Dihydroxy-acid dehydratase

Lastly, it is worth mentioning the case of the dihydroxy-acid dehydratase from spinach, in which the reduced form of a [2Fe-2S] center was found to exhibit a heterogeneous EPR spectrum of the gav === 1-91 type, indicating the presence of noncysteinyl ligands at the reducible site. This signal was converted into a gav 1-96 type spectrum upon addition of the substrate, which suggests that this center may be involved in the catalytic cycle 264). 

The second step in the synthesis, catalyzed by acetohydroxyacid isomeroreductase, involves shift of an alkyl group (Fig. 24-17). Neither this reaction nor the preceding one occurs in mammals. For this reason, the enzymes required are both attractive targets for herbicide design.343 346 The third enzyme, dihydroxy acid dehydratase, catalyzes dehydration followed by tautomerization, resembling 6-phospho-... 

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 In microorganisms and plants from pyruvic acid 2 pyruvate- 2-acetolactic acid (acetolactate synthase, EC 4.1.3.18 coenzyme thiamin(e) diphosphate)- 2,3-dihydroxyisovaleric acid (2-acetolactate mutase, EC 5.4.99.3)- 2-oxoisovaleric acid (dihydroxy acid dehydratase, EC 4.2.1.9). This is finally am-inated by branched chain amino acid aminotransferase (EC 2.6.1.42). 2-Oxoisovaleric acid is also a precursor of Leu. 

Figure 5.2 Isobutanol production via vaiine pathway and 1-butanol production via CoA-dependent pathway implemented. Common enzyme abbreviations or gene symbols ilvIH, acetolactate synthase ( . coli) aIsS, aceto-lactate synthase (Bacillus subtilis) ilvC, ace-tohydroxy acid isomeroreductase ( . coli) ilvD, dihydroxy acid dehydratase ( . coli) kivd, ketoisovalerate decarboxylase (Lactoccus...

Figure 15.4 Isobutanol and 2-methyl butanol (2MB) production via keto acid pathway in cyanobacteria. Gene/protein symbols are dmA, citramalate synthase leuCD, isopropylmalate isomerase leuB, 3-isopropylmalate dehydrogenase AHAS, acetohydroxyacid synthase ilvC, acetohydroxy acid isomeroreductase ilvD, dihydroxy acid dehydratase kivd, ketoisovalerate decarboxylase yqhD, alcohol dehydrogenase.

Scheme 12.16. A representation of the final stages in the biosynthesis of isoleucine (He, I). In the late stages shown here, the migration of the ethyl group is accompanied by the reduction of the carbonyl generated in that migration (the enzyme is a reductoisomerase (EC 1.1.1.86). Dehydration to an enol (a dihydroxy acid dehydratase, EC4.2.1.9), tautomeriza-tion to the corresponding ketone, and a final transamination ( branched-chain amino acid transaminase with pyridoxal as a cofactor) from glutamate (Glu,E) produces isoleuciue (He, I). EC numbers and some graphic materials provided in this scheme have been taken from appropriate links in a URL starting with http //www.chem.qmul.ac.uk/iubmb/enzyme/.

A dihydroxy-acid dehydratase (21) has been purified to homogeneity from spinach and shown to contain a (2Fe - 2S) cluster that is likely to be essential for catalysis (Hint and Emptage, 1988). Cangelosi et al. (1986) attempted to detect enzymes involved in leucine biosynthesis by complementation of a variety of . coli auxotrophs with cosmids containing Nostoc genomic DNA. This approach... 

---