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The Synthesis and Role of β-Alanine in Plants - PubMed

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Review

The Synthesis and Role of β-Alanine in Plants

Anutthaman Parthasarathy et al. Front Plant Sci. 2019.

Abstract

Most studies on amino acids are focused on the proteinogenic amino acids given their essential roles in protein synthesis among other pathways. In addition to 20 ubiquitous amino acids used in protein synthesis, plants synthesize over 250 non-proteinogenic amino acids that are involved in the synthesis of compounds that are anti-herbivory, anti-microbial, response to abiotic stresses, nitrogen storage, toxins against both vertebrates/invertebrates, and plant hormones among others. One such non-proteinogenic acid is β-alanine, which is known mainly for studies on humans. β-Alanine forms a part of pantothenate (vitamin B5), which is incorporated into the universal carbon shuttling compounds Coenzyme A and acyl carrier protein, in all organisms including plants. The focus of this review, however, is on the biosynthesis, metabolism, and the role of β-alanine in plants. There are several functions of β-alanine unique to plants. It is accumulated as a generic stress response molecule involved in protecting plants from temperature extremes, hypoxia, drought, heavy metal shock, and some biotic stresses. There is evidence of its participation in lignin biosynthesis and ethylene production in some species. It is further converted to the osmoprotective compound β-alanine betaine in some species and converted to the antioxidant homoglutathione in others. The polyamines spermine/spermidine, propionate and uracil have been shown to be precursors of β-alanine in plants. However, plants vary in terms of their biosynthetic pathways, and the primary metabolism of β-alanine is far from settled.

Keywords: L-alanine; amino acid; non-proteinogenic; secondary metabolites; β-alanine.

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Figures

Figure 1
Figure 1

The chemical structures of L-alanine and β-alanine.

Figure 2
Figure 2

Anabolism of β-Alanine via the spermine and spermidine pathways. The enzyme class (EC) numbers shown in figure correspond to the following enzymes: spermidine dehydrogenase [EC 1.5.99.6], polyamine oxidase [EC 1.5.3.14], diamine oxidase [EC 1.4.3.22], aldehyde dehydrogenase [EC 1.2.1.3], and pantoate-β-alanine ligase (AMP-forming) [EC 6.3.2.1]. The dotted lines show the pantothenate and CoA biosynthesis pathway into which β-alanine can be fed. BCAA, branched chain amino acid.

Figure 3
Figure 3

The anabolism of β-alanine via the propionate pathway. The enzymes shown in the figure correspond to the following EC numbers: propionate CoA ligase = [EC 6.2.1.17]; medium chain acyl-CoA dehydrogenase = [EC 1.3.8.7]; 3-hydroxypropionyl-CoA dehydratase = [EC 4.2.1.116]; 3-hydroxypropionyl-CoA hydrolase = [EC 3.1.2.4]; 3-hydroxypropionate dehydrogenase = [EC 1.1.1.59], and β-alanine-pyruvate transaminase = [EC 2.6.1.18].

Figure 4
Figure 4

The anabolism of β-Alanine biosynthesis via uracil and L-aspartate. (A) The nucleotide base uracil can be degraded to β-alanine. The enzymes in this pathway are dihydrouracil dehydrogenase [EC 1.3.1.2], dihydropyrimidinase [EC 3.5.2.2] and β-ureidopropionase [EC 3.5.1.6]. (B) Aspartate 1-decarboxylase [EC 4.1.1.11] can directly decarboxylate L-aspartate into β-alanine, which may feed into Coenzyme A biosynthesis via pantothenate.

Figure 5
Figure 5

The conversion of β-alanine to important secondary metabolites. (A) A trifunctional, S-adenosyl-L-methionine (SAM)-dependent N-methyltransferase [EC 2.1.1.49], performs iterative N-methylation of β-alanine to β-alanine betaine. (B) Homoglutathione biosynthesis involves the enzymes, glutamate cysteine ligase (GCL) or γ-ECS (γ-glutamyl-cysteine synthetase) [EC 6.3.2.2] and a β-alanine specific homoglutathione synthase (hGSHS) [EC 6.3.2.23], both of which perform specific amino acid condensations at the expense of ATP.

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