Feedback regulation enzyme multiplicity

Figure 9-5. Multiple feedback inhibition in a branched biosynthetic pathway. Superimposed on simple feedback loops (dashed, curved arrows) are multiple feedback loops (solid, curved arrows) that regulate enzymes common to biosynthesis of several end products.

Figure 6-3. The pentose phosphate pathway. In the oxidative phase of the pentose phosphate pathway, NADP is reduced to NADPH H, with feedback regulation by NADPH at the step catalyzed by glucose 6-phosphate dehydrogenase. In the nonoxidative phase, multiple sugar interconversions catalyzed by three different enzymes occur.

Sophisticated regulation can also evolve by duplication of the genes encoding the biosynthetic enzymes. For example, the phosphorylation of aspartate is the committed step in the biosynthesis of threonine, methionine, and lysine. Three distinct aspartokinases catalyze this reaction in E. coli, an example of a regulatory mechanism called enzyme multiplicity. (Figure 24.24). The catalytic domains of these enzymes show approximately 30% sequence identity. Although the mechanisms of catalysis are essentially identical, their activities are regulated differently one enzyme is not subject to feedback inhibition, another is inhibited by threonine, and the third is inhibited by lysine. 

Cholesterol can be obtained from the diet or it can be synthesized de novo. An adult on a low-cholesterol diet typically synthesizes about 800 mg of cholesterol per day. The liver is the major site of cholesterol synthesis in mammals, although the intestine also forms significant amounts. The rale of cholesterol formation by these organs is highly responsive to the cellular level of cholesterol. This feedback regulation is mediated primarily by changes in the amount and activity of 3-hydroxy 3 methylglutaryl CoA reductase. As described earlier (p. 739), this enzyme catalyzes the formation of meval-onate, the committed step in cholesterol biosynthesis. HMG CoA reductase is controlled in multiple ways ... 

Define the committed step of a metabolic pathway and recognize that it is often the target of feedback regulation. Note the main features of control of branched pathways by feedback inhibition and activation, enzyme multiplicity, and cumulative feedback. 

The synthesis of tryptophan in microorganisms is affected at several levels by end-product inhibition. Thus, end-product feedback inhibition partly regulates the synthesis of chorismic acid which is the final product of the common aromatic pathway and serves as a substrate for the first reaction in the tryptophan-synthesizing branch pathway (see Fig. 2). Regulation of the common aromatic pathway was recently reviewed by Doy [72]. The first enzyme of the common aromatic pathway, 3-deoxy-D-flrah/>jo-heptulosonate 7-phosphate synthetase (DAHPS), has been reported to exist as at least three isoenzymes, each specifically susceptible to inhibition by one of the aromatic amino acid end products (tyrosine, phenylalanine, and tryptophan), in E. coli (see reference [3]). It should be noted that many reports have indicated that in E. coli the DAHPS (trp), the isoenzyme whose synthesis is repressed specifically by tryptophan, was not sensitive to end-product inhibition by tryptophan. Recently, however, tryptophan inhibition of DAHPS (trp) activity has been demonstrated in E. coli [3,73,74]. The E. coli pattern, therefore, represents an example of enzyme multiplicity inhibition based on the inhibition specificity of isoenzymes. It is interesting to note the report by Wallace and Pittard [75] that even in the presence of an excess of all three aromatic amino acids enough chorismate is synthesized to provide for the synthesis of the aromatic vitamins whose individual pathways branch from this last common aromatic intermediate. In S. typhimurium, thus far, only two DAHPS isoenzymes, DAHPS (tyr) and DAHPS (phe) have been identified as sensitive to tyrosine and phenylalanine, respectively [76]. 

Tyrosine hydroxylase, the rate-hmiting enzyme, is a substrate for PKA, PKC, and CaM kinase phosphorylation may increase hydroxylase activity, an important acute mechanism whereby NE and Epi, acting at autoreceptors, enhance catecholamine synthesis in response to elevated nerve stimulation. In addition, there is a delayed increase in tyrosine hydroxylase gene expression after nerve stimulation, occurring at the levels of transcription, RNA processing, regulation of RNA stability, translation, and enzyme stability. Thus, multiple mechanisms maintain the content of catecholamines in response to increased transmitter release. In addition, tyrosine hydroxylase is subject to allosteric feedback inhibition by catecholamines. 

Isoenzymes. In this case, multiple enzymes are made each carries out the same reaction but is regulated by a different end product. This mechanism is used in both feedback inhibition and feedback repression. A well known example of such control is the aspartic acid family in E. coli where the three aspartokinases are regulated by lysine, threonine and methionine respectively (Stadtman, 1968). 

Studies on the regulation of the common pathway of aromatic biosynthesis in several micro-organisms have shown that control of the first reaction (Figure 1.2), the conversion of o-erythrose-4-phosphate (7) and phosphoenolpyruvate (8) to 3-deoxy-o-arabino-heptulosonic acid-7-phosphate (9, DAHP), catalysed by the enzyme DAHP synthetase (EC 4.1.2.15) is an important factor in the overall control of the pathway In a number of enteric bacteria this enzyme exists in multiple molecular forms each of which is under the feedback control of a specific end-product. Thus in Escherichia coli there are three DAHP synthetases (iso-enzymes), the activity and formation of which are controlled by the three aromatic amino acids The formation and activity of DAHP synthetase... 

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