Photorespiratory metabolism of glyoxylate and formate in glycine-accumulating mutants of barley and Amaranthus edulis

Abstract

Glycine-accumulating mutants of barley (Hordeum vulgare L.) and Amaranthus edulis (Speg.), which lack the ability to decarboxylate glycine by glycine decarboxylase (GDC; EC 2.1.2.10), were used to study the signi®cance of an alternative photorespiratory pathway of serine formation. In the normal photorespiratory pathway, 5,10-methylenetetrahydrofolate is formed in the reaction catalysed by GDC and transferred to serine by serine hydroxymethyltransferase. In an alternative pathway, glyoxylate could be decarboxylated to formate and formate could be converted into 5,10-methylenetetrahydrofolate in the C1-tetrahydrofolate synthase pathway. In contrast to wild-type plants, the mutants showed a light-dependent accumulation of glyoxylate and formate, which was suppressed by elevated (0.7%) CO 2 concentrations. After growth in air, the activity and amount of 10-formyltetrahydrofolate synthetase (FTHF synthetase; EC 6.3.4.4), the ®rst enzyme of the conversion of formate into 5,10-methylenetetrahydrofolate, were increased in the mutants compared to the wild types. A similar increase in FTHF synthetase could be induced by incubating leaves of wild-type plants with glycine under illumination, but not in the dark. Experiments with 14 C showed that the barley mutants incorporated [ 14 C]formate and [2-14 C]glycollate into serine. Together, the accumulation of glyoxylate and formate under photorespiratory conditions, the increase in FTHF synthetase and the ability to utilise formate and glycollate for the formation of serine indicate that the mutants are able partially to compensate for the lack of GDC activity by bypassing the normal photorespiratory pathway.

Peroxisomal Malate Dehydrogenase Is Not Essential for Photorespiration in Arabidopsis But Its Absence Causes an Increase in the Stoichiometry of Photorespiratory CO2 Release

“…Based on previous studies, the most obvious explanation for an increased CO 2 release stoichiometry would be an alternative oxidative decarboxylation of accumulating pools of glyxoylate and hydroxypyruvate through their nonenzymatic interactions with H 2 O 2 to produce formate and glycolate, respectively, plus CO 2 (Halliwel, 1974;Halliwel and Butt, 1974;Grodzinski, 1979;Walton and Butt, 1981;Wingler et al, 1999). In addition, the resulting formate can also be decarboxylated in the peroxisomes by its oxidation via catalase and H 2 O 2 (Halliwel and Butt, 1974) or can be oxidized to CO 2 by formate dehydrogenase in the mitochondrion (Hourton-Cabassa et al, 1998).…”

“…However, disruption of the photorespiratory pathway may lead to the accumulation of intermediate compounds, allowing for alternative enzymatic and spontaneous reactions to occur. For example, in barley Gly decarboxylase (GDC) mutants lacking the ability to decarboxylate Gly, the levels of glyoxylate and formate under photorespiratory conditions increased in comparison with those in wild-type plants (Wingler et al, 1999). The production of formate can come from glyoxylate reacting with H 2 O 2 nonenzymatically, and the formate can be further incorporated into Ser via the C1-tetrahydrofolate synthase/Ser hydroxymethyl transferase pathway, partially compensating for the lack of GDC activity in these plants (Wingler et al, 1999).…”

C1 Metabolism and Photorespiration of <i>Ficus deltoidea</i> based on Peptide Mass Fingerprinting Approach

“…Glyoxylate which is the precursor of glycine is decarboxylated to formate, and then further converted to 5,10 methylene-THF through series of reactions. The reactions involve two enzymes which are related to one carbon-metabolism; 10-formyl-THF synthase (FTHF synthase) and 5,10-methylenetetrahydrofolate dehydrogenase: 5,10-methenyltetrahydrofolate cyclohydrolase (DHY-CYC), followed by formation of serine by serine hydoxymethytransferase (SHMT) [20]. Some organisms have 10-formyl-THF synthase (FTHF synthase), 5,10-methylenetetrahydrofolate dehydrogenase (DHY) and 5,10-methenyltetrahydrofolate cyclohydrolase (CYC) forming a single dimeric trifunctional protein [21].…”

Tansley Review No. 112

“…In Euglena, serine is formed via an alternative pathway in which methylene-tetrahydrofolate is synthesized with formate derived from glyoxylate. Work with barley mutants with very low glycine decarboxylase activity has shown that this pathway, which allows the conversion of glycine to serine without the release of ammonia, can also occur in higher plants (Wingler et al, 1999b). Genetic transformation has provided good evidence that photorespiratory amino acids can be used in biosyntheses.…”