Biosynthesis of Amino Acids:- Amino Acid Biosynthesis Is under Allosteric Regulation
The most responsive regulation of amino acid synthesis takes place through feedback inhibition of the first re action in a sequence by the end product of the pathway. This first reaction is usually irreversible and catalyzed by an allosteric enzyme. As an example, Figure 1 shows the allosteric regulation of isoleucine synthesis from threonine . The end product, isoleucine, is an allosteric inhibitor of the first reaction in the sequence. In bacteria, such allosteric modulation of amino acid synthesis occurs as a minute to-minute response. Allosteric regulation can be considerably more com plex. An example is the remarkable set of allosteric controls exerted on glutamine synthetase of E.coli. Six products derived from glutamine serve as negative feedback modulators of the enzyme, and the overall effects of these and other modulators are more than additive. Such regulation is called concerted in hibition.
FIGURE 1 Allosteric regulation of isoleucine biosynthesis. The first reaction in the pathway from threonine to isoleucine is inhibited by the end product, isoleucine. This was one of the first examples of allosteric feedback inhibition to be discovered. The steps from ketobutyrate to isoleucine correspond to steps (five steps because 19 is a two-step reaction).
Because the 20 common amino acids must be made in the correct proportions for protein synthesis, cells have developed ways not only of controlling the rate of synthesis of individual amino acids but also of coordinating their formation. Such coordination is especially well developed in fast-growing bacterial cells. Figure 2 shows how E. coli cells coordinate the synthesis of lysine, methionine, threonine, and isoleucine, all made from aspartate. Several important types of inhibition patterns are evident. The step from aspartate to aspartyl-β-phosphate is catalyzed by three isozymes, each independently controlled by different modulators. This enzyme multiplicity prevents one biosynthetic end product from shutting down key steps in a pathway when other products of the same pathway are required. The steps from aspartate β-semialdehyde to homoserine and from threonine to -ketobutyrate are also catalyzed by dual, independently controlled isozymes. One isozyme for the conversion of aspartate to aspartyl- β -phosphate is allosterically inhibited by two different modulators, lysine and isoleucine, whose action is more than additive—another example of concerted inhibition. The sequence from aspartate to isoleucine undergoes multiple, overlapping negative feedback inhibition; for example, isoleucine inhibits the conversion of threonine to -ketobutyrate (as described above), and threonine inhibits its own formation at three points: from homoserine, from aspartate β-semialdehyde, and from aspartate . This overall regulatory mechanism is called sequential feedback inhibition. Similar patterns are evident in the pathways leading to the aromatic amino acids. The first step of the early pathway to the common intermediate chorismate is catalyzed by the enzyme 2-keto-3-deoxy-D-arabino heptulosonate 7-phosphate (DAHP) synthase . Most microorganisms and plants have three DAHP synthase isozymes. One is allosterically inhibited (feedback inhibition) by phenylalanine, another by tyrosine, and the third by tryptophan. This scheme helps the overall pathway to respond to cellular requirements for one or more of the aromatic amino acids. Additional regulation takes place after the path way branches at chorismate. For example, the enzymes catalyzing the first two steps of the tryptophan branch are subject to allosteric inhibition by tryptophan.
FIGURE 2 Interlocking regulatory mechanisms in the biosyn thesis of several amino acids derived from aspartate in E. coli. Three enzymes (A, B, C) have either two or three isozyme forms, indicated by numerical subscripts. In each case, one isozyme (A2, B1, and C2) has no allosteric regulation; these isozymes are regulated by changes in the amount synthesized. Synthesis of isozymes A2 and B1 is repressed when methionine levels are high, and synthesis of isozyme C2 is repressed when isoleucine levels are high. Enzyme A is aspartokinase; B, homoserine dehydrogenase; C, threonine dehydratase.
