, 12-[(4-azidosalicyl)amino]dodecanoic acid was synthesized, as were its CoA derivative (ASD-CoA) and the CoA derivative of 12-azidooleic acid. The CoA derivatives were synthesized from azido fatty acid analogs by acyl-CoA synthetase. The synthesized photolabile reagents were tested as photoaffinity labels for acyl-CoA oxidase (EC 1.3.99.3) from Arthrobacter species. When a mixture of oxidase and the acyl-CoA analogs were incubated in the absence of ultraviolet light, the analogs were recognized as substrate. Acyl-CoA oxidase was incubated in the presence of acyl-CoA analogs and immediately photolyzed, which resulted in irreversible inhibition. Oleoyl-CoA and dodecanoyl-CoA protect the enzyme from photoactivated inhibition by 12-azidooleoyl-CoA and ASD-CoA, respectively. Analysis of photolyzed enzyme preparations by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed that both analogs preferentially labeled a 54,000 molecular weight protein. These results demonstrate that the photoaffinity acyl-CoA analogs have potential application as probes to identify and characterize lipid biosynthetic enzymes and to identify the active site of these proteins.

Sensitivity of Escherichia coli succinyl-CoA mutants at Trp beta 76 to clostripain and to trypsin. ADP and ATP protect against cleavage by clostripain at Arg beta 80

Mutant forms of Escherichia coli succinyl-CoA synthetase, W76F (Trp beta 76 replaced by Phe) (Nishimura, J. S., Mann, C. J., Ybarra, J., Mitchell, T., and Horowitz, P. M. (1990) Biochemistry 29, 862-865), and W43,76,248F (all three Trp replaced by Phe) were found to be more sensitive to proteolysis by clostripain than the wild-type enzyme or other Trp mutant proteins. Like wild-type enzyme, sensitivity to trypsin was apparent when the enzyme forms were in the dephosphorylated state. Sensitivity to clostripain was the same, whether mutant or wild-type forms were in the phosphorylated or dephosphorylated state. The substrates ADP and ATP both protected the enzymes against inactivation by clostripain, with dissociation constants for protection of W76F of 33 and 125 microM, respectively. Polyacrylamide gel electrophoresis of clostripain digests revealed preferential digestion of the beta-subunit and the appearance of 40- and 31-kDa species, with amino termini corresponding to residues 15 and 81, respectively, of the beta-subunit. Mutagenic replacement of Arg beta 80, but not Arg beta 14, with Lys resulted in an enzyme that was as resistant to clostripain as wild-type enzyme. These results suggest that Arg beta 80 is the principal site of inactivation by clostripain and may be involved in the binding of ADP and ATP to succinyl-CoA synthetase.

Stereoselective interaction of 2-halo-acyl-CoA derivatives with medium chain acyl-CoA dehydrogenase from pig kidney

Several 2-halo-acyl-CoA derivatives have been synthesized to examine the interaction of these potential inhibitors of mitochondrial beta-oxidation with the purified medium chain acyl-CoA dehydrogenase from pig kidney. Racemic 2-bromo-, 2-chloro-, and 2-fluoro-octanoyl-CoA thioesters show 6.6, 33, and 3.5% of the activity of octanoyl-CoA in the standard assay system, respectively. Only the S-enantiomer of these 2-substituted analogues is a significant substrate of the dehydrogenase, with S-2-bromo-octanoyl-CoA showing a rate of 18% that of octanoyl-CoA, compared to about 1% for the R-isomer. The observations presented here suggest that a detailed understanding of the mode of action of 2-halo-fatty acids as inhibitors of mammalian beta-oxidation will require consideration of the metabolic fate and inhibitory effects of both enantiomers.

