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.

Structure and Function of Arginases

Abstract: The arginases catalyze the divalent cation dependent hydrolysis of L-arginine to produce L-ornithine and urea. Although traditionally considered in terms of its role as the final enzyme of the urea cycle, the enzyme is found in a variety of nonhepatic tissues. These findings suggest that the enzyme may have other functions in addition to its role in nitrogen metabolism. High-resolution crystal structures have been determined for recombinant rat liver (type I) arginase and for recombinant human kidney (type II) arginase, their variants, and complexes with products and inhibitors. Each identical subunit of the trimeric enzyme contains an active site that lies at the bottom of a 15 A deep cleft. The 2 essential Mn(II) ions are located at the bottom of this cleft, separated by approximately 3.3 A and bridged by oxygens derived from 2 aspartic acid residues and a solvent-derived hydroxide. This metal bridging hydroxide is proposed to be the nucleophile that attacks the guanidinium carbon of substrate arginine. On the basis of this proposed mechanism, boronic acid inhibitors of the enzyme have been synthesized and characterized kinetically and structurally. These inhibitors display slow-onset inhibition at the pH optimum of the enzyme, and are found as tetrahedral species at the active site, as determined by X-ray diffraction. The potent inhibition of arginases I and II by these compounds has not only delineated key enzyme-substrate interactions, but has also led to a greater understanding of the role of arginase in nonhepatic tissues.

[107] Enzymes involved in the biosynthesis of tryptophan

Abstract: Publisher Summary When a group of tryptophan auxotrophs are obtained after treatment of a tryptophan-nonrequiring microorganism with a mutagenic agent, many biochemically different mutant types are usually represented. Extracts of most of these mutants are unable to catalyze the formation of tryptophan from its precursor, anthranilic acid. One of the mutant extracts contains the enzymes required to convert the substrate supplied, anthranilic acid, to some intermediate, but lacks one of the enzymes required to further metabolize the intermediate. The extract of the second mutant supplies this deficient enzyme and allows the complete conversion to take place. Use of the proper mutant extract alone, therefore, with the appropriate substrate, permits the formation and the accumulation of an intermediate which otherwise might not have been detected. Thus mutant extracts provide a ready source of specific enzymatic activities, essentially free of the next sequentiallyrelated enzymatic activity. The chapter describes the formation, assay method, purification procedure, and properties of anthranilic deoxyribulotide. The assay procedure is based on the decrease of the characteristic fluorescence due to anthranilie acid during the course of the enzymatic reaction. With ATP and ribose-5-P as substrates for PP-ribose-P synthesis, anthranilie acid utilization is proportional to enzyme concentration over a range of enzyme dilutions. The same is true with PPribose- P as substrate.

Electron‐transfer coupling in microbial fuel cells. 2. performance of fuel cells containing selected microorganism—mediator—substrate combinations

Abstract: Various phenoxazine, phenothiazine, phenazine, indophenol and bipyridilium derivatives were tested for their effectiveness as redox mediators in microbial fuel cells containing Alcaligenes eutrophus, Bacillus subtilis, Escherichia coli, or Proteus vulgaris as the active biological agent, and glucose or succinate as the oxidisable substrate. A ferricyanide-Pt cathode was used. The open-circuit cell e.m.f.′s increased in the order of increasing negative formal redox potentials at pH 7(E7m) of the redox compounds. Several of the redox agents worked well as mediators, maintaining steady currents over several hours, and thionine was found to be particularly effective in maintaining relatively high cell voltages when current was drawn from the cell. A number of the compounds tested did not function well, either because they were incompletely or slowly reduced by the microorganisms or because of their instability. P. vulgaris, with thionine as mediator and glucose as substrate, showed the best performance in a fuel cell. This system was examined in some detail under various conditions of external load to establish the effects of organism concentration, mediator concentration, and substrate addition. Coulombic outputs from these cells were calculated by integration of the current-time plots. Coulombic yields of 30–60% were obtained, on the basis of (theoretical) complete oxidation of added substrate to CO2 and water.

Electrochemical production of hydroxyl radical at polycrystalline Nb-doped TiO2 electrodes and estimation of the partitioning between hydroxyl radical and direct hole oxidation pathways

Abstract: The use of TiO_2 as a photocatalyst for the destruction of organic chemical pollutants in aqueous systems has been extensively studied. One obstacle to the effective utilization of these systems is the relatively inefficient use of the solar spectrum by the photocatalyst. In addition, light delivery to the photocatalyst can be impeded by UV-absorbing components in mixed effluent streams. We present a novel use of TiO_2 as a catalyst for the oxidative degradation of organic compounds in water that uses a potential source instead of light to generate reactive oxidants. Application of an anodic bias of >+2 V vs NHE to titanium electrodes coated with niobium-doped, polycrystalline TiO_2 particles electrochemically generates hydroxyl radicals at the TiO_2 surface. This process has been demonstrated to efficiently degrade a variety of environmentally important pollutants. In addition, these electrodes offer a unique opportunity to probe mechanistic questions in TiO_2 catalysis. By comparing substrate degradation rates with increases in current density upon substrate addition, the extent of degradation due to direct oxidation and •OH oxidation can be quantified. The branching ratio for these two pathways depends on the nature of the organic substrate. Formate is shown to degrade primarily via a hydroxyl radical mechanism at these electrodes, whereas the current increase data for compounds such as 4-chlorocatechol indicate that a higher percentage of their degradation may occur through direct oxidation. In addition, the direct oxidation pathway is shown to be more important for 4-chlorocatechol, a strongly adsorbing substrate, than for 4-chlorophenol, which does not adsorb strongly to TiO_2.