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Amino Acids: The Forum for Amino Acid, Peptide and Protein Research (v.20, #3)

Plant amino acid research in retrospect: From Chibnall to Singh by L. Fowden (pp. 217-224).

Does phosphoenolpyruvate carboxykinase have a role in both amino acid and carbohydrate metabolism? by P. J. Lea; Z.-H. Chen; R. C. Leegood; R. P. Walker (pp. 225-241).

Phosphoenolpyruvate carboxykinase (PEPCK) catalyses the reversible decarboxylation of oxaloacetate to yield phosphoenolpyruvate and CO₂. The role of the enzyme in gluconeogenesis and anaplerotic reactions in a range of organisms is discussed, along with the important function in C₄ and CAM photosynthesis in higher plants. In addition, new data are presented indicating that PEPCK may play a key role in amino acid metabolism. It is proposed that PEPCK is involved in the conversion of the carbon skeleton of asparagine/aspartate (oxaloacetate) to that of glutamate/glutamine (2-oxoglutarate). This metabolism is particularly important in the transport system, seeds and fruits of higher plants.

Keywords: Keywords: Amino acid metabolism; Asparagine; Glutamate; Higher plants; Oxaloacetate; Phosphoenolpyruvate carboxykinase

Molecular biology of the plastidic phosphorylated serine biosynthetic pathway in Arabidopsis thaliana by C.-L. Ho; K. Saito (pp. 243-259).

Serine biosynthesis in plants proceeds by two pathways; the glycolate pathway which is associated with photorespiration and the pathway from 3-phosphoglycerate which is presumed to take place in the plastids. The 3-phosphoglycerate pathway (phosphorylated pathway) involves three enzymes catalyzing three sequential reactions: 3-phosphoglycerate dehydrogenase (PGDH), 3-phosphoserine aminotransferase (PSAT) and 3-phosphoserine phosphatase (PSP). cDNA and genomic clones encoding these three enzymes from spinach and Arabidopsis thaliana were isolated by means of heterologous probe screening, homologous EST clones and genetic complementation in an Escherichia coli mutant. The identity of the isolated cDNAs was confirmed by functional complementation of serine auxotrophy in E. coli mutants and/or the detection of catalytic activity in the recombinant enzymes produced in E. coli. Northern blot analyses indicated the most preferential expression of these three genes in light-grown roots. In contrast, the mRNAs of two proteins involved in the glycolate pathway (H-protein of glycine decarboxylase multienzyme complex and serine hydroxymethyltransferase) accumulated to high levels in light-grown shoots. Environmental stresses, such as high salinity, flooding and low temperature, induced changes in mRNA levels of enzymes in the plastidic phosphorylated serine biosynthetic pathway but not in that of the glycolate pathway. These results indicate that the plastidic 3-phosphoglycerate pathway plays an important role in supplying serine in non-photosynthetic tissues in plants and under environmental stresses.

Keywords: Keywords: Amino acids; Arabidopsis thaliana; Serine biosynthesis; Phosphorylated pathway; Molecular cloning; Plastid

Lysine metabolism in higher plants by R. A. Azevedo; P. J. Lea (pp. 261-279).

The essential amino acid lysine is synthesised in higher plants via a pathway starting with aspartate, that also leads to the formation of threonine, methionine and isoleucine. Enzyme kinetic studies and the analysis of mutants and transgenic plants that overaccumulate lysine, have indicated that the major site of the regulation of lysine synthesis is at the enzyme dihydrodipicolinate synthase. Despite this tight regulation, there is strong evidence that lysine is also subject to catabolism in plants, specifically in the seed. The two enzymes involved in lysine breakdown, lysine 2-oxoglutarate reductase (also known as lysine α-ketoglutarate reductase) and saccharopine dehydrogenase exist as a single bifunctional protein, with the former activity being regulated by lysine availability, calcium and phosphorylation/dephosphorylation.