3-Hydroxy-3-methylglutaryldithio-CoA: utility of an alternative substrate in elucidation of a role for HMG-CoA lyase's cation activator

(S)-(3-Hydroxy-3-methyl-1-thionoglutaryl)-Coenzyme A (HMG[= S]CoA), a dithioester analog of (S)-(3-hydroxy-3-methylglutaryl)-CoA (HMG-CoA), acts as an efficient alternative substrate for avian HMG-CoA lyase. Detection of product formation by HPLC, UV absorbance and coupled enzyme assays indicates that HMG[= S]CoA cleavage yields acetyl[= S]CoA and acetoacetate. HMG[= S]CoA binds to the lyase with a Km of 13 microM and undergoes the cleavage reaction at a maximal rate which is 20% of that observed with HMG-CoA. The enzyme-catalyzed cleavage of both HMG-CoA and HMG[= S]CoA is stimulated by the divalent cations Mg2+ and Mn2+. Mg2+ produces a 2-fold higher stimulation of HMG-CoA cleavage than that observed with Mn2+. In contrast, stimulation of HMG[= S]CoA cleavage is nearly seven times higher with Mn2+ than with Mg2+. Not only is the stimulation of enzymatic activity dependent on the cation, but also the Km values for Mg2+ and Mn2+ are dependent upon the substrate used. In contrast, the Km values for HMG-CoA and HMG[= S]CoA are not markedly dependent on the identity of the divalent cation. These results are compatible with the initial formation of a binary enzyme-substrate complex prior to binding of the divalent cation to produce a catalytically active enzyme-substrate-metal ternary complex.

Products of enzymatic reduction of benzoyl-CoA, a key reaction in anaerobic aromatic metabolism

Benzoyl-coenzyme A is the most common central intermediate of anaerobic aromatic metabolism. Studies with whole cells of different bacteria and in vitro had shown that benzoyl-CoA is reduced to alicyclic compounds, possibly via cyclohexadiene intermediates. This reaction is considered a 'biological Birch reduction'. We have elucidated by NMR techniques the structures of six products of [ring-13C6]benzoate reduction. The reaction is catalyzed by extracts from cells of a denitrifying Pseudomonas strain K172 anaerobically grown with benzoate and nitrate as sole carbon and energy sources. The assay mixture contained [ring-13C6]benzoate plus traces of [U-14C]benzoate, Mg2+, ATP, coenzyme A (CoA), and Ti(III) as reductant. The use of the multiply 13C-labelled precursor increases the sensitivity of NMR detection and allows the analysis of crude product mixtures by two-dimensional coherence transfer procedures such as total correlation 13C-NMR spectroscopy and 13C-filtered 1H-NMR spectroscopy. The time course of product formation is consistent with the following order of events. Benzoyl-CoA is formed from benzoate via benzoate-CoA ligase. The first ring reduction product observed is cyclohex-1,5-diene-1-carboxyl-CoA. The next intermediate is 6-hydroxycyclohex-1-ene-1-carboxyl-CoA which is derived from the diene by addition of water. Part of the diene seems to be reduced to cyclohex-1-ene-1-carboxyl-CoA which becomes hydrated to trans-2-hydroxycyclohexane-1-carboxyl-CoA; these two intermediates may be side products in vitro. The first non-cyclic intermediate formed by beta-oxidation is 3-hydroxypimelyl-CoA. This aliphatic C7 dicarboxylic acid is proposed to be oxidized via glutaryl-CoA and crotonyl-CoA to three molecules of acetyl-CoA and one molecule of CO2. A similar product pattern was observed in the benzoate-degrading phototrophic bacterium Rhodopseudomonas palustris. This indicates that the enzymatic reduction of benzoyl-CoA may be mechanistically similar in different anaerobes.

(R)-lactyl-CoA dehydratase from Clostridium propionicum. Stereochemistry of the dehydration of (R)-2-hydroxybutyryl-CoA to crotonyl-CoA

