Enzyme Catalysis
Mostrando 49-60 de 558 artigos, teses e dissertações.
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49. Substrate specificity of catechol 2,3-dioxygenase encoded by TOL plasmid pWW0 of Pseudomonas putida and its relationship to cell growth.
Catechol 2,3-dioxygenase encoded by TOL plasmid pWW0 of Pseudomonas putida consists of four identical subunits, each containing one ferrous ion. The enzyme catalyzes ring cleavage of catechol, 3-methylcatechol, and 4-methylcatechol but shows only weak activity toward 4-ethylcatechol. Two mutants of catechol 2,3-dioxygenases (4ECR1 and 4ECR6) able to oxidize
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50. Recognition and catalysis in nucleic acid chemistry.
The enzyme ribonuclease A catalyzes the cleavage of RNA, using the imidazole groups of histidine-12 and histidine-119 as its principal catalytic groups. Model studies show that RNA can be cleaved by imidazole buffer itself and that, as in the enzyme, a bell-shaped pH vs. rate profile is seen. This indicates that one imidazole functions as a base, while the o
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51. Covalent catalysis in nucleotidyl transfer reactions: essential motifs in Saccharomyces cerevisiae RNA capping enzyme are conserved in Schizosaccharomyces pombe and viral capping enzymes and among polynucleotide ligases.
Formation of the 5' cap structure of eukaryotic mRNAs occurs via transfer of GMP from GTP to the 5' terminus of the primary transcript. RNA guanylyltransferase, the enzyme that catalyzes this reaction, has been isolated from many viral and cellular sources. Though differing in molecular weight and subunit structure, the various guanylyltransferases employ a
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52. Snapshot of a key intermediate in enzymatic thiamin catalysis: Crystal structure of the α-carbanion of (α,β-dihydroxyethyl)-thiamin diphosphate in the active site of transketolase from Saccharomyces cerevisiae
Kinetic and spectroscopic data indicated that addition of the donor substrate hydroxypyruvate to the thiamin diphosphate (ThDP)-dependent enzyme transketolase (TK) led to the accumulation of the α-carbanion/enamine of (α,β-dihydroxyethyl) ThDP, the key reaction intermediate in enzymatic thiamin catalysis. The three-dimensional structure of this intermedia
The National Academy of Sciences.
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53. 3-hydroxy-3-methylglutaryl–CoA synthase intermediate complex observed in “real-time”
The formation of carbon–carbon bonds via an acyl-enzyme intermediate plays a central role in fatty acid, polyketide, and isoprenoid biosynthesis. Uniquely among condensing enzymes, 3-hydroxy-3-methylglutaryl (HMG)–CoA synthase (HMGS) catalyzes the formation of a carbon–carbon bond by activating the methyl group of an acetylated cysteine. This reaction
National Academy of Sciences.
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54. Communication between the active sites and dimer interface of a herpesvirus protease revealed by a transition-state inhibitor
Structurally diverse organophosphonate inhibitors targeting the active site of the enzyme were used to investigate the relationship of the active site and the dimer interface of wild-type protease in solution. Positional scanning synthetic combinatorial libraries revealed Kaposi's sarcoma-associated herpesvirus protease to be highly specific, even at sites d
National Academy of Sciences.
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55. How important are entropic contributions to enzyme catalysis?
The idea that enzymes accelerate their reactions by entropic effects has played a major role in many prominent proposals about the origin of enzyme catalysis. This idea implies that the binding to an enzyme active site freezes the motion of the reacting fragments and eliminates their entropic contributions, (ΔScat‡)′, to the activation energy. It
The National Academy of Sciences.
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56. Regulation of ribulose bisphosphate carboxylase activity in vivo by a light-modulated inhibitor of catalysis
The activity of ribulose 1,5-bisphosphate carboxylase [RuBPCase; 3-phospho-D-glycerate carboxylyase (dimerizing), EC 4.1.1.39] in leaf extracts of a number of species kept in the dark overnight was found to be very low. This was not the result of a change in the activation state or in the amount of enzyme that could be extracted from “dark” leaves. Rathe
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57. Unifying concept for the coupling between ion pumping and ATP hydrolysis or synthesis.
A mechanism is proposed for the coupling between ion transport and enzyme catalysis. The basic concept is that enzymes associated with transport exist in two possible conformations. Each conformation has the potential of catalyzing the enzymatic reaction, and pumping is associated with the conversion of one conformational form to the other. The conformationa
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58. Old Yellow Enzyme: Stepwise reduction of nitro-olefins and catalysis of aci-nitro tautomerization
The Old Yellow Enzyme has been shown to catalyze efficiently the NADPH-linked reduction of nitro-olefins. The reduction of the nitro-olefin proceeds in a stepwise fashion, with formation of a nitronate intermediate that is freely dissociable from the enzyme. The first step involves hydride transfer from the enzyme-reduced flavin to carbon 2 of the nitro
The National Academy of Sciences.
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59. Functional consequences of single amino acid substitutions in calmodulin-activated adenylate cyclase of Bordetella pertussis.
Calmodulin-activated adenylate cyclase of Bordetella pertussis and Bacillus anthracis are two cognate bacterial toxins. Three short regions of 13-24 amino acid residues in these proteins exhibit between 66 and 80% identity. Site-directed mutagenesis of four residues in B. pertussis adenylate cyclase situated in the second (Asp188, Asp190) and third (His298,
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60. Effect of amino acid substitutions on the catalytic and regulatory properties of aspartate transcarbamoylase.
Although intensive investigations have been conducted on the allosteric enzyme, aspartate transcarbamoylase, which catalyzes the first committed reaction in the biosynthesis of pyrimidines in Escherichia coli, little is known about the role of individual amino acid residues in catalysis or regulation. Two inactive enzymes produced by random mutagenesis have