Isopropylmalate synthase

ISOPROPYLMALATE DEHYDRATASE 3-ISOPROPYLMALATE DEHYDROGENASE 2-ISOPROPYLMALATE SYNTHASE ISOPULLULANASE ISOSBESTIC POINT... 

Apart from the true Claisen condensations discussed in the previous section in which the electrophilic reaction partner is another thioester, a number of enzymes also catalyze related Claisen-like condensations in which an acyl-CoA-based nucleophile reacts with other electrophilic carbonyl groups such as ketones, aldehydes, and the carboxylate group of carboxy-biotin. The most important examples of such enzymes are hydroxymethylglutaryl-CoA (HMG-CoA) synthase, citrate and homocitrate synthase (HCS), malate and ct-isopropylmalate synthase (ct-IPMS), and the biotin-dependent acetyl- and propionyl-CoA carboxylases. 

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... 

Biosynthesis Leu is formed from pyruvic acid - 2-acetolactic acid [acetolactate synthase (EC 4.1.3.18.)+(l-hydroxyethyl)-TPP] - 2,3-dihydroxy-isovaleric acid [reductase+NAD(P)H] - 2-oxoisova-leric acid [dihydroxyacid dehydratase] - 2-isopropyl-malate [2-isopropylmalate synthase + acetyl-CoA (EC 4.1.3.12)] -> 3-isopropylmalate [isopropylmalate dehydratase (EC 4.2.1.33) -HjO+HiO] 2-oxo-isocaproate [3-isopropylmalate dehydrogenase (EC 1.1.1.85) + NAD ] L. [leucine aminotransferase (EC... 

Although properties of enzymes associated with leucine biosynthesis in plants have not been examined, isopropylmalate synthase was identified as a regulatory enzyme (Oaks, 1965a). The conversion of [l- K Jacetate to leucine was inhibited by addition of leucine to extracts prepared from maize root tips or scutella. The results of a series of control experiments indicated that the only enzyme within the pathway (Fig. 5) subject to inhibition by the end product was isopropylmalate synthase. Inhibition was specific for the pathway product leucine as its biosynthesis was not influenced by valine, isoleucine, arginine, or lysine. 

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... 

Isopropylmalate synthase 2 isopropylmalate isomerase 3 carboxyhydroxyisocaproate decarboxylase 4 leucine aminotransferase... 

The 3-methyl-2-oxobutanoate, which undergoes tranamination from alanine (Ala, A) to lead to valine (Val, V), also serves as a point from which the biosynthesis of leucine (Leu, L) can begin to be discussed. Thus, as shown in Scheme 12.18, 3-methyl-2-oxobutanoate reacts with acetyl-CoA (CH3COSC0A) in the presence of 2-isopropylmalate synthase (EC 2.3.3.13) to generate (2S)-2-isopropylmalate, which then loses water (isopropylmalate dehydratase, EC 4.2.1.33) to generate 2-isopropylmaleate. Now, water readds under the influence of the same enzyme to... 

P-leuA (WT) Isopropylmalate synthase TACCCA TTGTATGCTTC - Patek et al. (2003b)... 

Figure 19.2 Keto-add pathways for the production of n-butanol, isobutanol, and related compounds. Enzyme abbreviations Pyk, pyruvate kinase Ppc, phosphoenolpyruvate carboxylase AspC, aspartate aminotransferase ThrA, apartate kinase/homoserine dehydrogenase asd, aspartate semialdehyde dehydrogenase ThrB, homoserine kinase ThrC, threonine synthase LeuA, 2-isopropylmalate synthase LeuCD, 2-isopropylmalate hydrolyase/3-isopropylmalate...

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