1. A new two-step method for purifying component E II of lactyl-CoA dehydratase was developed. The source of the enzyme was Clostridium propionicum grown on either D,L-alanine or L-threonine. No difference in these preparations was observed whether during purification or by SDS/PAGE of the pure enzymes. Both preparations exhibited similar activities towards (R)-lactyl-CoA as well as towards (R)-2-hydroxybutyryl-CoA, the latter being the superior substrate. 2. Three species of (2R)-2-hydroxybutyrate labelled with 3H at C3 were prepared containing 96%, 37% and 63% of the 3H in the 3S-position. By incubation of these species with acetyl-CoA, propionate CoA-transferase and lactyl-CoA dehydratase 104%, 32% and 70% of the 3H, respectively, was release as 3HOH. The data indicate that stereospecific abstraction of the 3Si hydrogen of (2R)-2-hydroxybutyryl-CoA during the dehydration. 3. The identity of the product of the dehydration as crotonyl-CoA was established by the combined action of the enzymes crotonase and (S)-3-hydroxyacyl-CoA dehydrogenase. The results indicate that the elimination of water from (R)-2-hydroxybutyryl-CoA occurs in a syn mode. 4. All enzyme activities necessary for the conversion of L-threonine via (R)-2-hydroxybutyryl-CoA to butyrate were detected in cell-free extracts of C. propionicum. 5. A new mechanism for the dehydration of lactyl-CoA is proposed.

Interactions of acyl-coenzyme A with phosphatidylcholine bilayers and serum albumin

Interactions of oleoyl- and octanoyl-coenzyme A (CoA) with phosphatidylcholine (PC) vesicles and bovine serum albumin (BSA) were investigated by NMR spectroscopy. Binding of acyl-CoA to small unilamellar PC vesicles and to BSA was detected by changes in 13C and 31P chemical shifts relative to the chemical shifts for aqueous acyl-CoA. When oleoyl-CoA (less than or equal to 15 mol %) was added to preformed vesicles, the 13C thioester signal (200.1 ppm) was upfield from the signal for micellar oleoyl-CoA (201.7 ppm), suggesting decreased H-bonding (partial dehydration) at the carbonyl group upon binding to the bilayer. When vesicles were prepared by cosonication of oleoyl-CoA and PC, a second peak (199.8 ppm) was seen. The major peak at 200.1 ppm broadened and shifted after addition of Dy(NO3)3 and was not seen after addition of BSA, while the peak at 199.8 ppm was unaffected by either perturbation. Thus, oleoyl-CoA in each bilayer leaflet was distinguished, and transbilayer movement was shown to be slow (t 1/2 greater than or equal to hours). PC vesicles remained intact with less than or equal to 15 mol % oleoyl-CoA, while higher oleoyl-CoA proportions produced mixed micelles. In contrast, 13C spectra revealed rapid exchange (ms) of octanoyl-CoA between the aqueous phase and PC vesicles and a low affinity for the bilayer. Thus, the binding affinity of acyl-CoA for PC bilayers is dependent on the acyl chain length. Oleoyl-CoA in the presence of BSA (1 mol/mol) gave rise to three carbonyl signals at 197.2-203.6 ppm. With 2-5 mol of oleoyl-CoA/BSA, 1-2 additional signals were observed. None of the signals corresponded to unbound oleoyl-CoA. Addition of [13C]carboxyl-enriched oleic acid to oleoyl-CoA/BSA mixtures revealed simultaneous binding of oleic acid and oleoyl-CoA to BSA, with some perturbation of binding interactions. Thus, BSA contains multiple binding sites for oleoyl-CoA and can bind fatty acid and acyl-CoA simultaneously.

Synthesis, characterisation and high-performance liquid chromatography of C6-C16 dicarboxylyl-mono-coenzyme A and -mono-carnitine esters

The synthesis and purification of the mono-coenzyme A and mono-carnitine esters of the homologous series of straight-chain even-numbered dicarboxylic acids (C6-C16) is described. The corresponding 3-hydroxyacyl- and 2-enoyl-CoA esters were prepared enzymatically. A reversed-phase high-performance liquid chromatographic (HPLC) system for the analysis of the intact CoA esters is described and their chromatographic behaviour documented. Reversed-phase HPLC systems for the analysis of the 4-bromophenacyl derivatives of the dicarboxylyl-mono-carnitines and the 4-nitrobenzyl derivatives of the free acids are also described. Some preliminary studies of the metabolism of [U-14C]hexadecanedionoyl-mono-CoA by rat liver peroxisomes and rat skeletal muscle mitochondria are described illustrating the application of these methods.

