Alanine Aminotransferase Homologs Catalyze the Glutamate:Glyoxylate Aminotransferase Reaction in Peroxisomes of Arabidopsis

Abstract

Plant peroxisomal glyoxylate aminotransferases play central roles within the photorespiratory pathway. Genes encoding glyoxylate aminotransferases have been isolated from several animals and microbes, but only recently have plant homologs been identified. Three Arabidopsis homologs of alanine (Ala):glyoxylate aminotransferase 2 (AGT2) contain a putative type 1 peroxisomal targeting signal (PTS1), but the metabolic significance of these AGT2 homologs is unknown. GGT1 and GGT2 are Ala aminotransferase (AlaAT) homologs from Arabidopsis that represent another type of glyoxylate aminotransferase. These proteins are class I aminotransferases, each containing a putative PTS1. GGT1 and GGT2 are members of a small family of AlaATs in Arabidopsis. When expressed as recombinant proteins in Escherichia coli, GGT1 and GGT2 displayed biochemical characteristics very similar to one another, and to the Arabidopsis protein purified from leaves. Four aminotransferase activities were specifically associated with GGT1 and GGT2, using the substrate pairs glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and Ala:2-oxoglutarate. GGT1 and GGT2 may have partially redundant functions; transcripts of both genes were detected in many of the same tissues. Although Glu:glyoxylate aminotransferase (GGT) activity has been observed in several locations in different plants and algae, including the cytoplasm and mitochondria, our subcellular fractionation data indicate that GGT activity was exclusively peroxisomal in Arabidopsis. Thus, glyoxylate aminotransferase reactions in plant peroxisomes appear to be catalyzed by at least two distinct types of aminotransferases: an AGT1 homolog with serine:glyoxylate aminotransferase activity Photorespiration is a metabolite salvage pathway coordinated among at least three organelles in plant cells: chloroplasts, peroxisomes, and mitochondria. In the process of recycling phosphoglycolate, generated by the oxygenase activity of Rubisco, into the photosynthetically useful metabolite phosphoglycerate, this pathway may consume a significant proportion of the energy generated by photosynthesis in C 3 plants (Ogren, 1984; Husic et al., 1987; Buchanan et al., 2000). Despite the obvious importance of photorespiration, genes encoding several enzymes of this pathway have not been identified.Leaf peroxisomes are hypothesized to contain at least four aminotransferase activities including Ser: glyoxylate aminotransferase (SGT), Glu:glyoxylate aminotransferase (GGT), Ala:glyoxylate aminotransferase (AGT), and Asp:2-oxoglutarate aminotransferase (AspAT; Rehfield and Tolbert, 1972). Studies in which isolated plant peroxisomes were fed labeled glycolate and various amino acids, in conjunction with studies of photorespiration mutants, have demonstrated that peroxisomal glyoxylate aminotransferases occupy central roles in photorespiration (Kisaki and Tolbert, 1969;Somerville and Ogren, 1980; Chang and Huang, 1981; Betsche, 1983;Liang and Huang, 1983;Yu et al., 1984).Although genes encoding glyoxylate aminotransferase...

Altered ABA, proline and hydrogen peroxide in an Arabidopsis glutamate:glyoxylate aminotransferase mutant

“…Strong evidence supports a role for GGT1 in photorespiratory metabolism. First, Glu-glyoxylate and/or Ala-glyoxoylate aminotransferases are required for photorespiration because these reactions generate glycine in the peroxisome (Wingler et al 2000;Liepman and Olsen 2003). A light sensitive phenotype which can be alleviated by addition of sucrose, as seen in ggt1, is consistent with impaired photorespiration and Igarashi et al (2003) have also shown that the impaired growth of a ggt1 [referred to as aoat1 in Igarashi et al (2003)] knockout mutant can be rescued by high CO 2 .…”

“…GGT1 contains a type I peroxisomal targeting signal and has been shown to be localized to peroxisomes (Liepman and Olsen 2003). Strong evidence supports a role for GGT1 in photorespiratory metabolism.…”

Arabidopsis 10-Formyl Tetrahydrofolate Deformylases Are Essential for Photorespiration

“…Levels of Ala, Asp, and Glu, amino acids that can donate their amino groups to photorespiratory glyoxylate instead of Ser (Liepman and Olsen, 2003), were depleted in both dKO and shm1-1 mutant leaves (Table 1). Levels of Ala and Asp in the leaves of dKO mutant decreased to ;30%, while those of Gln, Glu, and Asn to 39, 56, and 66% of the wild-type levels, respectively.…”

Seed-Specific Expression of a Feedback-Insensitive Form of CYSTATHIONINE-γ-SYNTHASE in Arabidopsis Stimulates Metabolic and Transcriptomic Responses Associated with Desiccation Stress

“…7). AGT3 converts Ala to glyoxylate, thus producing pyruvate and Gly, while AlaAT1 converts Ala to a-ketoglutarate, thus producing pyruvate and Glu (Liepman and Olsen, 2003). Interestingly, the products of these transamination reactions provide substrates for the tricarboxylic acid cycle.…”