Plants, transamination

PEP carboxylase occurs in yeast, bacteria, and higher plants, but not in animals. The enzyme is specifically inhibited by aspartate, which is produced by transamination of oxaloacetate. Thus, organisms utilizing this enzyme control aspartate production by regulation of PEP carboxylase. Malic enzyme is found in the cytosol or mitochondria of many animal and plant ceils and is an NADPIT-dependent enzyme. 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

The wild type ilvA gene was modified to target the protein to the plastid and expressed in A. thaliana. Transgenic plants showed a 20-fold increase in levels of 2-ketobutyrate as well as a large increase in 2-aminobutyrate, the transaminated product of 2-ketobutyrate [27, 41]. The levels of threonine remained stable whereas isoleucine concentration increased. Constitutive expression of the ilvA protein along with bktB, phaA, and phaC proteins in the plastids of A. thaliana led to the synthesis of poly(3HB-co-3HV) in the range of 0.2 - 0.8 % dry weight, with a HV level between 4-17 mol % [27,41]. Co-expression of the iso-... 

In plants of tropical origin, the first intermediate into which 14C02 is fixed is oxaloacetate, a four-carbon compound. This reaction, which occurs in the cytosol of leaf mesophyll cells, is catalyzed by phosphoenolpyru-vate carboxylase, for which the substrate is HC03, not C02. The oxaloacetate thus formed is either reduced to malate at the expense of NADPH (as shown in Fig. 20-23b) or converted to aspartate by transamination ... 

In plants and bacteria, phenylalanine and tyrosine are synthesized from chorismate in pathways much less complex than the tryptophan pathway. The common intermediate is prephenate (Fig. 22-19). The final step in both cases is transamination with glutamate. 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

Perhaps the simplest explanation at this stage for the results obtained with dopa is that material which is fed to plants is poorly transported to the appropriate transaminating enzyme that is normally involved in alkaloid biosynthesis. The early pathway to benzylisoquinoline alkaloids that has so far been deduced is illustrated in Scheme 6. The compound (65) is not involved in the biosynthesis of reticuline. For details of the later stages in the biosynthesis of (66)—(71), see refs. 1 and 2. 

Glutamate provides the amino group for the synthesis of many other amino acids through transamination reactions in all cells. These amino acids are then used for protein synthesis and other aspects of nitrogen metabolism. The majority of animals are dependent on plant or animal proteins for fixed nitrogen, for their nitrogen metabolism. 

Returning to the question in the title of this section, capsaicin does not fall into any of the three classic types of nitrogen-bearing plant natural products, being neither a true alkaloid, a protoalkaloid, or a pseudoalkaloid. Capsaicin is oflimited distribution in Nature and shows pharmacological activity, but is non-basic, structurally unsophisticated, and not directly derived from an amino acidic precursor. On the other hand, the lack of attributes such as basicity, complexity, and an amino acidic pedigree can also be found in compounds commonly perceived as alkaloids. Thus, colchicine is neutral, ephedrine is structurally unsophisticated, and the nitrogen atom of the potato alkaloid solanine is not derived from an amino acid, but rather incorporated into as non-amino acidic framework by a transamination reaction. For the sake of clarity and consistency, it seems therefore convenient to adopt the modern definition of alkaloids, and consider capsaicin, as well as alkylamides such as piperine (18) and pellitorine (19), as such. 

Elongation of amino acid side chains prior to glucosinolate biosynthesis has been studied in several plants. The mechanisms involved are believed to be similar to the formation of leucine from valine and acetate (Fig. 3.13). Through transamination, the amino acid is converted to the corresponding... 

Herbivore diets usually contain adequate amounts of protein as evidenced by the net growth of the animal but the mixture of amino acids derived from vegetation is probably different from that required by the animal Thus herbivores probably synthesize a large proportion of their required amino acids by transamination of keto->acids derived from the carbohydrate part of their diet As a result one would expect that the carbon isotopes of both apatite and gelatin in herbivores would show a direct relationship to the mixture of Co and plants consumed ... 

We end this section on enzyme inhibition with a case study about 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) and disorders in tyrosine catabolism. After transamination of tyrosine, 4-hydroxyphenylpyruvate (148) is formed which is then decarboxylated, isomerized and oxygenated by HPPD to yield homogentisate (149) or by hydroxyman-delate synthase (HMS) to yield p-hydroxymandelate (150). 149 serves as the precursor for plastoquinones and tocopherols in plants . Thus, inhibitors of HPPD have been designed... 

Two other types of C4 pathways are recognized. In type-2 plants, Atriplex spongiosa) and type-3 Panicum maximum) plants, malate is replaced by aspartate as the major C4 acid transported to the bundle sheath cells. After transport, aspartate is converted to OAA by transamination. In type-2 plants, OAA is reduced to malate, which in turn is decarboxylated by NAD-malic enzyme in the bundle sheath cell mitochondria to give NADH, CO2 and pyruvate. In type-3 plants, OAA is decarboxylated in the cytosol by PEP carboxykinase in the presence of ATP, yielding PEP, CO2 and ADP. The return of carbon to the mesophyll cells for regeneration of the CO2 acceptor occurs as pyruvate (or alanine to maintain nitrogen balance) in type-2 and as PEP (or again perhaps as alanine) in type-3. These variations in the C4 pathway are summarized in Table I (see also Ref. 14). 

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