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 beta-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 beta-hydroxypropionate by a modified beta-oxidation pathway. The chy1-3 mutation, creating a defect in a peroxisomal hydroxyacyl-CoA hydrolase, abolished the accumulation of beta-hydroxyisobutyrate from exogenous isobutyrate, but not the accumulation of beta-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 beta-hydroxypropionate. No evidence was observed for a modified beta-oxidation of valine. (13)C NMR analysis showed that valine was converted to leucine through the production of alpha-ketoisovalerate and isopropylmalate. These data suggest that peroxisomal enzymes for a modified beta-oxidation of isobutyryl-CoA and propionyl-CoA could function for metabolism of substrates other than valine.
Highlights
Tanic acid, derived from the degradation of chlorophyll [3]
Examination of genes coding for key enzymes of a modified -oxidation pathway for propionylCoA suggests that the catabolic disposal of propionyl-CoA may be exclusively peroxisomal in plants and that -hydroxypropionate may be the final product of this metabolism
We considered whether propionyl-CoA produced from mitochondrial metabolism of valine would lead to an accumulation of -hydroxypropionate
Summary
Examination of genes coding for key enzymes of a modified -oxidation pathway for propionylCoA suggests that the catabolic disposal of propionyl-CoA may be exclusively peroxisomal in plants and that -hydroxypropionate may be the final product of this metabolism. This concept is consistent with previous reports of -hydroxypropionate production in plants [2, 13]. The present study shows that -hydroxypropionate is produced from exogenous propionate but not from propionyl-CoA derived from valine and that mutation of a peroxisomal hydroxyacyl-CoA hydrolase results in increased sensitivity to propionate but not to valine This mutation abolished the accumulation of -hydroxyisobutyrate from exogenous isobutyrate. A better understanding of this pathway may be valuable for the manipulation of this metabolism in the engineered synthesis of polyhydroxyalkanoates containing these acids [4, 5]
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