Keywords: Keywords: Amino acids; Aspartate kinase; Aspartate; Lysine synthesis; Lysine 2-oxoglutarate reductase; Methionine; Threonine

Approaches towards understanding methionine biosynthesis in higher plants by H. Hesse; O. Kreft; S. Maimann; M. Zeh; L. Willmitzer; R. Höfgen (pp. 281-289).

Plants are able to synthesise all amino acids essential for human and animal nutrition. Because the concentrations of some of these dietary constituents, especially methionine, lysine, and threonine, are often low in edible plant sources, research is being performed to understand the physiological, biochemical, and molecular mechanisms that contribute to their transport, synthesis and accumulation in plants. This knowledge can be used to develop strategies allowing a manipulation of crop plants, eventually improving their nutritional quality.This article is intended to serve two purposes. The first is to provide a brief review on the physiology of methionine synthesis in higher plants. The second is to highlight some recent findings linked to the metabolism of methionine in plants due to its regulatory influence on the aspartate pathway and its implication in plant growth. This information can be used to develop strategies to improve methionine content of plants and to provide crops with a higher nutritional value.

Keywords: Keywords: Amino acids; Transgenic plants; Cystathionine gamma-synthase; Cystathionine beta-lyase; Methionine synthase; Methionine biosynthesis; Sulfur-rich proteins

Manipulation of thiol contents in plants by R. Höfgen; O. Kreft; L. Willmitzer; H. Hesse (pp. 291-299).

As sulfur constitutes one of the macronutrients necessary for the plant life cycle, sulfur uptake and assimilation in higher plants is one of the crucial factors determining plant growth and vigour, crop yield and even resistance to pests and stresses. Inorganic sulfate is mostly taken up as sulfate from the soil through the root system or to a lesser extent as volatile sulfur compounds from the air. In a cascade of enzymatic steps inorganic sulfur is converted to the nutritionally important sulfur-containing amino acids cysteine and methionine (Hell, 1997; Hell and Rennenberg, 1998; Saito, 1999). Sulfate uptake and allocation between plant organs or within the cell is mediated by specific transporters localised in plant membranes. Several functionally different sulfate transporters have to be postulated and have been already cloned from a number of plant species (Clarkson et al., 1993; Hawkesford and Smith, 1997; Takahashi et al., 1997; Yamaguchi, 1997). Following import into the plant and transport to the final site of reduction, the plastid, the chemically relatively inert sulfate molecule is activated through binding to ATP forming adenosine-5′-phosphosulfate (APS). This enzymatic step is controlled through the enzyme ATP-sulfurylase (ATP-S). APS can be further phosphorylated to form 3′-phosphoadenosine-5′-phosphosulfate (PAPS) which serves as sulfate donor for the formation of sulfate esters such as the biosynthesis of sulfolipids (Schmidt and Jäger, 1992). However, most of the APS is reduced to sulfide through the enzymes APS-reductase (APR) and sulfite reductase (SIR). The carbon backbone of cysteine is provided through serine, thus directly coupling photosynthetic processes and nitrogen metabolism to sulfur assimilation. L-serine is activated by serine acetyltransferase (SAT) through the transfer to an acetyl-group from acetyl coenzyme A to form O-acetyl-L-serine (OAS) which is then sulhydrylated using sulfide through the enzyme O-acetyl-L-serine thiol lyase (OAS-TL) forming cysteine. Cysteine is the central precursor of all organic molecules containing reduced sulfur ranging from the amino acid methionine to peptides as glutathione or phytochelatines, proteines, vitamines, cofactors as SAM and hormones. Cysteine and derived metabolites display essential roles within plant metabolism such as protein stabilisation through disulfide bridges, stress tolerance to active oxygen species and metals, cofactors for enzymatic reactions as e.g. SAM as major methylgroup donor and plant development and signalling through the volatile hormone ethylene. Cysteine and other metabolites carrying free sulfhydryl groups are com-monly termed thioles (confer Fig. 1). The physiological control of the sulfate reduction pathway in higher plants is still not completely understood in all details. The objective of this paper is to summarise the available data on the molecular analysis and control of cysteine biosynthesis in plants, and to discuss potentials for manipulating the pathway using transgenic approaches.