Decreased long-chain fatty acyl CoA elongation activity in quaking and jimpy mouse brain: deficiency in one enzyme or multiple enzyme activities?

Using long-chain fatty acyl CoAs (arachidoyl CoA and behenoyl CoA), a decrease in overall fatty acid chain elongation activity was observed in the quaking and jimpy mouse brain microsomes relative to controls. Arachidoyl CoA (20:0) and behenoyl CoA (22:0) elongation activities were depressed to about 50% and 80% of control values in quaking and jimpy mice, respectively. Measurement of the individual enzymatic activities of the elongation system revealed a single deficiency in enzyme activity; only the condensation activity was reduced to the same extent as total elongation in both quaking and jimpy mice. The activities of the other three enzymes, beta-ketoacyl CoA reductase, beta-hydroxyacyl CoA dehydrase, and trans-2-enoyl CoA reductase, in both mutants were similar to the activities present in the control mouse. In addition, the activities of these three enzymes were more than two to three orders of magnitude greater than the condensing enzyme activity in all three groups, establishing that the condensing enzyme catalyzes the rate-limiting reaction step of total elongation. When the elongation of palmitoyl CoA was measured, only a 25% decrease in total elongation occurred in both mutants; a similar percent decrease in the condensation of palmitoyl CoA also was observed. The activities of the other three enzymes were unaffected. These results support the concept of either multiple elongation pathways or multiple condensing enzymes.

Determination of the epimeric composition of ibuprofenyl-CoA

Ibuprofen [racemic2-(4-isobutylphenyl)propionic acid] is a 2-arylpropionic acid nonsteroidal anti-inflammatory drug which undergoes unidirectional, R to S chiral inversion in vivo. It has been proposed that this chiral inversion phenomenon occurs via a coenzyme A (CoA) thioester intermediate. To characterize the formation and metabolism of this metabolic intermediate, ibuprofenyl-CoA, reference standards were needed and thus the CoA derivatives of (R)-, (S)-, and racemic ibuprofen were chemically synthesized. An HPLC assay employing a C18 reverse-phase column was developed to quantitate "total" ibuprofenyl CoA. Samples collected from this assay were then analyzed for ibuprofenyl-CoA epimeric composition by chiral chromatography employing a Chiral-AGP alpha 1-acid glycoprotein column. The applicability of these methods was demonstrated by assessing (R)- and (S)-ibuprofenyl-CoA hydrolysis and epimerization following incubation with rat liver homogenates. Rat liver homogenate catalyzed the complete and rapid epimerization of ibuprofenyl-CoA and the rate constants for (R)- and (S)-ibuprofenyl-CoA hydrolysis were equal. ATP and CoA were found to inhibit rat liver-catalyzed ibuprofenyl-CoA hydrolysis by 70-80% with no effect on epimerization. Additionally, it was demonstrated that traditional indirect ibuprofenyl-CoA assays which employ basic hydrolysis result in erroneous epimeric ratio determinations due to chemical epimerization.

Synthesis and properties of (R)-2-hydroxyglutaryl-1-CoA. (R)-2-hydroxyglutaryl-5-CoA, an erroneous product of glutaconate CoA-transferase

1) (R)-2-Hydroxyglutaryl-1-CoA was synthesised starting from (R)-5-oxotetrahydrofuran-2-carboxylic acid (gamma-lactone of (R)-2-hydroxyglutarate) which was converted to the acylchloride and condensed with N-capryloylcysteamine. The lactone ring of the resulting thiolester was opened by acid hydrolysis and the CoA derivative was obtained by transesterification. 2) Pure glutaconate CoA-transferase from Acidaminococcus fermentans catalysed the formation of the 1- and the 5-isomer of (R)-2-hydroxyglutaryl-CoA from acetyl-CoA and (R)-2-hydroxyglutarate. The isomers were separated by HPLC and characterised by their reaction with acetate under the catalysis of the CoA-transferase. V/Km for the 1-isomer was 80 times higher than that for the 5-isomer. 3) Studies with cell-free extracts from A. fermentans showed that only (R)-2-hydroxyglutaryl-1-CoA but not its 5-isomer was dehydrated to glutaconyl-1-CoA. The data indicate that (R)-2-hydroxyglutaryl-5-CoA is an erroneous product of glutaconate CoA-transferase which only occurs in vitro.

