Plants amino acids synthesized

In bacterial cells and in yeast, which are, for obvious reasons, much more commonly studied than other animals and plants, amino acids, synthesized de novo or by degradation of proteins of various kinds and sizes, are found in the cytosol. The amino acids from these proteins are used for the synthesis of peptides. Much of the synthesis occurs either on/in the ribosomes of the endoplasmic reticulum (ER) or through nonribosomal pathways using acyl carrier protein-like condensation reactions. 

The tightly regulated pathway specifying aromatic amino acid biosynthesis within the plastid compartment implies maintenance of an amino acid pool to mediate regulation. Thus, we have concluded that loss to the cytoplasm of aromatic amino acids synthesized in the chloroplast compartment is unlikely (13). Yet a source of aromatic amino acids is needed in the cytosol to support protein synthesis. Furthermore, since the enzyme systems of the general phenylpropanoid pathway and its specialized branches of secondary metabolism are located in the cytosol (17), aromatic amino acids (especially L-phenylalanine) are also required in the cytosol as initial substrates for secondary metabolism. The simplest possibility would be that a second, complete pathway of aromatic amino acid biosynthesis exists in the cytosol. Ample precedent has been established for duplicate, major biochemical pathways (glycolysis and oxidative pentose phosphate cycle) of higher plants that are separated from one another in the plastid and cytosolic compartments (18). Evidence to support the hypothesis for a cytosolic pathway (1,13) and the various approaches underway to prove or disprove the dual-pathway hypothesis are summarized in this paper. 

A series of specific H+-linked cotransporters are found in the brush border membranes of small intestine and in kidney epithelial cells.440 441 In green plants H+-linked cotransport of amino acids is used in the distribution of amino acids synthesized in the roots and leaves to other parts of the plants.442 443... 

The different types of amino acids can be broadly classified as neutral, acidic or basic (see Box 2.5), depending on whether the number of carboxylic acid groups in a molecule equals, is greater than or is less than the number of amine groups respectively. Proteins are made up from some 20 different amino acids, shown in Fig 2.11b. Sulphur is an important component in some amino acids (e.g. cysteine). In plants amino acids are generally synthesized from glutamic acid (Fig. 

The trend in this field, as in other biosynthetic areas, is clearly towards investigations on the enzymic level, though traditional incorporation studies on intact plants still play an important role. It is apparent from recent results that the types of enzymic process involved in the biosynthesis of the ca. 200 known free plant amino-acids are rather small in number, most of them representing minor deviations from the biosynthesis of protein amino-acids. The operation of specific enzymes has been established in a number of cases, indicating that non-protein amino-acids are not generally the results of low specificity in the enzymic syntheses of protein amino-acids. 

Humans are able to synthesize only 11 of the 20 amino acids in proteins, called nonessential amino acids. The other 9, called essential amino acids, are biosynthesized only in plants and microorganisms and must be obtained in our diet. The division between essential and nonessential amino acids is not clearcut, however tyrosine, for instance, is sometimes considered nonessential because humans can produce it from phenylalanine, but phenylalanine itself is essential and must be obtained in the diet. Arginine can be synthesized by humans, but much of the arginine we need also comes from our diet. 

The synthesis of virtually all proteins in a cell begins on ribosomes in the cytosol (except a few mitochondrial, and in the case of plants, a few chloroplast proteins that are synthesized on ribosomes inside these organelles). The fate of a protein molecule depends on its amino acid sequence, which can contain sorting signals that direct it to its corresponding organelle. Whereas proteins of mitochondria, peroxisomes, chloroplasts and of the interior of the nucleus are delivered directly from the cytosol, all other organelles receive their set of proteins indirectly via the ER. These proteins enter the so-called secretory pathway (Fig. 1). 

Humans can synthesize 12 of the 20 common amino acids from the amphiboHc intermediates of glycolysis and of the citric acid cycle (Table 28-1). While nutritionally nonessenrial, these 12 amino acids are not nonessential. AH 20 amino acids are biologically essential. Of the 12 nutritionally nonessential amino acids, nine are formed from amphibolic intermediates and three (cysteine, tyrosine and hydroxylysine) from nutritionally essential amino acids. Identification of the twelve amino acids that humans can synthesize rested primarily on data derived from feeding diets in which purified amino acids replaced protein. This chapter considers only the biosynthesis of the twelve amino acids that are synthesized in human tissues, not the other eight that are synthesized by plants. 

Other than a nutritional role linked to mineralization processes, humic compounds have been hypothesized to directly affect plant nutrition, since it has been suggested that roots may take up low-molecular-weight humic molecules (21). Interestingly, plants have been ob.served to express carriers for amino acids (22) and small peptides (23) at the root level. Certain components of the humic fraction have been found inside root cells and were, moreover, translocated to the shoots (24,25). Recent experiments performed on rice cells in suspension culture seem to suggest that they may use carbon skeletons from humic molecules to synthesize proteins and DNA (26). 

