The synthesis of leucine in maize embryos

Two Arabidopsis genes (IPMS1 and IPMS2) encode isopropylmalate synthase, the branchpoint step in the biosynthesis of leucine

Heterologous expression of the Arabidopsis (Arabidopsis thaliana) IPMS1 (At1g18500) and IPMS2 (At1g74040) cDNAs in Escherichia coli yields isopropylmalate synthases (IPMSs; EC 2.3.3.13). These enzymes catalyze the first dedicated step in leucine (Leu) biosynthesis, an aldol-type condensation of acetyl-coenzyme A (CoA) and 2-oxoisovalerate yielding isopropylmalate. Most biochemical properties of IPMS1 and IPMS2 are similar: broad pH optimum around pH 8.5, Mg2+ as cofactor, feedback inhibition by Leu, Km for 2-oxoisovalerate of approximately 300 microM, and a Vmax of approximately 2 x 10(3) micromol min(-1) g(-1). However, IPMS1 and IPMS2 differ in their Km for acetyl-CoA (45 microM and 16 microM, respectively) and apparent quaternary structure (dimer and tetramer, respectively). A knockout insertion mutant for IPMS1 showed an increase in valine content but no changes in Leu content; two insertion mutants for IPMS2 did not show any changes in soluble amino acid content. Apparently, in planta each gene can adequately compensate for the absence of the other, consistent with available microarray and reverse transcription-polymerase chain reaction data that show that both genes are expressed in all organs at all developmental stages. Both encoded proteins accept 2-oxo acid substrates in vitro ranging in length from glyoxylate to 2-oxohexanoate, and catalyze at a low rate the condensation of acetyl-CoA and 4-methylthio-2-oxobutyrate, i.e. a reaction involved in glucosinolate chain elongation normally catalyzed by methylthioalkylmalate synthases. The evolutionary relationship between IPMS and methylthioalkylmalate synthase enzymes is discussed in view of their amino acid sequence identity (60%) and overlap in substrate specificity.

Different elongation pathways in the biosynthesis of acyl groups of trichome exudate sugar esters from various solanaceous plants

Two common pathways are known for elongation of aliphatic acids via acetate in biological organisms: the fatty acid synthase (FAS) and the alpha-ketoacid elongation (alphaKAE) pathways. The alphaKAE route is utilized in many biosynthetic pathways, including the tricarboxylic acid cycle, leucine biosynthesis, and in formation of coenzyme B, glucosinolates, alpha-ketoadipate, sugar-ester acyl acids, short-chain alcohols of yeast and Clostridium species, 2-amino-4-methylhex-4-enoic acid, and l-gamma-phenyl butyrine. In the FAS route, both carbons from acetyl-acyl carrier protein are retained per elongation cycle, while in the alphaKAE route only one carbon from acetyl-coenzyme A is retained. Available evidence indicates that different members of the family Solanaceae may use one or the other of these elongation mechanisms in the synthesis of acyl groups of trichome-exuded sugar esters. In both, precursors for elongation are derived from branched-chain amino acid metabolism. Here we compared radiolabeling patterns in sugar-ester acyl groups from trichomes (the specific tissue in which sugar esters are synthesized) of the tobaccos, Nicotiana benthamiana, N. gossei, N. glutinosa, of Petunia x hybrida cv. Falcon Red & White, and Datura metel, and epidermal peels of Lycopsersicon pennellii after their synthesis from [2-(14)C]-, [1-(14)C]- and [1,2-(14)C]acetate. Recovered acyl acids were purified and then degraded to determine label distribution between the carboxyl termini and the remainder of the molecules. Six- and 20-h incubations were studied, and membrane fatty acids were monitored as internal controls for FAS-mediated elongation. Results are consistent with participation of alphaKAE in synthesis of sugar-ester acyl groups of tobaccos and petunia, but apparently FAS is utilized in the formation of these groups in L. pennellii and D. metel.

