The shikimate pathway provides the basic building blocks for the synthesis of the three aromatic amino acids as well as an array of other aromatic compounds required for functions as different as UV protection, electron transport, signalling, communication, plant defence, structural components and the wound response (Schmid and Amrhein, 1995; Radwanski and Last, 1995; Herrmann and Weaver, 1999). The pathway is firmly rooted in primary metabolism and forms a major link between primary and secondary metabolism in higher plants. From just this brief survey of products derived from the shikimate pathway it is not surprising to find that up to 35% of the ultimate plant mass in dry weight has its metabolic origins in this pathway as for example lignins are derived from the shikimate pathway. The first seven reactions of the pathway lead from erythrose 4-phosphate and PEP via shikimate to chorismate and are also referred to as the main trunk of the shikimate pathway, or the pre-chorismate pathway.The shikimate pathway is present only in bacteria, fungi and plants. The absence of the pathway in all other genera has rendered the enzymes catalysing these reactions potentially useful targets for the development of new antibiotics and herbicides (Siehl, 1992).
Some of these reactions are unique in nature: for example, 5-enolpyruvylshikimate 3-phosphate synthase (EPSP-synthase), the sixth enzyme of the pre-chorismate pathway, catalyses the transfer of the intact enolpyruvate to shikimate 3-phosphate.
The first step of the synthesis of these three amino acids is the condensation of erythrose 4′-phosphate (derived from the oxidative pentose phosphate pathway or the Calvin cycle) with phosphoenolpyruvate (from glycolysis) to produce 3′-deoxy D-arabino heptulosonic acid 7′-phosphate (DAHP). This undergoes a series of reactions, including loss of a phosphate, ring closure and a reduction to give shikimic acid, which is then phosphorylated by shikimate kinase. Shikimate phosphate is combined with a further molecule PEP to give 3-enolpyruvylshikimate 5-phosphate (EPSP). The enzyme EPSP synthase, which has received considerable attention because it is inhibited by the herbicide, glyphosate, catalyses this latter reaction. EPSP is converted to chorismic acid, which is at a branch point in this pathway, and can undergo two different reactions, one leading to tryptophan, and the other to phenylalanine and tyrosine.
Anthranilate synthase (AS) catalyses the first reaction in the multi-step tryptophan biosynthesis branch by converting chorismate to anthranilate. AS is feedback inhibited by the end product tryptophan, which binds to an allosteric site on the AS catalytic αα-subunit. The fact that AS is the control point in the tryptophan branch in plant cells is indicated by pathway intermediate feeding and many other studies. But conversion of chorismate to tryptophan has significance beyond amino acid biosynthesis. This is the branch point from which the essential aromatic amino acids as well as many important secondary plant metabolites are derived. Plants use this pathway to produce precursors for numerous secondary metabolites, including the hormone auxine (e.g. indoleacetic acid), indole alkaloids, phytoalexins, cyclic hydroxamic acids, indole glucosinolates, acridone alkaloids, tetrahydrofolate, ubiquinone and vitamine K. These metabolites serve as growth regulators, defence agents and signals for insect pollinators and herbivores. Some of these alkaloids have great pharmacological value, including the anticancer drugs vinblastine and vincristine.
The synthesis of tryptophan from chorismate begins with the reaction of chorismate with the amide group of glutamine to produce anthranilic acid, which subsequently condenses with phosphoribosyl pyrophosphate (derived from ribose 5′-phosphate) to give phosphoribosyl anthranilate. This molecule undergoes a further series of reactions to produce indole glycerol phosphate, which then reacts with serine to produce tryptophan (catalysed by tryptophan synthase).
The synthesis of phenylalanine and tyrosine starts with the rearrangement of chorismate by chorismate mutase to prephenic acid, whose further metabolism has been subject to some debate. For some time the synthesis of phenylalanine and tyrosine from prephenate in plants was assumed to be the same as in bacteria, where the prephenate is either dehydrated to phenylpyruvate (prephenate dehydratase) or oxidatively decarboxylated to hydroxyphenylpyruvate (prephenate dehydratase). Both of these keto acids are subsequently aminated by tranaminases, the former to phenylalanine and the latter to tyrosine. In addition, although phenylalanine, tyrosine and tryptophan are necessary for protein biosynthesis, phenylanine is also a substrate for the phenylpropanoid pathway that produces numerous secondary plant products, such as anthocyanins, lignin, growth promoters, growth inhibitors and phenolics.
Although some of the enzymes involved in this route have been found in plants, there is a growing body of evidence which suggests that another route is either also, or in some plants solely, in operation. Formerly called the ‘pretyrosine’ pathway, it is now generally referred to as the ‘arogenate pathway’, and involves the transamination of prephenate to arogenate, which is then directly converted to either phenylalanine (arogenate dehydratase) or tyrosine (arogenate dehydratase). Arogenate dehydratase has been purified from sorghum and is activity shown to be inhibited by phenylalanine and stimulated by tyrosine, as might be expected from its position in the pathway.
Many of the enzymes of tryptophan synthesis have been found in the chloroplast, and labelling studies with
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