Desaturation of trans-octadecenoyl-acyl carrier protein by stearoyl-acyl carrier protein delta9 desaturase

Cloning and expression of stearoyl-CoA desaturase 1 (SCD-1) in the liver of the Sichuan white goose and landes goose responding to overfeeding

The EST sequence of goose (Anser cygnoides) Stearoyl-CoA desaturase 1(SCD-1) was obtained from a subtractive cDNA library. To further investigate the role of SCD-1 in lipid metabolism in geese, 5'-RACE and 3'-RACE were carried out in this study to obtain the complete cDNA sequence of goose SCD-1, which contained a 29-bp 5' UTR, a 1074-bp open reading frame (ORF) encoding 357 amino acids, and a 125-bp 3' UTR. The expression of SCD-1 was measured in several tissues, and the effects of overfeeding on the expression of SCD-1 were studied. The results of real time RT-PCR demonstrated that, compared to the brain, goose SCD-1 mRNA was more abundant in the liver. Overfeeding markedly increased the mRNA expression of SCD-1 in the liver of Sichuan White and Landes geese, and gene expression was markedly higher in the Sichuan White goose than in the landes goose. The mRNA abundance of SCD-1 in the liver had significant positive correlations with triacylglycerol (TG) content in liver lipids and in the levels of plasma insulin and very low-density lipoproteins (VLDL) levels in Sichuan white geese. However, the mRNA abundance of SCD-1 in the livers of Landes geese had only significant positive correlations with the TG content in liver lipids. In conclusion, SCD-1 is not only critical for hepatic steatosis in geese but is also important for the difference in lipid deposition in the livers of the two breeds.

Biochemical and physiological function of stearoyl-CoA desaturase

A key and highly regulated enzyme that is required for the biosynthesis of monounsaturated fatty acids is stearoyl-CoA desaturase (SCD), which catalyzes the D(9)-cis desaturation of a range of fatty acyl-CoA substrates. The preferred substrates are palmitoyl- and stearoyl-CoA, which are converted into palmitoleoyl- and oleoyl-CoA respectively. Oleate is the most abundant monounsaturated fatty acid in dietary fat and is therefore readily available. Studies of mice that have a naturally occurring mutation in the SCD-1 gene isoform as well as a mouse model with a targeted disruption of the SCD gene (SCD-1(-/-)) have revealed the role of de novo synthesized oleate and thus the physiological importance of SCD-1 expression. SCD-1 deficiency results in reduced body adiposity, increased insulin sensitivity, and resistance to diet-induced obesity. The expression of several genes of lipid oxidation are upregulated, whereas lipid synthesis genes are downregulated. SCD-1 was also found to be a component of the novel metabolic response to the hormone leptin. Therefore, SCD-1 appears to be an important metabolic control point, and inhibition of its expression could be of benefit for the treatment of obesity, diabetes, and other metabolic diseases. In this article, we summarize the recent and timely advances concerning the important role of SCD in the biochemistry and physiology of lipid metabolism.

Desaturases: emerging models for understanding functional diversification of diiron-containing enzymes

Desaturases and related enzymes perform O(2)-dependent dehydrogenations initiated at unactivated C-H groups with the use of a diiron active site. Determination of the long-sought oxidized desaturase crystal structure facilitated structural comparison of the active sites of disparate diiron enzymes. Experiments on the castor desaturase are discussed that provide experimental support for a hypothesized ancestral oxidase enzyme in the context of the evolution of the diiron enzyme diverse functionality. We also summarize recent analysis of a castor mutant desaturase that provides valuable insights into the relationship of proposed substrate-binding modes with respect to a range of catalytic outcomes.

Revealing the catalytic potential of an acyl-ACP desaturase: tandem selective oxidation of saturated fatty acids

It is estimated that plants contain thousands of fatty acid structures, many of which arise by the action of membrane-bound desaturases and desaturase-like enzymes. The details of "unusual" e.g., hydroxyl or conjugated, fatty acid formation remain elusive, because these enzymes await structural characterization. However, soluble plant acyl-ACP (acyl carrier protein) desaturases have been studied in far greater detail but typically only catalyze desaturation (dehydrogenation) reactions. We describe a mutant of the castor acyl-ACP desaturase (T117R/G188L/D280K) that converts stearoyl-ACP into the allylic alcohol trans-isomer (E)-10-18:1-9-OH via a cis isomer (Z)-9-18:1 intermediate. The use of regiospecifically deuterated substrates shows that the conversion of (Z)-9-18:1 substrate to (E)-10-18:1-9-OH product proceeds via hydrogen abstraction at C-11 and highly regioselective hydroxylation (>97%) at C-9. (18)O-labeling studies show that the hydroxyl oxygen in the reaction product is exclusively derived from molecular oxygen. The mutant enzyme converts (E)-9-18:1-ACP into two major products, (Z)-10-18:1-9-OH and the conjugated linolenic acid isomer, (E)-9-(Z)-11-18:2. The observed product profiles can be rationalized by differences in substrate binding as dictated by the curvature of substrate channel at the active site. That three amino acid substitutions, remote from the diiron active site, expand the range of reaction outcomes to mimic some of those associated with the membrane-bound desaturase family underscores the latent potential of O(2)-dependent nonheme diiron enzymes to mediate a diversity of functionalization chemistry. In summary, this study contributes detailed mechanistic insights into factors that govern the highly selective production of unusual fatty acids.