Synthesis and characterization of inhibitors of myristoyl-CoA:protein N-myristoyltransferase

Several substrate and product analogs were synthesized and tested as in vitro inhibitors of bovine brain N-myristoyl-CoA:protein N-myristoyltransferase (NMT; EC 2.3.1.97). At 40 microM, the acyl CoA analog, S-(2-ketopentadecyl)-CoA, completely inhibited NMT in the presence of 80 microM myristoyl CoA. Decreasing but marked inhibition was also observed with the acyl CoA analogs, S-(2-bromo-tetradecanoyl)-CoA and S-(3-(epoxymethylene)dodecanoyl)-CoA, and the multisubstrate derivative N-(2-S-CoA-tetradecanoyl)glycinamide in the presence of 40 microM myristoyl CoA. Inhibition was also observed with the non-coenzyme A myristoyl analog, 1-bromo-2-pentadecanone. All of the above compounds exhibited reversible competitive inhibition kinetics with respect to myristoyl CoA with Ki values of 0.11 to 24 microM. Two additional acyl CoA analogs, S-(cis-3-tetradecenoyl)-CoA and S-(3-tetradecynoyl)-CoA, functioned as alternative substrates for NMT.

An acyl-coenzyme A chain length dependent assay for 3-oxoacyl-coenzyme A thiolases employing acetyldithio-coenzyme A

An assay for 3-oxoacyl-coenzyme A (3-oxoacyl-CoA) thiolases is described. The reaction utilizes acetyldithio-CoA as the nucleophile and variable chain length saturated acyl-CoA's as the electrophiles. The properties of the 3-oxoacyl-CoA dithioester product, notably a pKa of 6.6 +/- 0.1 and an extinction coefficient of 21,600 cm-1 M-1 for the enethiolate at 357 nm, make it possible to spectrophotometrically follow the reaction in the thermodynamically unfavorable carbon-carbon bond-forming direction. These properties eliminate both the background decomposition and the dependence on Mg2+, chain length, and pH that complicate assays with 3-oxoacyl-CoA substrates. Purified thiolase I from pig liver was 140-fold more active with butyryl-CoA as the electrophile than with acetyl-CoA and 38-fold more reactive with hexanoyl-CoA than with myristoyl-CoA. Beef liver homogenate showed a much greater relative activity with myristoyl-CoA as the electrophile than either purified pig heart thiolase I or pig heart homogenate. The analysis of the separation of thiolases by anion-exchange chromatography is simplified and further suggests the existence of isozymes with varying chain length specificities.

Aliphatic molecules (C-6 to C-8) containing double or triple bonds as potential penicillin side-chain precursors

Three different hexenoyl-CoA derivatives (trans-2-hexenoyl-CoA, trans-3-hexenoyl-CoA and trans-trans-2,4-hexadienoyl-CoA), two octenoyl-CoA (trans-2-octenoyl-CoA, trans-3-octenoyl-CoA) and 2-octynoyl-CoA were tested as substrates of the enzyme acyl-CoA: 6-Aminopenicillanic acid acyltransferase (AT) from Penicillium chrysogenum. Only trans-3-hexenoyl-CoA and trans-3-octenoyl-CoA were recognized by AT and efficiently converted into penicillin F and octenoylpenicillin, respectively. The Km values for these substrates were 0.6 and 0.5 mM, suggesting that the affinity of AT for these molecules is similar to that reported for phenyl acetyl-CoA, octanoyl-CoA and hexanoyl-CoA (0.5, 0.6, and 1 mM, respectively). The absence of enzymatic activity shown by AT with the other acyl-CoA derivatives tested is due to the different position of the double or triple bond(s) in their aliphatic chains. The influence of the free rotation round the bond C-2-C-3 and possibility of planar conformation in such molecules and the importance in the formation of the enzyme-substrate complex is discussed.