Keywords: Keywords: Amino acids; Thioles; Cysteine; Sulfur; Sulfate; Glutathione

Biosynthesis, oxidation and conjugation of aliphatic polyamines in higher plants by N. Bagni; A. Tassoni (pp. 301-317).

This chapter will focus on polyamine biosynthesis, oxidation, conjugation processes, mainly to hydroxycinnamic acids, and compartmentation of enzymes, substrates and products, giving an overview about recent results especially in higher plants. New research advances regarding the cloning of the main cDNA encoding for polyamine biosynthetic and oxidative enzymes, will be taken into consideration.

Keywords: Keywords: Amino acids; Arginine; Ornithine; Putrescine; Spermidine; Spermine; Polyamines

Nonprotein amino acids in edible lentil and garden pea seedlings by P. Rozan; Y. H. Kuo; F. Lambein (pp. 319-324).

Commercial edible seedlings of garden pea (Pisum sativum L.) and lentil (Lens culinaris L.) contain high concentration of nonprotein amino acids and trigonelline. Both seedlings grown in the laboratory or purchased in a supermarket were studied by HPLC. Samples from both origins contained trigonelline, α-aminoadipic acid, homoserine, β-(isoxazolin-5-on-2-yl)-alanine (BIA), and γ-glutamyl-BIA. Garden pea seedlings also contained a uracil-alanine derivative (isowillardiine) in substantial amount. Some of these compounds such as BIA and α-aminoadipic acid have neurotoxic activity.

Keywords: Keywords: Amino acids; Nonprotein amino acids; Trigonelline; Inherent toxicant; Leguminosae; Edible seedlings

Influence of heavy metals on the accumulation of trimethylglycine, putrescine and spermine in food plants by H. Bergmann; B. Machelett; B. Lippmann; Y. Friedrich (pp. 325-329).

Increased contents of cadmium (Cd), lead (Pb), zinc (Zn) and other heavy metals in barley plants enhanced the accumulation of trimethylglycine (betaine), putrescine and spermine. Higher contents of heavy metals in barley were caused by soil enrichment with heavy metals and by soil salinity. The highest accumulation of spermine and betaine (increase 3-fold or 5-fold in comparison to untreated soil substrates) was obtained at the highest concentration of heavy metals in plants. Consequently the betaine-N / protein-N-ratio and the spermine-N / protein-N-quotient increased 3-fold in plants with high heavy metal contents. The biomass formation was not changed significantly by the different experimental treatments.

Keywords: Keywords: Amino acids; Heavy metals; Salinity; Barley; Trimethylglycine; Putrescine; Spermine

Cycling treatment of anaerobic and aerobic incubation increases the content of γ-aminobutyric acid in tea shoots by Y. Sawai; Y. Yamaguchi; D. Miyama; H. Yoshitomi (pp. 331-334).

γ-Aminobutyric acid (GABA), a hypotensive compound, is formed from glutamic acid under anaerobic condition in tea shoots. Glutamic acid was exhausted in the first three hours of anaerobic incubation and the increase of GABA stopped. After that, when tea shoots were released under aerobic condition, glutamic acid reproduced rapidly. After one hour of aerobic incubation, tea shoots were given three hours of anaerobic incubation again and then accumulated glutamic acid changed to GABA. The content of GABA increased much more than usual anaerobic incubation. GABA was more in the tea stem than in the leaf.

Keywords: Keywords: Amino acids; γ-Aminobutyric acid (GABA); Glutamic acid; Tea; Anaerobic incubation; Aerobic incubation; Stem

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Amino Acids: The Forum for Amino Acid, Peptide and Protein Research (v.20, #3)