Allosteric properties

Because of the crucial role of this enzyme in one of the most important bicxrhemical pathways in the cell, its allosteric properties have been studied extensively in solution. Interpretation of these studies in terms of the theory of allosteric enzymes led Monod and coworkers to conclude that ... 

In addition to enzymes, noncatalytic proteins may exhibit many of these properties hemoglobin is the classic example. The allosteric properties of hemoglobin are the subject of a Special Focus beginning on page 480. 

THE ALLOSTERIC PROPERTIES OF HEMOGLOBINS RESULT FROM THEIR QUATERNARY STRUCTURES... 

The allosteric properties of FDPases present an interesting subject for future study. In the case of the liver enzyme the substrate shows positive cooperativity in binding, but no evidence for cooperativity in catalytic activity has been obtained. Perhaps this is because of the high affinity of the enzyme for the substrate, which prevents precise kinetic measurement at low substrate concentration. On the other hand, the substrate has been shown to increase the affinity of the enzyme for AMP, the allosteric inhibitor. 

Many aspects of metabolic control are not readily apparent when isolated enzymes are characterized in vitro. The regulatory function of allosteric properties and putative enzyme complexes are best studied in vivo. For this purpose, metabolic engineering can be used as a basic research tool to investigate the effect of specific perturbations on the overall flux through an alkaloid pathway. The first application of metabolic engineering to plant alkaloid biosynthesis involved the transformation... 

Leung, P., Lee, Y. M., Greenberg, E., Esch, K., Boylan, S., and Preiss, J. 1986. Cloning and expression of the Escherichia coli gig C gene from a mutant containing an ADPglucose pyrophosphorylase with altered allosteric properties. J. Bacteriol. 167, 82-88. 

The enzyme phosphofructokinase is allosteric, that is, it is made up of equivalent units that possess specific reaction sites for the fixation of the substrate and product. Each unit exists in two conformational states one active with more affinity for the substrate, and one inactive. The reaction products of phosphofructokinase (FDP and ADP) displace the conformational equilibrium in favor of the active form of the enzyme. This may create a destabilizing effect on the excess entropy production. In the glycolytic cycle, the allosteric properties of the phosphofructokinase may lead to oscillations. Consider the following simple model... 

In plants, only one conserved small (catalytic) subunit and several large (regulatory) subunits are distributed in different parts of the plant (56, 57). This finding is of physiological significance as expression of different large subunits in different plant tissues may confer distinct allosteric properties to the ADP-Glc PPase needed for the plant tissue s distinct need for starch. 

As previously indicated. Table 1 summarizes the kinetic and regulatory properties of purified potato tuber and spinach leaf ADP-Glc PPases. As reviewed in References 5 and 53, the properties of many other plant, algal, and cyanobacterial ADP-Glc PPases are similar. However, some ADP-Glc PPases within plant reserve tissue show some differences with respect to allosteric properties and their nonplastidic location. These examples are summarized below. 

Furthermore, the reconstituted enzyme has the same allosteric properties as the native enzyme. Thus, ATCase is composed of discrete catalytic and regulatory suhunits, which interact in the native enzyme to produce its allosteric behavior. 

Virtually all the metabolic processes that we will examine are regulated in part by covalent modification. Indeed, the allosteric properties of many enzymes are modified by covalent modification. Table 10.1 lists some of the common covalent modifications. 

B25. Benesch, R., and Benesch, R. E., The effect of organic phosphates from the human erythrocyte on the allosteric properties of hemoglobin. Biochem. Biophys. Res. Commun. 26, 162-167 (1967). 

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