Molecular cloning and expression of Spirulina platensis acetohydroxy acid synthase genes in Escherichia coli

Bacterial Branched-Chain Amino Acid Biosynthesis: Structures, Mechanisms, and Drugability

The eight enzymes responsible for the biosynthesis of the three branched-chain amino acids (l-isoleucine, l-leucine, and l-valine) were identified decades ago using classical genetic approaches based on amino acid auxotrophy. This review will highlight the recent progress in the determination of the three-dimensional structures of these enzymes, their chemical mechanisms, and insights into their suitability as targets for the development of antibacterial agents. Given the enormous rise in bacterial drug resistance to every major class of antibacterial compound, there is a clear and present need for the identification of new antibacterial compounds with nonoverlapping targets to currently used antibacterials that target cell wall, protein, mRNA, and DNA synthesis.

Engineering Spirulina for Enhanced Medicinal Application

Cyanobacteria are prokaryotes which can perform photosynthesis like higher plants. Their genomic organisation is very simple and thus is suitable for the study of detailed photosynthesis mechanism at a molecular level also for many other genomic manipulations relevant to benefit of living organisms. This unicellular alga, Spirulina has a thin thread like elongated structure and classified under Cyanobacteriaceae which is blue green in colour. Under microscope it looks like bunch of bright helical threads (Fig. 11.1).

Regulation by external pH and stationary growth phase of the acetolactate synthase from Synechocystis PCC6803

Several characteristics identify the protein encoded by the alsS gene [sll1981 in Cyanobase (http://www.kazusa.or.jp/cyano/cyano. html)] of Synechocystis PCC6803 as an acetolactate synthase. The AlsS protein is about 60% homologous to the AlsS from Bacillus subtilis or other bacteria. These enzymes condense two pyruvates to form acetolactate, implicated in pH homeostasis via the acetoin-2, 3-butanediol pathway or in valine biosynthesis. Transcriptional fusions revealed that alsS was induced at the onset of stationary phase, as in B. subtilis, a situation leading to an increase in the pHout to above 11 in Synechocystis. This is the first cyanobacterial gene showing a dependence on pH for its expression. Induction was also obtained by the presence of > 100 mM Na+, the effect being prevented by amiloride, in agreement with Na+/H+ exchange in the pH homeostasis process. Homology of the Synechocystis AlsS protein to the close family of acetohydroxy acid synthases (including one in Synechocystis) is around 30%. These enzymes are involved in the parallel routes for valine/leucine and isoleucine biosynthesis. No phenotype of auxotrophy for any of these amino acids was associated with a null mutation in the Synechocystis alsS gene. The AlsS enzyme did not complement the isoleucine deficiency of an acetohydroxy acid synthase-deficient Escherichia coli mutant.

Biosynthesis of 2-aceto-2-hydroxy acids: acetolactate synthases and acetohydroxyacid synthases

Two groups of enzymes are classified as acetolactate synthase (EC 4. 1.3.18). This review deals chiefly with the FAD-dependent, biosynthetic enzymes which readily catalyze the formation of acetohydroxybutyrate from pyruvate and 2-oxobutyrate, as well as of acetolactate from two molecules of pyruvate (the ALS/AHAS group). These enzymes are generally susceptible to inhibition by one or more of the branched-chain amino acids which are ultimate products of the acetohydroxyacids, as well as by several classes of herbicides (sulfonylureas, imidazolinones and others). Some ALS/AHASs also catalyze the (non-physiological) oxidative decarboxylation of pyruvate, leading to peracetic acid; the possible relationship of this process to oxygen toxicity is considered. The bacterial ALS/AHAS which have been well characterized consist of catalytic subunits (around 60 kDa) and smaller regulatory subunits in an alpha2beta2 structure. In the case of Escherichia coli isozyme III, assembly and dissociation of the holoenzyme has been studied. The quaternary structure of the eukaryotic enzymes is less clear and in plants and yeast only catalytic polypeptides (homologous to those of bacteria) have been clearly identified. The presence of regulatory polypeptides in these organisms cannot be ruled out, however, and genes which encode putative ALS/AHAS regulatory subunits have been identified in some cases. A consensus sequence can be constructed from the 21 sequences which have been shown experimentally to represent ALS/AHAS catalytic polypeptides. Many other sequences fit this consensus, but some genes identified as putative 'acetolactate synthase genes' are almost certainly not ALS/AHAS. The solution of the crystal structures of several thiamin diphosphate (ThDP)-dependent enzymes which are homologous to ALS/AHAS, together with the availability of many amino acid sequences for the latter enzymes, has made it possible for two laboratories to propose similar, reasonable models for a dimer of catalytic subunits of an ALS/AHAS. A number of characteristics of these enzymes can now be better understood on the basis of such models: the nature of the herbicide binding site, the structural role of FAD and the binding of ThDP-Mg2+. The models are also guides for experimental testing of ideas concerning structure-function relationships in these enzymes, e.g. the nature of the substrate recognition site. Among the important remaining questions is how the enzyme suppresses alternative reactions of the intrinsically reactive hydroxyethylThDP enamine formed by the decarboxylation of the first substrate molecule and specifically promotes its condensation with 2-oxobutyrate or pyruvate.