Purification and properties of the polymeric fatty acid synthetase from a filamentous fungus

Purification and characterization of fatty acid synthetase from Cryptococcus neoformans

Fatty acid synthetase has been purified from Cryptococcus neoformans 450 fold to a specific activity of 3.6 units per mg protein with an overall yield of 23%. The purified enzyme contained two non-identical subunits, Mr approximately 2.1 x 10(5) and 1.8 x 10(5). Under optimum conditions, 100 mM KCl and pH 7.5, apparent K(m) values for the substrates were: Acetyl CoA, 19 microM; Malonyl CoA, 5 microM; and NADPH, 6 microM. Product inhibition patterns were determined to be: CoA, competitive versus acetyl CoA and malonyl CoA, uncompetitive versus NADPH; NADP, competitive versus NADPH, uncompetitive versus acetyl CoA and malonyl CoA; Palmitoyl CoA, competitive versus malonyl CoA, noncompetitive versus acetyl CoA and NADPH; Bicarbonate, uncompetitive versus malonyl CoA. These product inhibition patterns are consistent with the multisite ping-pong mechanism previously proposed for the avian fatty acid synthetase complex. The cryptococcal fatty acid synthetase was inhibited by the polyanionic polymers, heparin and dextran sulfate, an effect never before demonstrated for a fatty acid synthetase. This inhibition exhibited a marked dependence on the length of the polymer chain, with dextran sulfate fractions with Mr of 6 x 10(5) and above having Ki values below 100 nanomolar. A model is presented that involves initial binding of the anionic polymer to the enzyme complex at a region of high positive charge density, followed by interaction of the end of the tethered polymer with the catalytic site. This study represents the first purification of fatty acid synthetase from a basidiomycete.

Structure of bacterial fatty acid synthetase from Brevibacterium ammoniagenes

Hydrodynamic measurements and a cross-linking study with dimethyl suberimidate have shown that the native fatty acid synthetase from Brevibacterium ammoniagenes is a hexameric protein having a molecular weight of 1.56 . 10(6). The subunits of the enzyme are identical in size (Mr 2.6 . 10(5). The negatively stained fatty acid synthetase had an electron microscopic image of ellipsoidal structure with major and minor axes approximately equal to 270 A and 180 A, respectively. The electron microscopic image is similar to that of the yeast enzyme, which is quite distinct from the B. ammoniagenes enzyme with respect to the subunit composition. The inactivated enzyme prepared by dialysis against a lower ionic strength solution was partially reactivated by raising the ionic strength. Ellipsoidal images similar to those of the native enzyme were found in the electron micrograph of the reactivated enzyme. Sucrose density gradient centrifugation of the reactivated enzyme sample showed that the active component had almost the same sedimentation coefficient as the native hexamer. These results indicate that the enzyme is active only in its hexameric state.

Purification and properties of the fatty acid synthetase complex from the marine dinoflagellate, Crypthecodinium cohnii

De novo biosynthesis of fatty acids in the heterotrophic marine dinoflagellate, Crypthecodinium cohnii, has been studied in vitro. Fatty acid synthetase was located in the cytosol and its activity was dependent on acetyl-CoA, malonyl-CoA, NADPH2 and NADH2. The enzyme was purified 100-fold using ion-exchange chromatography on DEAE-Sephadex A-25, adsorption to hydroxyapatite and gel filtration on Sepharose 4B columns. Very active endogenous proteases were separated from the fatty acid synthetase at the first step of purification. The purified enzyme had a molecular weight of about 400000, as judged from gel filtration, sucrose density gradient centrifugation and polyacrylamide gel electrophoresis under non-denaturing conditions. Polyacrylamide gel electrophoresis under denaturing conditions in the presence of SDS and urea revealed one major protein band of Mr 180000, suggesting that the enzyme is composed of two multifunctional subunits of apparently identical molecular weight. Reaction products of the C. cohnii fatty acid synthetase are free fatty acids due to the presence of a thioesterase activity in the purified enzyme complex. The main product is palmitate. Docosahexaenoic acid (C22:6, n-3), the major fatty acid component of C. cohnii lipids, is not directly synthesized by the enzyme.

Characterization of subspecies from a fungal fatty acid synthetase

Fatty acid synthetase from the filamentous fungus, Aspergillus fumigatus dissociates during gel filtration on Sepharose 6B into two differently sized subspecies with mol. wt. approx. 1.5 x 10(6) and 8 x 10(5). After elution, they readily reform intact molecules, as determined by their enzymic activity (overall synthetase and 3-oxoreductase activities were measured), sedimentation coefficient and appearance in the electron microscope. Synthetase was cross-linked with dimethyl suberimidate and the resultant protein did not dissociate on Sepharose 6B. The two smaller species which were eluted after chromatography of untreated enzyme were also fixed by reaction with this reagent. They did not reform intact molecules of synthetase and were characterized by electron microscopy as large and small circular aggregates; the low molecular weight form also contained tetrameric structures which exhibited cyclic symmetry. The composition of the two species derived during dissociation was, therefore, confirmed as eight and four polypeptides, respectively; each contained polypeptides A and B. It is proposed that the intact fungal synthetase of composition A6B6 comprises three equivalent loops of protein, each of which contain four polypeptides, presumably with composition A2B2; the molecular weights of A and B are 207 000 and 201 000, respectively. During filtration on Sepharose 6B, two such loops remain associated to form a large circle, leaving the other four polypeptides ro rearrange themselves into a small circle or tetramer.

Mechanism of chain length determination in biosynthesis of milk fatty acids

Fatty acid synthetases isolated from all mammalian tissues synthesize predominately palmitic acid. However, in vivo the mammary gland fatty acid synthetases of some species are responsible for the synthesis of medium chain fatty acids. The objective of this presentation is to outline the mechanism which regulates the product specificity of fatty acid synthetases in general and to illustrate how this control is modified in the mammary gland. Fatty acid synthetases isolated from mammalian tissues are composed of two similar, probably identical, polypeptides, each carrying as many as seven enzyme components. Thioesterase I, the component which functions to terminate growth of acyl chains on the multienzyme, is located at one terminus of each polyfunctional polypeptide and can be detached by limited proteolysis with trypsin. By studying separately the kinetics of chain elongation by the core of the trypsinized complex and of chain termination by the isolated thioesterase I component, it has been possible to establish that the specificities of the elongation and termination reactions account for the synthesis of predominantly the carbon-16 fatty acid by purified fatty acid synthetases. Mammary glands of some species contain an additonal enzyme, thioesterase II, which can modify the product specificity of fatty acid synthetase by hydrolyzing medium chain acyl moieties from thioester linkage to the 4'phosphopantetheine of the multienzyme. At all stages of development of rat mammary gland, the amount of theoesterase II present correlates well with the proportion of medium chain fatty acids synthesized by the gland. This mammary gland-specific thioesterase appears responsible for the ability of this tissue to synthesize medium chain fatty acids characteristic of milk fat.