Functional characterization of a desaturase from the tobacco hornworm moth (Manduca sexta) with bifunctional Z11- and 10,12-desaturase activity

The pheromone blend produced by the tobacco hornworm moth (Manduca sexta) (L.) female is unusually complex and contains two conjugated dienals and trienals together with two monounsaturated alkenals. Here, we describe the identification and construction of two genes encoding MsexKPSE and MsexAPTQ desaturases from a cDNA library prepared from the total RNA of the M. sexta pheromone gland. The MsexKPSE desaturase shares a high degree of similarity with Delta(9)-desaturases from different moth species. The functional expression of MsexAPTQ desaturase in Saccharomyces cerevisiae followed by a detailed GC-MS analysis of fatty acid methyl esters (FAME) and their derivatized products and gas-phase Fourier transform infrared (FTIR) spectroscopy of the extracted FAME confirms that this enzyme is a bifunctional Z-Delta(11)-desaturase. MsexAPTQ desaturase catalyses the production of Z11-hexadecenoate (Z11-16) and Z10E12- and E10E12-hexadecadienoates (Z10E12-16) via 1,4-desaturation of the Z11-16 substrate. The stereochemistry of 1,4-desaturation and formation of isomers is discussed.

Reactions of the diiron enzyme stearoyl-acyl carrier protein desaturase

Stearoyl-acyl carrier protein Delta(9) desaturase (Delta9D) produces oleic acid, a nutritionally valuable fatty acid containing a cis double bond between C-9 and C-10. This multiprotein diiron enzyme complex reacts with stearoyl-acyl carrier protein, reduced [2Fe-2S] ferredoxin, and O(2) to complete the highly regiospecific and stereoselective desaturation reaction. Interactions with the acyl chain provide stability to the enzyme-substrate complex, give an energetic contribution to catalytic selectivity, and help to order the electron transfer, O(2) binding, and C-H bond cleavage steps of catalysis. Reactions with natural acyl chains indicate the involvement of a highly reactive diiron intermediate capable of oxidizing secondary C-H bonds (bond dissociation energy approximately 95 kcal/mol), but also capable of diagnostic O-atom transfer reactions with the appropriate substrate analogues. For soluble Delta9D, the natural reaction may initiate at the C-10 position, in contrast to the well-established initial reactivity of the membrane enzyme homologue stearoyl-coenzyme A (CoA) Delta(9) desaturase at the C-9 position.

Role of stearoyl-coenzyme A desaturase in lipid metabolism

Stearoyl-CoA desaturase (SCD) (EC 1.14.99.5) is an endoplasmic reticulum-bound enzyme that catalyzes the delta9-cis desaturation of saturated fatty acyl-CoAs, the preferred substrates being palmitoyl- and stearoyl-CoA, which are converted to palmitoleoyl- and oleoyl-CoA, respectively. These monounsaturated fatty acids are used as substrates for the synthesis of triglycerides, wax esters, cholesteryl esters and membrane phospholipids. The saturated to monounsaturated fatty acid ratio affects membrane phospholipid composition and alteration in this ratio has been implicated in a variety of disease states including cardiovascular disease, obesity, diabetes, neurological disease, skin disorders and cancer. Thus, the expression of SCD is of physiological importance in normal and disease states. Several mammalian SCD genes have been cloned. A single human, three mouse and two rat are the best characterized SCD genes. The physiological role of each SCD isoform and the reason for having three or more SCD gene isoforms in the rodent genome are currently unknown. A clue as to the physiological role of the SCD, at least SCD1 gene and its endogenous products came from recent studies of asebia mouse strains that have a natural mutation in the SCD1 gene and a mouse model with a targeted disruption of the SCD1 gene. In this review we discuss our current understanding of the physiological role of SCD in lipid synthesis and metabolism.

Effects of sterculic acid on stearoyl-CoA desaturase in differentiating 3T3-L1 adipocytes

The effects of sterculic acid on cell size, adiposity, and fatty acid composition of differentiating 3T3-L1 adipocytes are correlated with stearoyl-CoA desaturase (SCD) expression (mRNA and protein levels) and enzyme activity. Fluorescence-activated cell scanning (FACS) analysis showed that adipocytes differentiated with methylisobutylxanthine, dexamethasone, and insulin (MDI) plus 100 microM sterculic acid comprised a population of predominantly large cells with reduced adiposity compared to MDI-treated cells. Although both groups had similar amounts of total fat, their fatty acid profiles were strikingly different: MDI-treated cells had high levels of the unsaturated palmitoleic (Delta(9)-16:1) and oleic (Delta(9)-18:1) acids, whereas the cells cultured with MDI plus sterculic acid accumulated palmitic (16:0) and stearic (18:0) acids together with a marked reduction in Delta(9)-16:1. Although the cells treated with MDI plus sterculic acid had similar levels of scd1 and scd2 mRNAs and antibody-detectable SCD protein as the MDI-treated cells, the SCD enzyme activity was inhibited more than 90%. The accumulation of 16:0 and 18:0, together with normal levels of fatty acid synthase (FAS) and aP2 mRNAs, shows that de novo synthesis and elongation of fatty acids, as well as cell differentiation, were not affected by sterculic acid. Because of the increase in cell size in the sterculic acid-treated cells, the insulin-stimulated 2-deoxyglucose (2-DOG) uptake was determined. Compared to MDI-treated cells, the 2-DOG uptake in the cells treated with sterculic acid was not affected. These results indicate that sterculic acid directly inhibits SCD activity, possibly by a turnover-dependent reaction, without affecting the processes required for adipocyte differentiation, scd gene expression or SCD protein translation.

