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N Use By Plants
Nitrate Assimilation
Ammonia Assimilation
Glu, Gln, Asn, Gly, Ser
Asp, Ala, GABA
Val, Leu, Ileu, Thr, Lys
Pro, Arg, Orn
Non-protein AAs
Sulfate Assimilation
Cys, Met, AdoMet, ACC
His, Phe, Tyr, Tryp
Secondary Products
Onium Compounds
HORT640 - Metabolic Plant Physiology

Ammonia Assimilation and Recycling

Tetrahydrofolate involvement in glycine and serine metabolism

Tetrahydrofolate-dependent glycine and serine metabolism are closely linked. Tetrahydrofolate (THF) allows transfer of the alpha-carbon of glycine through glycine decarboxylase (GDC) activity for serine biosynthesis by the serine hydroxymethyltransferase (SHMT) [serine transhydroxymethylase or glycine hydroxymethyltransferase] reaction:

Glycine decarboxylase complex [EC]

Gly + NAD+ + THF <---> 5,10-methylene-THF + NADH + NH3 + CO2

Serine hydroxymethyltransferase [EC]

5,10-methylene-THF + Gly <---> Ser + THF

In C3 plants, the GDC activity is greater than that of SHMT activity in leaf mitochondria, which results in the maintenance of high levels of 5,10-methylene-THF; thus the equilibrium of the SHMT reaction is shifted in favor of serine synthesis, which enables recycling of THF for continuous operation of the GDC reaction during photorespiration.

Formate is a potential alternative single-carbon source for the production of the 5,10-methylene-THF required for serine synthesis.

10-Formyl-THF synthetase (SYN) [EC]

Formate + THF + ATP <---> 10-formyl-THF + ADP + Pi

5,10-Methenyl-THF cyclohydrolase (CYC) [EC]

10-formyl-THF + H+ <---> 5,10-methenyl-THF + H2O

5,10-Methylene-THF dehydrogenase (DHY) [EC and EC]

5,10-methenyl-THF + NADPH <---> 5,10-methylene-THF + NADP+ (EC

5,10-methenyl-THF + NADH <---> 5,10-methylene-THF + NAD+ (EC

These three enzymes are predominantly cytosolic; mitochondria contain <1% of total SYN, CYC and DHY activities (Chen et al, 1997). In most eukaryotes these three activities occur on a single polypeptide called C1-THF synthase; in plants, however, the synthetase activity occurs on a separate protein moiety from that of the cyclohydrolase and reductase, which occur on a bifunctional protein (Prabhu et al, 1996). Possible pathways of formate production in higher plants are described by Hourton-Cabass et al (1998). These include non-enzymic, hydrogen peroxide mediated decarboxylation of glyoxylate during photorespiration, synthesis from methanol (via formaldehyde), synthesis from glycolytic products, and the reverse of the reactions depicted above. Formate dehydrogenase (FDH) [EC] catalyzes the oxidation of formate to CO2 and may thus play a key role in regulating formate levels. FDH is a major mitochondrial protein in potato, and is induced by formate, methanol, and various stresses, including anoxia (Hourton-Cabassa et al, 1998), and iron deficiency (Suzuki et al, 1998).

5,10-Methylene-THF is further reduced to 5-methyl-THF by 5,10-methylenetetrahydrofolate reductase [EC and EC]. 5-Methyl-THF is exclusively used in methionine synthesis in the reaction catalyzed by methionine synthase [EC] (see also methionine synthesis and the activated methyl cycle under Sulfate uptake and assimilation).

Tetrahydrofolic acid (THF) is synthesized from 6-hydroxymethydihydropteridin via the intermediates 6-hydroxymethyldihydropteridin diphosphate, dihydropteroic acid, and dihydrofolic acid (Prabhu et al, 1998). The enzymes catalyzing this reaction sequence are: 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK) [EC], dihydropteroate synthase (DHPS) [EC], dihydrofolate synthase (DHFS) [EC], and dihydrofolate reductase (DHFR) [EC], respectively (Prabhu et al, 1998). Folylpolyglutamate synth(et)ase (FPGS) [EC] then adds a polyglutamyl tail to THF (Prabhu, 1998). Methotrexate and aminopterin are potent inhibitors of DHFR in Arabidopsis (Prabhu et al, 1998).

see also: Interfaces between photorespiration, one-carbon, methionine and S-adenosylmethionine metabolism


Hourton-Cabassa C, Ambard-Bretteville F, Moreau F, de Virville JD, Remy R, des Francs-Small CC 1998 Stress induction of mitochondrial formate dehydrogenase in potato leaves. Plant Physiol. 116: 627-635.

Prabhu V, Chatson KB, Abrams GD, King J 1996 13C Nuclear magnetic resonance detection of interactions of serine hydroxymethyltransferase with C1-tetrahydrofolate synthase and glycine decarboxylase complex activities in Arabidopsis. Plant Physiol. 112: 207-216.

Prabhu V, Chatson KB, Lui H, Abrams GD, King J 1998 Effects of sulfanilamide and methotrexate on 13C fluxes through the glycine decarboxylase/serine hydroxymethyltransferase enzyme system in Arabidopsis. Plant Physiol. 116: 137-144.

Suzuki K, Itai R, Suzuki K, Nakanishi H, Nishizawa NK, Yoshimura E, Mori S 1998 Formate dehydrogenase, an enzyme of anaerobic metabolism, is induced by iron deficiency in barley roots. Plant Physiol. 116: 725-732.

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Last Update: 10/01/09