Enoyl coenzyme A reduction by bovine mammary fatty acid synthetase. Specificity and other characteristics

Activation of acyl condensation reaction of monomeric 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme, by free coenzyme A

6-Hydroxymellein (6HM) synthase is a multifunctional polyketide enzyme induced in carrot cells, whose fully active homodimer catalyzes condensation of acyl-CoAs and the NADPH-dependent ketoreduction of the enzyme-bound intermediate. 6HM-forming activity of the synthase was markedly decreased when the reaction mixture pH was adjusted from 7.5 to 6.0. However, under these slightly acidic conditions, the acyl condensation catalyzed by the dissociated monomer enzyme was appreciably stimulated by addition of free coenzyme A (CoA). In contrast, the condensation reaction at pH 6.0 was significantly inhibited in the presence of CoA when the reaction was carried out with the NADPH-omitted dimer synthase. Among the kinetic parameters of the acyl condensation, velocity of the monomer-catalyzing reaction at the acidic pH was appreciably increased upon addition of CoA while K(m)s did not show any significant change in the presence and absence of the compound. These results suggest that CoA associates with a specific site in the dissociated monomeric form of 6HM synthase, and the velocity of the acyl condensation reaction catalyzed by the CoA-synthase complex appreciably increases in acidic conditions.

Purification of crotonyl-CoA reductase from Streptomyces collinus and cloning, sequencing and expression of the corresponding gene in Escherichia coli

A crotonyl-CoA reductase (EC 1.3.1.38, acyl-CoA:NADP+ trans-2-oxidoreductase) catalyzing the conversion of crotonyl-CoA to butyryl-CoA has been purified and characterized from Streptomyces collinus. This enzyme, a dimer with subunits of identical mass (48 kDa), exhibits a Km = 18 microM for crotonyl-CoA and 15 microM for NADPH. The enzyme was unable to catalyze the reduction of any other enoyl-CoA thioesters or to utilize NADH as an electron donor. A highly effective inhibition by straight-chain fatty acids (Ki = 9.5 microM for palmitoyl-CoA) compared with branched-chain fatty acids (Ki > 400 microM for isopalmitoyl-CoA) was observed. All of these properties are consistent with a proposed role of the enzyme in providing butyryl-CoA as a starter unit for straight-chain fatty acid biosynthesis. The crotonyl-CoA reductase gene was cloned in Escherichia coli. This gene, with a proposed designation of ccr, is encoded in a 1344-bp open reading frame which predicts a primary translation product of 448 amino acids with a calculated molecular mass of 49.4 kDa. Several dispersed regions of highly significant sequence similarity were noted between the deduced amino acid sequence and various alcohol dehydrogenases and fatty acid synthases, including one region that contains a putative NADPH binding site. The ccr gene product was expressed in E. coli and the induced crotonyl-CoA reductase was purified tenfold and shown to have similar steady-state kinetics and electrophoretic mobility on sodium dodecyl sulfate/polyacrylamide to the native protein.

Decarboxylation of malonyl-CoA by lactating bovine mammary fatty acid synthase

1. A pronounced malonyl-CoA decarboxylase activity of bovine mammary fatty acid synthase results in the formation of acetyl-CoA and not of triacetic acid lactone as in the reaction by yeast and pigeon liver synthase. 2. This activity is unaffected by the dissociation of the enzyme and is insensitive to its modification by iodoacetamide, N-ethylmaleimide, p-hydroxymercuribenzoate or 2-chloroacetyl-CoA. 3. A 50% inhibition of the activity observed on the depletion of free CoA from the medium indicates that at least part of the reaction occurs only after the acylation of the enzyme with the malonyl group. 4. A parallel reaction without such a transfer also appears to occur simultaneously.

Transacylase activity of lactating bovine mammary fatty acid synthase

An assay for the transacylation reaction catalyzed by fatty acid synthase was developed which does not require model substrates or labelled acyl-derivatives of CoA. It involves the transfer of the acyl group from unlabelled CoA to [3H]CoA. This assay shows the occurrence of transacylation at a relatively high rate with a variety of substrates that the enzyme is able to utilize. The activity is unaffected by dissociation of the enzyme or modification by iodoacetamide or 2-chloroacetyl-CoA.

Fatty acid chain elongation by microsomal enzymes from the bovine meibomian gland

The composition of meibomian gland lipids suggested that fatty acid chain elongation might play a major role in the synthesis of such lipids. A fatty acid synthase preparation from the bovine meibomian gland catalyzed the formation of C16 acid and the enzyme was immunologically quite similar to that in the mammary gland. The microsomal fraction from the gland, on the other hand, catalyzed elongation of endogenous fatty acids in the presence of ATP and Mg2+ and of exogenous C18-CoA using malonyl-CoA and NADPH as the preferred reductant. The elongated products, ranging up to C28 in chain length, were found mainly as CoA esters and products derived from them. With C18-CoA as the exogenous primer, the elongation rate was linear with incubation time up to 20 min but the rate changed in a sigmoidal manner with increasing protein concentration. The elongation rate was maximal at a pH around 7.0. Typical Michaelis-Menten-type substrate saturation patterns were observed with both malonyl-CoA and NADPH. From linear double-reciprocal plots, the Km values for the two substrates were calculated to be 52 and 11 microM, respectively, with a V of about 340 pmol min-1 mg protein-1 with respect to malonyl-CoA. Exogenous CoA esters of C16 to C22 fatty acids were elongated to give products up to C28 without exhibiting any preference for the primer. The present elongation system could account for the formation of most of the very long chains found in meibomian lipids.

Inhibition in vitro of lipogenic enzymes from bovine (Bos taurus) mammary tissue by methylmalonyl-coenzyme A and coenzyme A

Bovine mammary fatty acid synthetase was inhibited by approximately 50% by 40 microM methylmalonyl-CoA; this inhibition was competitive with respect to malonyl-CoA (apparent Ki = 11 microM). Similarly, 6.25 microM coenzyme A inhibited the synthetase by 35% and this inhibition was again competitive (apparent Ki = 1.7 microM). Apparent Km for malonyl-CoA was 29 microM. The short-chain dicarboxylic acids malonic, methylmalonic and ethylmalonic at high concentrations (160-320 microM) and ATP (5 mM) enhanced the synthetase activity by about 50% respectively; the activating effects of methylmalonic acid and ATP on the synthetase were additive. Methylmalonyl-CoA at 50 microM concentration inhibited the partially purified acetyl-CoA carboxylase uncompetitively by 10% and the propionyl-CoA carboxylase activity of the enzyme preparation competitively (apparent Ki = 21 microM) by 40%. Malonyl-CoA also inhibited the acetyl-CoA carboxylase activity competitively (apparent Ki = 7 microM) by 35% and the propionyl-CoA carboxylating activity of the preparation competitively (apparent Ki = 4 microM) by 82%. The possibility that methylmalonyl-CoA may be a causal factor in the aetiology of the low milk-fat syndrome in high yielding dairy cows is discussed.