Fatty acid desaturation: variations on an oxidative theme

Significant progress in our understanding of the mechanism of fatty acid desaturation has been achieved. The site of initial oxidation has been determined for several membrane-bound desaturases and a common cryptoregiochemical theme has been revealed. The results of several studies, including a detailed analysis of a soluble plant desaturase system, point to a close mechanistic relationship between dehydrogenation and hydroxylation pathways.

Desaturation, chain scission, and register-shift of oxygen-substituted fatty acids during reaction with stearoyl-ACP desaturase

Stearoyl acyl carrier protein Delta(9) desaturase catalyzes the NADPH- and O(2)-dependent insertion of a cis double bond between the C-9 and C-10 positions of the acyl chain in the kinetically preferred natural substrate 18:0-ACP. In this work, substrate analogues with an oxygen atom singly replacing the methylene groups at the 8, 9, 10, and 11 positions of the stearoyl chain were synthesized, converted to acyloxy-ACPs, and used as probes of desaturase reactivity. Evidence for desaturation, acyloxy chain scission, and register-shift in binding prior to chain scission was obtained. Reactions with acyloxy-ACPs having either O-8 or O-11 substitutions gave a single desaturation product consistent with the insertion of a cis double bond between C-9 and C-10. The k(cat)/K(M) values for the O-8- and O-11-substituted acyloxy-ACPs were comparable to that of the natural substrate, indicating that the presence of an ether group adjacent to the site of reactivity did not significantly interfere either with the desaturation reaction or with the binding of substrate in the proper register for desaturation between C-9 and C-10. For reactions with the O-9 and O-10 acyloxy-ACPs, the k(cat) values were decreased to approximately 3% of that observed for 18:0-ACP, and upon reaction, the acyloxy chain was broken to yield an omega-hydroxy fatty alkanoyl-ACP and a volatile long-chain aldehyde. For the O-9 substitution, 8-hydroxyoctanoate and 1-nonanal were obtained, corresponding to the anticipated binding register and subsequent reaction between the O-9 and C-10 positions. In contrast, the O-10 substitution yielded 9-hydroxynonanoyl-ACP and 1-octanal, corresponding to an obligate "register-shift" of acyloxy chain binding prior to reaction between the O-10 and C-11 positions. Register-shift is thus defined as a mechanistically relevant misalignment of acyl chain binding that results in reaction at positions other than between C-9 and C-10. The inability of the O-10 acyloxy probe to undergo reaction between the C-9 and O-10 positions provides evidence that the Delta9D-catalyzed desaturation of stearoyl-ACP may initiate at C-10. Possible mechanisms of the acyl chain scission and implications of these results for the desaturation mechanism are considered.

Crystal structures of the soluble methane monooxygenase hydroxylase from Methylococcus capsulatus (Bath) demonstrating geometrical variability at the dinuclear iron active site

The oxidation of methane to methanol is performed at carboxylate-bridged dinuclear iron centers in the soluble methane monooxygenase hydroxylase (MMOH). Previous structural studies of MMOH, and the related R2 subunit of ribonucleotide reductase, have demonstrated the occurrence of carboxylate shifts involving glutamate residues that ligate the catalytic iron atoms. These shifts are thought to have important mechanistic implications. Recent kinetic and theoretical studies have also emphasized the importance of hydrogen bonding and pH effects at the active site. We report here crystal structures of MMOH from Methylococcus capsulatus (Bath) in the diiron(II), diiron(III), and mixed-valent Fe(II)Fe(III) oxidation states, and at pH values of 6.2, 7.0, and 8.5. These structures were investigated in an effort to delineate the range of possible motions at the MMOH active site and to identify hydrogen-bonding interactions that may be important in understanding catalysis by the enzyme. Our results present the first view of the diiron center in the mixed-valent state, and they indicate an increased lability for ferrous ions in the enzyme. Alternate conformations of Asn214 near the active site according to redox state and a distortion in one of the alpha-helices adjacent to the metal center in the diiron(II) state have also been identified. These changes alter the surface of the protein in the vicinity of the catalytic core and may have implications for small-molecule accessibility to the active site and for protein component interactions in the methane monooxygenase system. Collectively, these results help to explain previous spectroscopic observations and provide new insight into catalysis by the enzyme.