A-Isopropylmalate

The leucme-reversible toxicity of 5,5, -trifluoroleudne (10) to microorganisms, such as Escherichia coli, has been attributed to incorporation of JO into protem [32] and to the action of 10 as a false feedback inhibitor of a-isopropylmalate... 

Scheme 10 General reaction mechanism of the CoA-dependent Claisen-type condensing enzymes, malate synthase, a-isopropylmalate synthase, citrate synthase, and homocitrate synthase.

Ahpiperidine-2,6-dicarboxylate dehydrogenase (Q) N-succinyl-2-amino-6-ketopimelate synthase succinyl diaminopimelate aminotransferase succinyl diaminopimelate desuccinylase diaminopimelate epimerase diaminopimelate decarboxylase (Q threonine dehydratase (serine dehydratase) acetolactate synthase acetohydroxy acid isomeroreductase dihydroxy acid dehydratase valine aminotransferase a-isopropylmalate synthase isopropylmalate isomerase -isopropylmalate dehydrogenase leucine aminotransferase... 

In leucine biosynthesis, the intermediate 218 on the valine pathway reacts with acetyl-CoA to yield a-isopropylmalate 225 whose configuration has been shown to be S by X-ray crystallography (200). This reaction is analogous to that catalyzed by citrate synthase, and indeed the subsequent reaction, dehydration/rehydration giving ) -isopropylmalate 226, is analogous to the conversion of citrate to isocitrate. The configuration of / -isopropylmalate 226 had been shown to be 2i ,3S (201), and so the stereochemistry of the citrate and isopropylmalate reactions was identical. j -Isopropylmalate 226 was finally converted to leucine 205 by a 4-pro-R NADH specific dehydrogenase (EC 1.1.1.85) (202) and transamination (Scheme 61). 

As expected, among the mutants excreting leucine there was a mutant constitutively derepressed with respect to the formation of the enzyme a-isopropylmalate synthetase, which is the first enzyme in the leucine biosynthetic pathway. In another mutant, this enzyme is insensitive to endproduct inhibition by leucine. However, contrary to our expectations, we found mutants carrying regulatory defects in the control of the valine-isoleucine biosynthetic pathway several mutants are constitutively derepressed with respect to the formation of aceto-hydroxy acid synthase and in one mutant this enzyme is insensitive to endproduct inhibition by valine. The selection and the existence of... 

Figure 7 Other dehydratases that may use a [4Fe-4S] cluster in an analogous manner to aconitase. A. Isopropylmalate isomerase. B. Fumarase. C. Dihydroxyacid dehydratase. D. Maleic acid hydratase.

The sequence of reactions leading to the synthesis of leucine originates with the condensation of 2-oxoisovalerate and acetyl-CoA to form 3-carboxy-3-hydroxyisocaproate (a-isopropylmalate) (Fig. 5). An isomerization which entails dehydration and rehydration results in formation of a... 

The conversion of [ C]valine to [ CJleucine would be predicted on the basis of the reversibility of most reactions catalyzed by aminotransferases and the fact that the oxoanalogue of valine is also a leucine precursor (Figs. 4 and 5). This has been observed to occur in flax and sorghum seedlings (Butler and Shen, 1963), and a duckweed (Borstlap, 1975). D,L-a-Isopropylmalate reduced the incorporation of from valine into bound (protein) but not free leucine (Butler and Shen, 1963). These results indicate that isopropylmalate is an intermediate in leucine synthesis and suggest that multiple pools of metabolites are present in plant cells (Section III,C). 

The keto-group is subsequently reduced and water is eliminated. By aldol addition, a-isopropylmalic acid is obtained. This is first dehydrated, followed by another addition of water in this way, )S-isoprop)dmalic acid is formed. Oxidative decarboxylation yields a-ketoisocaproic add, which is finally converted into leucine by reductive amination. 

The entire gene cluster responds to the signal repress or derepress as a single functional unit [12]. Departures from the predicted coordinate response can probably be explained adequately by the relatively lesser stabihty of a-isopropylmalate (a-IPM) synthetase (enzyme L-1) and a-IPM isomerase (enzyme L-2) under conditions of substrate limitation. 

Strassman, M. and Ceci, L.N. (1963) Enzymic formation of a-isopropylmalic acid, an intermediate in leucine biosynthesis. /. Biol Chem., 238,2445-2452. 

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