Amino acid synthesis branched pathways

Herbicides also inhibit 5- (9/-pymvylshikiniate synthase, a susceptible en2yme in the pathway to the aromatic amino acids, phenylalanine, tyrosine and tryptophan, and to the phenylpropanes. Acetolactate synthase, or acetohydroxy acid synthase, a key en2yme in the synthesis of the branched-chain amino acids isoleucine and valine, is also sensitive to some herbicides. Glyphosate (26), the sulfonylureas (136), and the imida2oles (137) all inhibit specific en2ymes in amino acid synthesis pathways. 

Identification of the mode of action of the imidazolinones occurred while resistant cell lines were being isolated. Imidazolinones inhibit acetohydroxyacid synthase (AHAS EC 4.1.3.18), the first enzyme in the pathway of branched chain amino acid synthesis (8). Imidazolinone-resistant cell lines provide proof that inhibition of AHAS is the site of action of the imidazolinones AHAS activities in extracts from resistant corn cell lines are highly resistant to inhibition by imidazolinone herbicides (7). 

Herbicides that inhibit enzymes important for amino acid synthesis account for 28% of the herbicide market. Just three enzymes are involved the enzyme that adds phosphoenolpyruvate to shikimate-3-phoshate in the pathway leading to aromatic compounds, the enzyme that makes glutamine from glutamate and ammonia, and the first common enzyme in the biosynthesis of the branched-chain amino acids. 

Branched chain amino acid synthesis has been extensively studied (, Figure 3). In branched chain amino acid biosynthesis ALS Is the second enzyme of Isoleuclne biosynthetic pathway and the first enzyme of valine synthesis. Commonly the first enzyme of a synthetic pathway is Inhibited by the end-product thus preventing waste of carbon skeletons and energy. Two ALS Isozymes exist In... 

Amino acid degradation is important for the synthesis of volatile compounds and the transamination of some amino acids methionine, branched-chain, and aromatic amino acids. Transamination is the main degradation pathway that leads to the formation of a-keto acids, which are then degraded into various aromatic compounds. The conversion of amino acids to keto- and hydroxyl acids is initiated by lactobacilli, and Lactococcus strains further convert these products to carboxylic acid. This cooperation between LAB and non-starter LAB can enhance cheese flavor. 

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. 

Although the mitochondria are the primary site of oxidation for dietary and storage fats, the peroxisomal oxidation pathway is responsible for the oxidation of very long-chain fatty acids, jS-methyl branched fatty acids, and bile acid precursors. The peroxisomal pathway also plays a role in the oxidation of dicarboxylic acids. In addition, it plays a role in isoprenoid biosynthesis and amino acid metabolism. Peroxisomes are also involved in bile acid biosynthesis, a part of plasmalogen synthesis and glyoxylate transamination. Furthermore, the literature indicates that peroxisomes participate in cholesterol biosynthesis, hydrogen peroxide-based cellular respiration, purine, fatty acid, long-chain... 

Investigation of Mutant Strains. Murray (14) suggested that the amino acids valine, leucine and isoleucine were precursors of 2-methoxy-3-isopropyl, 3-isobutyl and 3-secbutyl pyrazines, respectively. The effects of mutations in the branched chain amino acid pathways on the synthesis of these compounds was investigated. 

The mutant which was blocked in the synthesis of branched chain amino acids produced very low levels of methoxy pyrazines. Cultures of this mutant did generate a new N peak and produced a strong butter-like aroma. TVo compounds were identified in these cultures as 2,3,5,6-tetramethy1 pyrazine and diacetyl. The synthesis of tetramethylpyrazine by a Corynebacterium glutamicum that was also metabolically blocked in the branched chain amino acid pathway has previously been reported (24). 

Chorismate synthase (CS) catalyzes the formation of chorismate, the last step in the shikimate pathway. Chorismate is a branch-point metabolite used for the synthesis of aromatic amino acids, p-aminobenzoic acid, folate, and other cyclic metabolites such as ubiquinone. The shikimate pathway is found only in plants, fungi, and bacteria, making the enzymes of the pathway potential targets for herbicides, antifungals, and antibiotics. 

Besides showing the unbranched pathway from erythrose-4-phosphate and phosphoenolpyruvate to shikimic acid. Figure 2L13 also shows the sequence of reactions from shikimic acid to chorismate, the first major branch point in the synthesis of the aromatic amino acids and histidine. The sixth reaction of the shikimic acid pathway is inhibited specifically by glyphosate (see here), which is the active ingredient in the broad spectrum herbicide known as Roundup. 

Synthesis of glutamate removes a-ketoglutarate from the TCA cycle, thereby decreasing the regeneration of oxaloacetate in the TCA cycle. Because oxaloacetate is necessary for the oxidation of acetyl CoA, oxaloacetate must be replaced by anapierotic reactions. There are two major types of anapierotic reactions (1) pyruvate carboxylase and (2) the degradative pathway of the branched-chain amino acids, valine and isoleucine, which contribute succinyl CoA to the TCA cycle. This pathway uses B12 (but not folate) in the reaction catalyzed by methylmalonyl CoA mutase. 

Feedback Regulation. Feedback regulation also appears to play a role in secondary metabolism. It was shown many years ago that chloramphenicol inhibits its own production at concentrations nontoxic for growth of Streptomyces venezuelae (Legator and Gottlieb, 1953). Addition of 6-methylsalicylic acid to idiophase mycelium of P. urticae inhibits its own synthesis (Bu Lock and Shepherd, 1968). Stimulation of carotenoid overproduction by )8-ionone in Phycomyces blakesleeanus appears to be due to its ability to interfere with normal feedback inhibition (Reyes et al, 1964). The inhibition of penicillin production by lysine (Demain, 1957) seems to be due to feedback regulation by the amino acid of a branched pathway (Fig. 6) leading to both lysine and... 

Plant secondary metabolites are biosynthesized from rather simple building blocks supplied by primary metabolism. Two important metabolic routes in this are the shikimate pathway and the isoprenoid biosynthesis. The shikimate pathway leads to the synthesis of phenolic compounds and the aromatic amino acids phenylalanine, tyrosine and tryptophan. The isoprenoid biosjmthesis is a heavily branched pathway leading to a broad spectrum of compounds (fig. 1). From plants and microorganisms more than 37,000 isoprenoid compounds have been isolated so far [1]. 

Since the substrate specificity of individual aminotransferases may vary widely, it is not known whether the nitrogen atoms of the aspartate family and branched-chain amino acids are derived from a single, or from multiple precursors (Table I). Utilization of a common amino donor in aminotransferase catalyzed reactions would strengthen the biosynthetic relationship among the pathway products, whereas multiple precursors could tend to balance the synthesis of these amino acids with that of other protein precursors in a type of crosspathway or interfamily regulation. 

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