Plants synthesize all the amino acids they require. They do so using as raw material carbohydrates, which they make during photosynthesis, and nitrogen, derived from nitrate ions absorbed from the soil. Animals cannot synthesize all the amino acids required for their regular living, health, and growth. Those they cannot synthesize, known as the essential amino acids, are acquired from plants and/or animals they consume as food. Human beings, for example, acquire nine essential amino acids from their diet. 

Precursors of phenylpropanoids are synthesized from two basic pathways the shikimic acid pathway and the malonic pathway (see Fig. 3.1). The shikimic acid pathway produces most plant phenolics, whereas the malonic pathway, which is an important source of phenolics in fungi and bacteria, is less significant in higher plants. The shikimate pathway converts simple carbohydrate precursors into the amino acids phenylalanine and tyrosine. The synthesis of an intermediate in this pathway, shikimic acid, is blocked by the broad-spectrum herbicide glyphosate (i.e., Roundup). Because animals do not possess this synthetic pathway, they have no way to synthesize the three aromatic amino acids (i.e., phenylalanine, tyrosine, and tryptophan), which are therefore essential nutrients in animal diets. 

The nonprotein amino acid, 1-aminocyclopropane-l-carboxylic acid, is an intermediate of ethylene biosynthesis in plants. This amino acid is synthesized from the L-a-amino acid methionine through the intermediate 5 -adenosyl-L-methionine (SAM) (Scheme 8). ... 

Figure 22.11. Cellulose is a structural and rather chemically inert component of terrestrial plants. Alginic acid is synthesized by marine algae and has medicinal properties. Some polysaccharides contain nitrogen in the form of amino sugars. The primary example of this is chitin, which composes the exoskeletons of Crustacea. An amino sugar contains an amine in place of a hydroxyl group. In other polysaccharides, sugars combine with lipids and proteins. These form glycoUpids and glycoproteins, respectively.

The successful expression of recombinant plant peroxidases such as HRP C has been a major focus of research in a number of laboratories. Three synthetic HRP C genes based on the amino acid sequence determined by Welinder (36, 47) were synthesized independently in order to initiate this work (63-65). A number of different expression systems have been evaluated (64, 66-73), a summary of which is presented in Table I. Refolding of recombinant HRP C isolated from inclusion... 

Plants and microorganisms are able to synthesize all of the amino acids from scratch, but during the course of evolution, mammals have lost the ability to synthesize approximately half of the 20 proteinogenic amino acids. These essential amino acids therefore... 

In plants, this non-protein amino acid is derived from L-glutamate and in animals from L-arginine. Moreover, Figure 23 demonstrates that synthesis of alkaloids is complicated by the ability of the same amino acid to synthesize many different alkaloids. 

Ornithine is a metabolically quite active amino acid, and the important precursor of pyrrolidine nucleus, which is found in pyrrolizidine alkaloids. Ornithine itself is a non-protein amino acid formed mainly from L-glumate in plants, and synthesized from the urea cycle in animals as a result of the reaction catalyzed by enzymes in arginine. 

Inhibition of Chorismate Synthase Shikimic and quinic acids are used by microorganisms, fungi, and superior plants for the synthesis of essential aromatic amino acids from acyclic sugars. Fluorinated analogues of substrates and reaction intermediates have been synthesized in order to inhibit enzymes involved in... 

L-Canavanine and L-canaline are non-protein amino acids of certain leguminous plants, that function as protective allelochemicals. L-Canavanine is incorporated into de novo synthesized proteins in place of arginTne there is suggestive evidence that formation of such anomalous proteins figures significantly in canavanine s adverse biological effects. Canavanine, however, does not appear to inhibit overall protein synthesis. Thus, an important basis for canavanine s antimetabolic properties resides in the sustained production of biologically aberrant proteins. 

Like histamine, serotonin is widely distributed in nature, being found in plant and animal tissues, venoms, and stings. It is synthesized in biologic systems from the amino acid l -tryptophan by hydroxylation of the indole ring followed by decarboxylation of the amino acid (Figure... 

Organisms vary greatly in their ability to synthesize the 20 common amino acids. Whereas most bacteria and plants can synthesize all 20, mammals can synthesize only about half of them—generally those with simple pathways. These are the nonessential amino acids, not needed in the diet (see Table 18-1). The remainder, the essential amino acids, must be obtained from food. Unless otherwise indicated, the pathways for the 20 common amino acids presented below are those operative in bacteria. 

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