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The rates of most physical, chemical and biological processes vary with temperature and numerous instances have been reported in which Arrhenius plots of the experimental data appear to consist of two straight line segments joined by a relatively sharp break. An empirical model, based on a general hyperbola, is shown to be applicable to such systems; a(More)
Sulfonation catalyzed by sulfotransferase enzymes plays an important role in chemical defense mechanisms against various xenobiotics but also bioactivates carcinogens. A major human sulfotransferase, SULT1A1, metabolizes and/or bioactivates many endogenous compounds and is implicated in a range of cancers because of its ability to modify diverse promutagen(More)
The branched-chain amino acids are synthesized by plants, fungi and microorganisms, but not by animals. Therefore, the enzymes of this pathway are potential target sites for the development of antifungal agents, antimicrobials and herbicides. Most research has focused upon the first enzyme in this biosynthetic pathway, acetohydroxyacid synthase (AHAS)(More)
Three-dimensional structures have been determined for 13 different enzymes that use thiamine diphosphate (ThDP) as a cofactor. These enzymes fall into five families, where members within a family have similar structures. In different families, there are similarities between some domains that clearly point to a common ancestor for all of these enzymes. Where(More)
The sulfonylureas and imidazolinones are potent commercial herbicide families. They are among the most popular choices for farmers worldwide, because they are nontoxic to animals and highly selective. These herbicides inhibit branched-chain amino acid biosynthesis in plants by targeting acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This report describes the(More)
Sulfonation is an important reaction in the metabolism of numerous xenobiotics, drugs, and endogenous compounds. A supergene family of enzymes called sulfotransferases (SULTs) catalyze this reaction. In most cases, the addition of a sulfonate moiety to a compound increases its water solubility and decreases its biological activity. However, many of these(More)
Acetohydroxyacid synthase (AHAS) catalyses the first step in the synthesis of the branched-chain amino acids and is the target of several classes of herbicides. Four mutants (A122V, W574S, W574L and S653N) of the AHAS gene from Arabidopsis thaliana were constructed, expressed in Escherichia coli, and the enzymes were purified. Each mutant form and wild-type(More)
Plants and microorganisms synthesize valine, leucine and isoleucine via a common pathway in which the first reaction is catalysed by acetohydroxyacid synthase (AHAS, EC 2.2.1.6). This enzyme is of substantial importance because it is the target of several herbicides, including all members of the popular sulfonylurea and imidazolinone families. However, the(More)
When the progress curve for an enzyme catalysed reaction follows the integrated Michaelis-Menten equation, the maximum velocity and Michaelis constant can be determined from a single such curve. In this paper, an experimental design for collecting the data is proposed which is close to optimum in the sense that it produces the smallest standard error in the(More)