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

ABSTRACT: 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.

SUBMITTER: Whittle EJ

PROVIDER: S-EPMC2567156 | biostudies-literature | 2008 Sep

REPOSITORIES: biostudies-literature

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

Whittle Edward J EJ Tremblay Amy E AE Buist Peter H PH Shanklin John J

Proceedings of the National Academy of Sciences of the United States of America 20080916 38

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 descri ...[more]

PMID: 18796606

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Project description:Enzyme and metabolic engineering offer the potential to develop biocatalysts for converting natural resources into a wide range of chemicals. To broaden the scope of potential products beyond natural metabolites, methods of engineering enzymes to accept alternative substrates and/or perform novel chemistries must be developed. DNA synthesis can create large libraries of enzyme-coding sequences, but most biochemistries lack a simple assay to screen for promising enzyme variants. Our solution to this challenge is structure-guided mutagenesis in which optimization algorithms select the best sequences from libraries based on specified criteria (i.e. binding selectivity). Here, we demonstrate this approach by identifying medium-chain (C6-C12) acyl-ACP thioesterases through structure-guided mutagenesis. Medium-chain fatty acids, products of thioesterase-catalyzed hydrolysis, are limited in natural abundance compared to long-chain fatty acids; the limited supply leads to high costs of C6-C10 oleochemicals such as fatty alcohols, amines, and esters. Here, we applied computational tools to tune substrate binding to the highly-active 'TesA thioesterase in Escherichia coli. We used the IPRO algorithm to design thioesterase variants with enhanced C12- or C8-specificity while maintaining high activity. After four rounds of structure-guided mutagenesis, we identified three thioesterases with enhanced production of dodecanoic acid (C12) and twenty-seven thioesterases with enhanced production of octanoic acid (C8). The top variants reached up to 49% C12 and 50% C8 while exceeding native levels of total free fatty acids. A comparably sized library created by random mutagenesis failed to identify promising mutants. The chain length-preference of 'TesA and the best mutant were confirmed in vitro using acyl-CoA substrates. Molecular dynamics simulations, confirmed by resolved crystal structures, of 'TesA variants suggest that hydrophobic forces govern 'TesA substrate specificity. We expect that the design rules we uncovered and the thioesterase variants identified will be useful to metabolic engineering projects aimed at sustainable production of medium-chain oleochemicals.

Project description:IntroductionObesity during pregnancy increases the risk for maternal complications like gestational diabetes, preeclampsia, and maternal inflammation. Maternal obesity also increases the risk of childhood obesity, intrauterine growth restriction (IUGR) and diabetes to the offspring. Increased circulating free fatty acids (FFAs) in obesity due to adipose tissue lipolysis induces lipoapoptosis to hepatocytes, cholangiocytes, and pancreatic-β-cells. During the third trimester of human pregnancy, there is an increase in maternal lipolysis and release of FFAs into the circulation. It is currently unknown if increased FFAs during gestation as a result of maternal obesity cause placental cell lipoapoptosis. Increased exposure of FFAs during maternal obesity has been shown to result in placental lipotoxicity. The objective of the present study is to determine saturated FFA-induced trophoblast lipoapoptosis and also to test the protective role of monounsaturated fatty acids against FFA-induced trophoblast lipoapoptosis using in vitro cell culture model. Here, we hypothesize that saturated FFAs induce placental trophoblast lipoapoptosis, which was prevented by monounsaturated fatty acids.MethodsBiochemical and structural markers of apoptosis by characteristic nuclear morphological changes with DAPI staining, and caspase 3/7 activity was assessed. Cleaved PARP and cleaved caspase 3 were examined by western blot analysis.ResultsTreatment of trophoblast cell lines, JEG-3 and JAR cells with palmitate (PA) or stearate (SA) induces trophoblast lipoapoptosis as evidenced by a significant increase in apoptotic nuclear morphological changes and caspase 3/7 activity. We observed that saturated FFAs caused a concentration-dependent increase in placental trophoblast lipoapoptosis. We also observed that monounsaturated fatty acids like palmitoleate and oleate mitigates placental trophoblast lipoapoptosis caused due to PA exposure.ConclusionWe show that saturated FFAs induce trophoblast lipoapoptosis. Co-treatment of monounsaturated fatty acids like palmitoleate and oleate protects against FFA-induced trophoblast lipoapoptosis.

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

Publications

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

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