Substrate (chemistry)

About: Substrate (chemistry) is a research topic. Over the lifetime, 35902 publications have been published within this topic receiving 740722 citations. The topic is also known as: enzyme substrate.

The structural basis for specificity in lipoxygenase catalysis

Abstract: Many intriguing facets of lipoxygenase (LOX) catalysis are open to a detailed structural analysis. Polyunsaturated fatty acids with two to six double bonds are oxygenated precisely on a particular carbon, typically forming a single chiral fatty acid hydroperoxide product. Molecular oxygen is not bound or liganded during catalysis, yet it is directed precisely to one position and one stereo configuration on the reacting fatty acid. The transformations proceed upon exposure of substrate to enzyme in the presence of O2 (RH + O2 → ROOH), so it has proved challenging to capture the precise mode of substrate binding in the LOX active site. Beginning with crystal structures with bound inhibitors or surrogate substrates, and most recently arachidonic acid bound under anaerobic conditions, a picture is consolidating of catalysis in a U-shaped fatty acid binding channel in which individual LOX enzymes use distinct amino acids to control the head-to-tail orientation of the fatty acid and register of the selected pentadiene opposite the non-heme iron, suitably positioned for the initial stereoselective hydrogen abstraction and subsequent reaction with O2 . Drawing on the crystal structures available currently, this review features the roles of the N-terminal β-barrel (C2-like, or PLAT domain) in substrate acquisition and sensitivity to cellular calcium, and the α-helical catalytic domain in fatty acid binding and reactions with O2 that produce hydroperoxide products with regio and stereospecificity. LOX structures combine to explain how similar enzymes with conserved catalytic machinery differ in product, but not substrate, specificities.

Detoxification of substituted phenols by oxidoreductive enzymes through polymerization reactions

Abstract: Laccases from the fungiRhizoctonia praticola andTrametes versicolor as well as horseradish peroxidase and tyrosinase were evaluated for their ability to polymerize phenolic contaminants. The removal of phenols through polymerization depended on the chemical structure and concentration of the substrate, pH of the reaction mixture, activity of the enzyme, length of incubation, and temperature. The enzymes retained their activity throughout a broad range of pH (pH 3.0 to 10) and temperature (5 to 55°C). The removal of halogenated phenols decreased with increasing number of chlorines and increasing molecular weight of the substituent. Laccases fromR. praticola andT. versicolor removed 2,4-dichlorophenol at initial concentrations of up to 1,600 mg/L. The amount of the substrate removed increased with increasing enzyme activity. The precipitates formed during polymerization of 2,4-dichlorophenol constituted a mixture of oligomers with average molecular weights of up to 800 for the fraction soluble in dioxane. Mass spectra revealed the loss of chlorine atoms during enzymatic polymerization. The release of chloride ions into solution during polymerization amounted to up to 20% of the chlorine initially associated with the 2,4-dichlorophenol molecule. Dechlorination contributes to the overall detoxification effect which results from enzymatic polymerization.

Catalytic oxidations of steroid substrates by artificial cytochrome p-450 enzymes.

Abstract: Catalysts comprising manganese-porphyrins carrying cyclodextrin binding groups are able to perform hydroxylations with substrate selectivity and regio- and stereoselectivity and high catalytic turnovers. The geometries of the catalyst/substrate complexes override intrinsic substrate reactivities, permitting attack on geometrically accessible saturated carbons of steroids in the presence of secondary carbinol groups and carbon-carbon double bonds, as in enzymatic reactions. Selective hydroxylations of steroid carbon 9 positions are of particular practical interest.