BOTANY VOLUME 2 - PLANT PHYSIOLOGY - 2007

6. PHYSIOLOGY OF METABOLISM

6.13. Amino Acid Synthesis

Plants synthesize all Proteinogenic Amino Acids independently (see Fig. 1.11), among them humans' essential aromatic amino acids (phenylalanine, Tyrosine, Tryptophan), as well as valine, leucine, and isoleucine. Carbon skeletons are provided by Photosynthesis. It is entirely possible (though not yet definitively proven) that all Amino acids are synthesized within METABOLISM/14.html">Chloroplasts, while many additionally arise in other compartments (e.g., Glycine in Peroxisomes, Serine in Cell/35.html">Mitochondria during Photorespiration, see Fig. 6.75).

6.13.1. Amino Acid Families

Based on THE ORIGIN OF their carbon skeletons, Amino acids can be categorized into several groups (Fig. 6.106): the Pyruvate, 2-oxoglutarate, and oxaloacetate families; the 2-phosphoglycolate and shikimate families; and Histidine separately, which is derived from ribose-5-phosphate. The synthesis of glycine and serine from 2-phosphoglycolate during photorespiration was already discussed (see Fig. 6.75), as was The formation of Cysteine from serine (via O-acetylserine, see Fig. 6.90). Histidine Biosynthesis in plants is not yet fully understood, but it traces back to ribose-5-phosphate. The remaining amino acids are derived partially or entirely from 3-phosphoglycerate via the formation of phosphoenolpyruvate. The formation of pyruvate or oxaloacetate from phosphoenolpyruvate, and the generation of oxaloacetate from pyruvate in mitochondria, were discussed earlier in a different context (see Figs. 6.79; 6.92; 6.94). In mitochondria, 2-oxoglutarate can be synthesized from oxaloacetate via citrate formation (see Fig. 6.94), but citrate exported to the Cytoplasm can also be converted into 2-oxoglutarate, as previously mentioned (see 6.10.3.4).

6.13.2. Aromatic Amino Acids

Due to their special significance for plant metabolism—which extends far beyond merely satisfying the demand for amino acids during Protein Synthesis—the three aromatic amino acids (tryptophan, phenylalanine, and tyrosine) have long been the focus of intense scientific attention. The biosynthetic pathway beginning with phosphoenolpyruvate and erythrose-4-phosphate (see Fig. 6.106) was named the shikimate pathway after its characteristic intermediate metabolite. The shikimate pathway occurs in plants, Fungi, and Bacteria (though not in animals), and in plants it is localized within Plastids. This pathway also yields intermediates for The biosynthesis of numerous other plant compounds, essentially acting as a metabolic "boundary" between Primary and secondary metabolism.

Class="center">Fig. 6.106. Synthesis of carbon skeletons for various amino acid families during CO₂ assimilation

Erythrose-4-phosphate is an intermediate of The Calvin Cycle and the oxidative Pentose Phosphate Pathway, while phosphoenolpyruvate is synthesized via Glycolysis and imported into chloroplasts. The reaction sequence is illustrated in Fig. 6.107.

Fig. 6.107. Reaction sequence of the shikimate pathway for the biosynthesis of the aromatic amino acids phenylalanine, tyrosine, and tryptophan

The enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase) serves as the primary target protein for the world's most widely used herbicide, glyphosate (N-phosphonomethylglycine, Fig. 6.108), which acts as a potent competitive inhibitor preventing the binding of phosphoenolpyruvate to the catalytic site. Nevertheless, plants die not from a deficiency of aromatic amino acids, but rather from the toxic accumulation of shikimic acid within their Tissues (particularly in Meristems).

The shikimate pathway provides an excellent illustration of end-product feedback regulation in branching metabolic pathways (Fig. 6.108).

Fig. 6.108. Regulation of shikimate pathway enzyme activity by reaction products. The Role of the shikimate pathway as a precursor supplier for numerous other metabolic pathways, alongside protein synthesis, is not depicted here. Glyphosate (N-phosphonomethylglycine) is a herbicide and a strong competitive inhibitor of the enzyme 3-phospho-5-enolpyruvylshikimate (EPSP) synthase

Tryptophan inhibits its own synthesis while stimulating the synthesis of tyrosine and phenylalanine. Phenylalanine and Tyrosine similarly inhibit their respective pathways. This feedback mechanism prevents the overaccumulation of unneeded amino acids while Synthesis of the others continues.

6.13.3. Non-Proteinogenic Amino Acids and Amino Acid Derivatives

Alongside the 20 proteinogenic amino acids, plants contain more than 400 other non-proteinogenic amino acids, which are frequently (though not always) derived from proteinogenic ones (Fig. 6.109, A). This group also often includes biogenic amines (Fig. 6.109, B), which are synthesized from amino acids via decarboxylation. Non-proteinogenic amino acids can function as TRANSPORT AND STORAGE metabolites for reduced nitrogen: for example, the aforementioned citrulline (see Fig. 6.91) in Betulaceae and Juglandaceae also serves as an intermediate in Arginine biosynthesis. Canavanine in legumes likewise fulfills a transport and storage function (see Fig. 6.109, A). At the same time, this compound—which can constitute up to 10% or more of a seed's dry weight and contains up to 50% bound nitrogen—acts as a defense substance toxic to herbivores. The toxicity of canavanine stems from its structural similarity to L-arginine (see Fig. 1.11). As a result, herbivore organisms may incorporate it into aberrant Proteins because their Aminoacyl-tRNA synthetases, unlike those of plants, fail to distinguish L-arginine from its analog. Subsequent Metabolic Transformations in the animal's body convert canavanine into canaline, a neurotoxic, non-proteinogenic amino acid. Larvae of the beetle Caryedes brasiliensis, whose sole food source is legume seeds, can detoxify canaline by converting it via reductive deamination into homoserine (see Fig. 6.109, A), a natural intermediate in Threonine biosynthesis.

Fig. 6.109. Examples of plant non-proteinogenic amino acids and their metabolism (A); synthesis of biogenic amines via AMINO ACID DECARBOXYLATION (B)

The non-proteinogenic amino acids of onions (Allium cepa) and garlic (Allium sativum)—propenylalliin and alliin, respectively (see Fig. 6.109, A)—are cysteine derivatives and precursors of compounds that protect the plant against herbivory. When Cells storing alliin (in garlic) and propenylalliin (in onions) in their vacuoles are damaged, these compounds are cleaved by the enzyme alliinase into pyruvate, ammonia, and onion oils. Allicin and propanethial S-oxide are potent deterrents (propanethial S-oxide being the lachrymatory factor in onions!) with concurrent bactericidal activity (inhibiting Bacterial growth in damaged tissue), while diallyl disulfide is responsible for the characteristic aroma of onions.

Many biogenic amines are produced through the decarboxylation of their homologous amino acids; for instance, cadaverine from Lysine, tryptamine from tryptophan, and histamine from histidine (see Fig. 6.109, B). Biogenic amines can act as biosynthetic precursors for Alkaloids (see 6.16.3); tryptamine serves as the Starting Material for the synthesis of the phytohormone indole-3-acetic acid (see 7.6.1.2); and histamine, alongside serotonin and acetylcholine, is a component of the fluid in stinging hairs (see Fig. 3.15) of nettles, provoking itching and painful tissue reactions (blister formation) on vertebrate Skin. This potent active substance protects plants of this taxonomic group from being eaten by animals (defense against herbivores, see 9.4.1)!