Carboxylic acids organic acid biosynthesis

Especially in dicotyledonous plant species such as tomato, chickpea, and white lupin (82,111), with a high cation/anion uptake ratio, PEPC-mediated biosynthesis of carboxylates may also be linked to excessive net uptake of cations due to inhibition of uptake and assimilation of nitrate under P-deficient conditions (Fig. 5) (17,111,115). Excess uptake of cations is balanced by enhanced net re-lea,se of protons (82,111,116), provided by increased bio.synthesis of organic acids via PEPC as a constituent of the intracellular pH-stat mechanism (117). In these plants, P deficiency-mediated proton extrusion leads to rhizosphere acidification, which can contribute to the. solubilization of acid soluble Ca phosphates in calcareous soils (Fig. 5) (34,118,119). In some species (e.g., chickpea, white lupin, oil-seed rape, buckwheat), the enhanced net release of protons is associated with increased exudation of carboxylates, whereas in tomato, carboxylate exudation was negligible despite intense proton extrusion (82,120). 

The possibility that many organic compounds could potentially be precursors of ethylene was raised, but direct evidence that in apple fruit tissue ethylene derives only from carbons of methionine was provided by Lieberman and was confirmed for other plant species. The pathway of ethylene biosynthesis has been well characterized during the last three decades. The major breakthrough came from the work of Yang and Hoffman, who established 5-adenosyl-L-methionine (SAM) as the precursor of ethylene in higher plants. The key enzyme in ethylene biosynthesis 1-aminocyclopropane-l-carboxylate synthase (S-adenosyl-L-methionine methylthioadenosine lyase, EC 4.4.1.14 ACS) catalyzes the conversion of SAM to 1-aminocyclopropane-l-carboxylic acid (ACC) and then ACC is converted to ethylene by 1-aminocyclopropane-l-carboxylate oxidase (ACO) (Scheme 1). 

Carboxylic acids, compounds of the type RCOH, constitute one of the most frequently encountered classes of organic compounds. Countless natural products are carboxylic acids or are derived from them. Some carboxylic acids, such as acetic acid, have been known for centuries. Others, such as the prostaglandins, which are powerful regulators of numerous biological processes, remained unknown until relatively recently. Still others, aspirin for example, are the products of chemical synthesis. The therapeutic effects of aspirin, welcomed long before the discovery of prostaglandins, are now understood to result from aspirin s ability to inhibit the biosynthesis of prostaglandins. 

Using ether-treated cells of P. aureofaciens, dicarbonyl-14C2 phenazme-l,6-dicarboxylic acid las (121)1 was found to be an efficient and specific precursor for phenazine-1-carboxylic acid (123), and also for 2-hydroxyphenazine-l-carboxylic acid (124). The rate of growth of the organism appeared to be important, because an incorporation was also recorded of the labelled (121) into (123), albeit at a lower level, with cultures that had been grown rapidly. The position of phenazine-1,6-dicarboxylic acid (121) as a universal intermediate in the biosynthesis of phenazines now seems secure. The previously reported failure of dimethyl phenazine-1,6-dicarboxylate (122) to act as a precursor of phenazines cf. Vol. 10, p. 28 Vol. 9, p. 29) has been confirmed with ether-treated cells of P. aureofaciens. Efficient hydrolysis of (125) to (123) did, however, occur.101... 

Amino acids are small organic molecules that posses both an amino and a carboxyl group. Amino acids occur in nature in a multitude of biological forms, either free or conjugated to various types of compounds, or as the building blocks of proteins. The amino acids that occur in proteins are named a-amino acids and have the empirical formula RCH(NH2)COOH. Only 20 amino acids are used in nature for the biosynthesis of the proteins, because only 20 amino acids are coded by the nucleic acids. 

The synthesis of compounds by biological organisms is called biosynthesis. Acyl halides and acid anhydrides are too reactive to be used as reagents in biological systems. Cells live in a predominantly aqueous environment, and acyl halides and acid anhydrides are rapidly hydrolyzed in water. So living organisms must activate carboxylic acids in a different way. 

One of these amino acids is azetidine-2-carboxylic acid (D 12.1), which in most organisms is accepted as substrate of prolyl-tRNA synthetase, an enzyme obligatory for protein biosynthesis. Azetidine-2-carboxylic acid is incorporated in proteins instead of L-proline, giving protein molecules with reduced biological efficiency. Prolyl-tRNA in plants which store azetidine-2-carbo-xylic acid, however, has a much higher substrate specificity and does not react with this compound. Even in the presence of azetidine-2-carboxylic acid therefore normal proteins will be formed. 

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