Regulatory specificity, enzyme

In some multienzyme systems, the regulatory enzyme is specifically inhibited by the end product of the pathway whenever the concentration of the end product exceeds the cell s requirements. When the regulatory enzyme reaction is slowed, all subsequent enzymes operate at reduced rates as their substrates are depleted. The rate... 

The activities of allosteric enzymes (cf. 2.5.1.3) are affected by specific regulators or effectors. Thus, the activities of such enzymes show an additional regulatory specificity. 

The widespread occurrence of mitochondria in eukaryotic cells indicates that segregation of some of the most important aspects of energy metabolism has been evolutionarily useful. Such segregation has been accompanied by modifications of regulatory interrelationships which depend upon the movement of metabolites into and out of mitochondria and upon the specific modification of regulatory enzymes by specific allosteric modifiers. For example, the activity of pyruvate carboxylase, an enzyme which fulfills an anaplerotic function in a wide variety of cells, depends upon allosteric activation by acetyl CoA. Since the inner mitochondrial membrane is virtually impermeable to coenzyme A and its esters, the activation of pyruvate carboxylase may be looked upon as a compartmentation phenomenon in which this acetyl CoA is separated from the cytosol acetyl CoA pool and must be formed intramitochondrially. In this case movement across the membrane is not involved, since both the activation and the formation of the activator occur within the same compartment. Although a distinct possibility. 

Enzymes are used as feed digestibiUty enhancers for chicken and pigs. They must comply with purity specifications comparable to food-grade enzyme specifications. European Community (EC) guidelines for the assessment of additives in animal nutrition are being revised to make them appHcable for enzymes. Upon completion of these guidelines, the regulatory status of feed enzymes will be estabUshed in EC directive 70/524/EEC. 

The metabolic control is exercised on certain key regulatory enzymes of a pathway called allosteric enzymes. These are enzymes whose catalytic activity is modulated through non-covalent binding of a specific metabolite at a site on the protein other than the catalytic site. Such enzymes may be allosterically inhibited by ATP or allosterically activated by ATP (some by ADP and/or AMP). 

Most of these enzymes have steroids or fatty acids as their substrates (Table 1). Many P450s in endogenous biotransformation pathways are characterized by usually very narrow substrate and product specificity and by tight regulatory systems, especially those involved in steroid hormone biosynthesis. 

Little information is available on the specific control of fructan metabolism in higher plants73 or in microorganisms.35 If a regulatory role is envisaged for the di-D-fructose dianhydride enzymes, as has been suggested for microorganisms,71 then more detailed studies are required. 

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