Peroxisomal Metabolism of Propionic Acid and Isobutyric Acid in Plants

Abstract

The subcellular sites of branched-chain amino acid metabolism in plants have been controversial, particularly with respect to valine catabolism. Potential enzymes for some steps in the valine catabolic pathway are clearly present in both mitochondria and peroxisomes, but the metabolic functions of these isoforms are not clear. The present study examined the possible function of these enzymes in metabolism of isobutyryl-CoA and propionyl-CoA, intermediates in the metabolism of valine and of odd-chain and branched-chain fatty acids. Using 13 C NMR, accumulation of ␤-hydroxypropionate from [2-13 C]propionate was observed in seedlings of Arabidopsis thaliana and a range of other plants, including both monocots and dicots. Examination of coding sequences and subcellular targeting elements indicated that the completed genome of A. thaliana likely codes for all the enzymes necessary to convert valine to propionyl-CoA in mitochondria. However, Arabidopsis mitochondria may lack some of the key enzymes for metabolism of propionyl-CoA. Known peroxisomal enzymes may convert propionyl-CoA to ␤-hydroxypropionate by a modified ␤-oxidation pathway. The chy1-3 mutation, creating a defect in a peroxisomal hydroxyacyl-CoA hydrolase, abolished the accumulation of ␤-hydroxyisobutyrate from exogenous isobutyrate, but not the accumulation of ␤-hydroxypropionate from exogenous propionate. The chy1-3 mutant also displayed a dramatically increased sensitivity to the toxic effects of excess propionate and isobutyrate but not of valine.13 C NMR analysis of Arabidopsis seedlings exposed to [U-13 C]valine did not show an accumulation of ␤-hydroxypropionate. No evidence was observed for a modified ␤-oxidation of valine.13 C NMR analysis showed that valine was converted to leucine through the production of ␣-ketoisovalerate and isopropylmalate. These data suggest that peroxisomal enzymes for a modified ␤-oxidation of isobutyryl-CoA and propionyl-CoA could function for metabolism of substrates other than valine.Propionate, in the form of propionyl-CoA, is produced from a number of metabolic precursors in higher eukaryotes. It is the final product of odd-chain fatty acid ␤-oxidation (1). It is also produced during the catabolism of several amino acids, including isoleucine, methionine, and valine (1, 2). Propionyl-CoA is also a final product of metabolism of the branched acid, phytanic acid, derived from the degradation of chlorophyll (3). Aside from a basic understanding of metabolic biochemistry, the anabolic and catabolic pathways for propionyl-CoA are also of considerable importance in metabolic engineering of polyhydroxyalkanoates in plants, especially in the production of mixed polyhydroxyalkanoate polymers that have relied on the use of propionyl-CoA as a metabolic intermediate (4, 5). Several pathways have been confirmed for the catabolism of propionylCoA (6 -8). Bacteria and yeast utilize a 2-methylcitrate pathway with reactions analogous to those of the tricarboxylic acid cycle and glyoxylate cycle (6). Mammals use a well established b...

Methylcrotonyl-CoA Carboxylase Regulates Triacylglycerol Accumulation in the Model Diatom Phaeodactylum tricornutum

“…Although several propionyl-CoA catabolic pathways have been confirmed in bacteria, yeast, and mammals, their roles in plants and algae are not clearly understood (Lucas et al, 2007;Kroth et al, 2008). In this study, silencing of MCM had no influence on growth and lipid content, indicating that propionyl-CoA may be directed toward other metabolic pathways besides the TCA cycle.…”

“…Although some carbon skeletons from Val and Ile degradation are likely to enter the TCA cycle through succinyl-CoA, MCM nonetheless appears to play only a minor role during TAG accumulation. Transcript levels of genes encoding enzymes involved in conversion of Val to Leu were upregulated during TAG accumulation in P. tricornutum (Supplemental Figure 10), suggesting that catabolism of Val takes place first through conversion to Leu (Lucas et al, 2007).…”

Identification and Characterization of Arabidopsis Indole-3-Butyric Acid Response Mutants Defective in Novel Peroxisomal Enzymes

“…CHY1 encodes a peroxisomal b-hydroxybutyryl-CoA hydrolase similar to mammalian mitochondrial enzymes acting in valine catabolism (Zolman et al 2001a). chy1 mutants are sucrose dependent (Zolman et al 2001a), resistant to IBA (Zolman et al 2001a) and 2,4-DB (Lange et al 2004), and display heightened sensitivity to root elongation inhibition caused by isobutyrate and propionate treatment (Lucas et al 2007). These defects are thought to result from accumulation of toxic catabolic intermediates in the mutant.…”

“…Error bars show standard errors of mean root lengths (n $ 11). chy1-3 is included as a propionate-and isobutyrate-hypersensitive control (Lucas et al 2007). identical to IBR1, followed by 50 amino acids translated in a different frame and a stop codon; the C-terminal peroxisomal-targeting signal in this protein is lost.…”

“…Such a mechanism might result from accumulation of toxic intermediates, as in the chy1 mutant (Zolman et al 2001a). CHY1 encodes a b-hydroxyisobutyryl-CoA hydrolase acting in peroxisomal valine (Zolman et al 2001a;Lange et al 2004) or isobutyrate (Lucas et al 2007) catabolism. chy1 mutants are sucrose dependent (Zolman et al 2001a), IBA resistant (Zolman et al 2001a), 2,4-DB resistant (Lange et al 2004), and hypersensitive to isobutyrate and propionate (Lucas et al 2007).…”