Allosteric enzymes ATCase

Although the Michaelis-Menten model provides a very good model of the experimental data for many enzymes, a few enzymes do not conform to Michaelis-Menten kinetics. These enzymes, such as aspartate transcarbamoylase (ATCase), are called allosteric enzymes (see Topic C5). 

Aspartate transcarbamoylase (aspartate carbamoyltransferase ATCase), a key enzyme in pyrimidine biosynthesis (see Topic FI), provides a good example of allosteric regulation. ATCase catalyzes the formation of N-carbamoylaspar-tate from aspartate and carbamoyl phosphate, and is the committed step in pyrimidine biosynthesis (Fig. 2). The binding of the two substrates aspartate and carbamoyl phosphate is cooperative, as shown by the sigmoidal curve of V0 against substrate concentration (Fig. 3). 

Aspartate transcarbamoylase (ATCase) is an allosteric enzyme of the bacterium Escherichia coli, which has been extensively studied. This enzyme catalyzes the transfer of the carbamoyl group from carbamoyl phosphate to the amino group of aspartate ... 

Why do chymotrypsin and ATCase have different velocity curves Chymotrypsin and aspartate transcar-bamoylase exhibit different types of kinetics. Chymotrypsin is a nonallosteric enzyme and exhibits hyperbolic kinetics. ATCase is an allosteric enzyme. It has multiple subunits, and the binding of one molecule of substrate affects the binding of the next molecule of substrate. It exhibits sigmoidal kinetics. 

AP (apuiinic) site a site, lacking a purine base in DNA, that IS targeted by repair enzymes (10.5) arachidonic acid a fatty acid that contains 20 carbon atoms and 4 double bonds the precursor of prostaglandins and leukotrienes (8.8) aspartate transcarbamo)lase (ATCase) a classic example of an allosteric enzyme that catalyzes an early reaction in pyrimidine biosynthesis (6.5)... 

Aspartate transcarbamoylase (ATCase, EC 2.1.3.2) is an allosteric enzyme which controls the first step of p5oimidine de novo biosynthesis. It catalyzes the condensation of L-aspartic acid with carbamyl phosphate to produce carbamoyl-L-aspartate... 

The fact that ATP and CTP bind to the same site follows from the observation that adding ATP to the inhibited enzyme by CTP reduces or reverses the inhibition, presumably because ATP competes with CTP for the same site. The fact that CTP binds to an allosteric site (i.e., it is not a competitive inhibitor) follows from the so-called desensitization effect. Addition of mercurials [e.g., p-mercuribenzoate (PMB)] reduces or eliminates the inhibition by CTP. However, it has no effect on the enzymatic activity of ATCase, presumably because the mercurials affect the regulatory subunits but not the catalytic site. As for the mechanism of cooperativity (both positive and negative), it is known that CTP does induce changes in the quaternary structure of the enzyme. 

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. 

The allosteric kinetic effects of ATCase are shown in Figure 7-6. The interaction of substrates with the enzyme is cooperative (an example of homotropic cooperativity), as indicated by the sigmoidal shapes of the v versus [S] plots, CTP being an inhibitor and ATP an activator. These modulators compete for the same regulatory site and modulate the affinity of the enzyme for its... 

The inhibitory ability of CTP is remarkable because CTP is structurally quite different from the substrates of the reaction (see Figure 10.1). Thus CTP must bind to a site distinct from the active site where substrate binds. Such sites are called allosteric or regulatory sites. CTP is an example of an allosteric ibitoY. In ATCase (but not all allosterically regulated enzymes), the catalytic sites and the regulatory sites are on separate polypeptide chains. 

Allosteric Effectors (substrates, products, or coenzymes) of a pathway inhibit or activate an enzyme. (Responds rapidly to external stimuli.) ATCase (Section 7.2) Phosphofructokinase (Section 17.2)... 

Homotropic effects are allosteric interactions that occur when several identical molecules are bound to a protein. The binding of substrate molecules to different sites on an enzyme, such as the binding of aspartate to ATCase, is an example of a homotropic effect. Heterotropic effects are allosteric interactions that occur when different substances (such as inhibitor and substrate) are bound to the protein. In the ATCase reaction, inhibition by CTP and activation by ATP are both heterotropic effects. 

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