Leucine biosynthesis in fungi: entering metabolism through the back door

After exploring evolutionary aspects of branched-chain amino acid biosynthesis, the review focuses on the extended leucine biosynthetic pathway as it operates in Saccharomyces cerevisiae. First, the genes and enzymes specific for the leucine pathway are considered: LEU4 and LEU9 (encoding the alpha-isopropylmalate synthase isoenzymes), LEU1 (isopropylmalate isomerase), and LEU2 (beta-isopropylmalate dehydrogenase). Emphasis is given to the unusual distribution of the branched-chain amino acid pathway enzymes between mitochondrial matrix and cytosol, on the newly defined role of Leu5p, and on regulatory mechanisms governing gene expression and enzyme activity, including new evidence for the metabolic importance of the regulation of alpha-isopropylmalate synthase by coenzyme A. Next, structure-function relationships of the transcriptional regulator Leu3p are addressed, defining its dual role as activator and repressor and discussing evidence in support of the self-masking model. Recent data pointing at a more extended Leu3p regulon are discussed. An overview of the layered controls of the extended leucine pathway is provided that includes a description of the newly recognized roles of Ilv5p and Bat1p in maintaining mitochondrial integrity. Finally, branched-chain amino acid biosynthesis and its regulation in other fungi are summarized, the question of leucine as metabolic signal is addressed, and possible directions of future research in this area are outlined.

Auxin biosynthesis in maize kernels

Auxin biosynthesis was analyzed in a maize (Zea mays) kernel culture system in which the seeds develop under physiological conditions similar to the in vivo situation. This system was modified for precursor feeding experiments. Tryptophan (Trp) is efficiently incorporated into indole-3-acetic acid (IAA) with retention of the 3, 3' bond. Conversion of Trp to IAA is not competed by indole. Labeling with the general precursors [U-(13)C(6)]glucose and [1, 2-(13)C(2)]acetate followed by retrobiosynthetic analysis strongly suggest that Trp-dependent IAA synthesis is the predominant route for auxin biosynthesis in the maize kernel. The synthesis of IAA from indole glycerol phosphate and IAA formation via condensation of indole with an acetyl-coenzyme A or phosphoenolpyruvate derived metabolite can be excluded.

Cloning and expression analysis of beta-isopropylmalate dehydrogenase from potato

A full-length cDNA clone for beta-isopropylmalate dehydrogenase from potato has been isolated and sequenced. The open reading frame is 1071 bp in length encoding a protein of 357 amino acids which includes a 29 amino acid, putative chloroplastic transit peptide. The amino acid sequence shows 33.3% and 28.6% identity to beta-isopropylmalate dehydrogenases from rape and Bacillus subtilis, respectively. Southern analysis shows that the gene is present in low copy number in potato, and in single copy in tomato and Arabidopsis. The gene is expressed in all tissues of the potato plant and its expression is increased by leucine, and leucine plus threonine, in contrast to the situation in yeast and prokaryotes. The gene is also induced by sucrose in a manner similar to that seen with genes involved in carbohydrate metabolism, which indicates that there may be some interaction at the transcriptional level between genes involved in carbon and nitrogen metabolism.

Alpha-Isopropylmalate synthase from Alcaligenes eutrophus H 16 I. Purification and general properties

alpha-Isopropylmalate (IPM) synthase, the first enzyme in the biosynthesis of L-leucine, was purified to a specific activity of 12 micronmole/min x mg protein from the valine-isoleucine double auxotrophic mutant A-81 of the hydrogen bacterium Alcaligenes eutrophus H16. The activity in crude extracts of derepressed cells was 0.106 micronmoles of isopropylmalate formed per min and per mg protein. Gel electrophoresis and regel electrophoresis of the isolated main band resulted in several distinct bands, which were not altered by the additions of substrate alpha-ketoisovalerate, feedback inhibitor leucine or other effectors. The isoelectric points of the enzyme protein was between 3.9 and 4.0. The molecular weight was 114 500 daltons and 100 000 respectively in the absence and presence of the feedback inhibitor leucine. The enzyme activity depended strongly on the pH, the optimum is at pH 8.2. The enzyme was could labile and exhibits temperature anomalies.

Distribution of the isopropylmalate pathway to leucine among diverse bacteria

alpha-Isopropylmalate synthase and beta-isopropylmalate dehydrogenase activities were detected in extracts of the following organisms: Chromatium D, Rhodopseudomonas spheroides, Hydrogenomonas H16, Pseudomonas aeruginosa, Pseudomonas fluorescens, Vibrio extorquens, Rhizobium japonicum, Alcaligenes viscolactis, Escherichia coli B, Proteus vulgaris, Aerobacter aerogenes, Salmonella typhimurium, Micrococcus sp., Micrococcus lysodeikticus, Bacillus polymyxa, Bacillus subtilis, and Nocardia opaca. The alpha-isopropylmalate synthase activity in these extracts was inhibited by low concentrations of l-leucine. Taken together with other data, these results suggest that the isopropylmalate pathway is widespread among organisms that can synthesize leucine.