2-oxobutyrate synthase

This enzyme [EC 1.2.7.2], also known as 2-oxobutyrate synthase, catalyzes the reaction of o -ketobutyrate (or,... 

Serine hydro xymethyltransferase Threonine aldolase 8-Aminolevulinate synthase Serine palmitoyltransferase 2-Amino-3-oxobutyrate-CoA ligase... 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

As indicated in Fig. 24-17, pyruvate is the starting material for the formation of both l- and D-alanine and also the branched chain amino acids valine, leucine, and isoleucine.339,340 The chemistry of the reactions has been discussed in the sections indicated in the figure. The first step is catalyzed by the thiamin diphosphate-dependent acetohydroxyacid synthase (acetolactate synthase), which joins two molecules of pyruvate or one of pyruvate and one of 2-oxobutyrate (Fig. 24-17 Fig. 14-3).340a b In E. coli there are two isoenzymes encoded by genes ilv B and ilv HI. Both are regulated by feedback inhibition by valine, probably... 

A more direct y replacement of the hydroxyl of homocysteine or 0-phosphohomoserine by a sulfide ion has also been reported for both Neurospora and green plants.Methylation of homocysteine to methionine (Fig. 24-13) has been considered previously, as has the conversion of homoserine to threonine by homoserine kinase and the PLP-dependent threonine synthase (p. 746, Fig. i4-7).254-255a standard PLP-requiring P elimination converts threonine to 2-oxobutyrate, a precursor to isoleucine (Fig. 24-13). ... 

Cystathionine-y-synthase isolated from Salmonella typhimurium is a tetramer (molecular weight 160000) and catalyses, in vivo, the y-replacement of O-suc-cinylhomoserine with cysteine [79] to yield cystathionine. The latter, by way of homocysteine, is involved in the biosynthesis of methionine. In other species of bacteria and plants the succinyl moiety may be replaced by acetyl, phosphoryl, or malonyl moieties [80]. In the absence of cysteine the enzyme catalyses an abnormal reaction resulting in the formation of a-oxobutyrate. The latter reaction has been utilised for mechanistic investigations pertinent to the y-eUmination-deamination process (vide infra). 

The subsequent conversion of 2-oxobutyrate to isoleucine involves four enzymes. The same enzymes are considered to participate in the biosynthesis of valine (Fig. 4). Thus, 2-oxobutyrate and its three carbon analogue, pyruvate, would be alternate substrates of acetohydroxyacid synthase. This parallel reaction sequence (Fig. 4) is initiated by the addition of a two-carbon fragment to the 2-carbon of the 2-oxobutyrate or pyruvate. The resultant acetohydroxyacids are reduced with concomitant isomerization to form dihydroxy acids. Dehydration yields oxoacids which are then transaminated to synthesize isoleucine and valine. Both 2-oxoisovalerate and 2-oxo-3-methyl-valerate have been identified as components of plant extracts (Kretovich and Gejko, 1964). 

Although pyruvate and 2-oxobutyrate are substrates of acetohydroxyacid synthase, measurements of the activity of this enzyme have been almost exclusively based on the production of acetolactate from pyruvate. This reaction product is readily decarboxylated under acidic conditions and the acetoin produced can be measured spectrophotometrically. However, ace-toin can be formed during reactions which need not be related to amino acid biosynthesis. Therefore it is unclear whether the enzyme activity characterized by Saytanarayana and Radhakrishnan (1963) can be completely ascribed to acetohydroxyacid synthase. Only a portion of the acetolactate forming activity measured in pea extracts was considered to represent the activity of this enzyme (Davies, 1964). However, the enzyme(s) isolated from barley was shown to facilitate formation of acetohydroxy derivatives of 2-oxobutyrate and pyruvate (Miflin, 1971). Mg or Mn " " as well as the substrate, hydroxyethylthiamine-pyrophosphate, was required for maximum enzyme activity. The fact that the acetolactate forming activity of the barley... 

Regulation of the synthesis of the branched-chain amino acids, like that of the aspartate family, can be viewed in a temporal framework (Fig. 8). However, the nature of the controls associated with the pathway enzymes do not necessarily suggest an obligatory sequence of regulatory interactions. The sequence illustrated in Fig. 8 assumes that each of the end-products would initially be synthesized from its respective precursors. As isoleucine biosynthesis is reduced by inhibition of threonine dehydratase, the competition between pyruvate and 2-oxobutyrate for the active site of acetohydroxyacid synthase would be diminished. This could result in an increased rate of synthesis of leucine and valine (Fig. 8, 2). Leucine would eventually inhibit isopropylmalate synthase and, to a lesser extent, acetohydroxyacid synthase (Fig. 8, 3). The reduced flow of carbon through the pathway would be utilized for the synthesis of valine. As the concentration of valine increased, the activity of acetohydroxyacid synthase would be sharply curtailed due to... 

From a biochemical point of view, the transfer of the hydroxethyl group to 2-oxoisovalerate (Fig. 5 B, reaction 3) should proceed analogously to its transfer to pyruvate and 2-oxobutyrate in valine and isoleucine biosynthesis (Fig. 5 A, B, reaction 1) which is catalyzed by acetolactate synthase. One just has to assume an extension of the substrate specificity of the enzyme [43]. 

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