Plant cells amino acid synthesis

In higher plants aromatic amino acids are required not only for protein synthesis, but as precursors for hormones, and a vast diversity of phenylpropanoid or other secondary metabolites. Thus, the availability of aromatic amino acids in a number of the spatially separate compartments of the plant-cell microenvironment is essential. 

Tribenuronmethyl (Sulfonyl urea) CO.CH, OCH, n C >-SOjNHCON—< N CH- CHj Branched chain amino acid synthesis (ALS and A HAS) inhibitor. Acts by inhibiting biosynthesis of the essential amino acids valine and isoleucine, hence stopping cell division and plant growth... 

In order to find and manipulate the genes that code for resistance to herbicides, it is essential that we understand the mechanisms of action of herbicides in plant cells. It is fortunate that most herbicides interact with enzymes and other proteins in metabolic pathways specific to plants, such as amino acid synthesis and photosynthesis. Therefore, they are generally non-toxic to animals, and genetic engineering for these traits is specific for plants. 

Ethylene. It seems likely that ethylene is another second messenger, typical of plant material. It can be formed in all cells, even if in varying concentrations. In most instances in higher plants the amino acid methionine is the starting material for the synthesis (Fig. 176c). Special... 

Synthetic processes proceeding in respiring cells not only consume the energy stored in ATP and other phosphorylated compounds but draw upon the intermediates of respiration. For instance, the enhanced rate of respiration of cells actively synthesising proteins is, in part, due to the utilisation in amino acid synthesis of organic acids which are Krebs cycle intermediates. It is clearly a major objective in the study of plant metabolism to understand the quantitative aspects of such inter-relationships between the metabolic processes of the cell. 

Biotransformations are carried out by either whole cells (microbial, plant, or animal) or by isolated enzymes. Both methods have advantages and disadvantages. In general, multistep transformations, such as hydroxylations of steroids, or the synthesis of amino acids, riboflavin, vitamins, and alkaloids that require the presence of several enzymes and cofactors are carried out by whole cells. Simple one- or two-step transformations, on the other hand, are usually carried out by isolated enzymes. Compared to fermentations, enzymatic reactions have a number of advantages including simple instmmentation reduced side reactions, easy control, and product isolation. 

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). 

FIGURE 1-2 Diverse living organisms share common chemical features. Birds, beasts, plants, and soil microorganisms share with humans the same basic structural units (cells) and the same kinds of macromolecules (DNA, RNA, proteins) made up of the same kinds of monomeric subunits (nucleotides, amino acids). They utilize the same pathways for synthesis of cellular components, share the same genetic code, and derive from the same evolutionary ancestors. Shown here is a detail from "The Garden of Eden," by Jan van Kessel the Younger (1626-1679). 

D-Amino acid oxidase D-Amino acids (see p. 5) are found in plants and in the cell walls of microorganisms, but are not used in the synthesis of mammalian proteins. D-Amino acids are, hew ever, present in the diet, and are efficiently metabolized by 1he liver. D-Amino acid oxidase is an FAD-dependent enzyme that catalyzes the oxidative deamination of these amino acid isomers. The resulting a-ketoacids can enter the general pathways of amino acid metabolism, and be reaminated to L-isomers, or cafe balized for energy. 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

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