1.) Gerhart and Pardee measured ATCase activity in the presence of a variety of
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Question
1.) Gerhart and Pardee measured ATCase activity in the presence of a variety of purine and pyrimidine derivatives. Their results are presented in Table 16.2. What compound(s) were the most effective inhibitors? activators? Explain the significance of the metabolites that served as inhibitors or activators in the context of the biosynthetic pathway presented in Figure 16.1.
Table 16.2: Effect of nitrogen bases, nucleosides and nucleotides on ATCase activity. *Indicates stimulation. (Based on Gerhart and Pardee, 1962.)
Table 16.2: Effect of nitrogen bases, nucleosides and nucleotides on ATCase activity. Indicates stimulation. (Based on Gerhart and Pardee, 1962.) Compound Inhibition, (Conc 2 mM) Cytosine 24 Cytidine 38 CMP dCMP 48 86 CTP 88 dCTP 35 GTP dGTP 31 -180* ATP dATP -162*Explanation / Answer
In the above question ,the CTP acts as an inhibitor. Moreover the ,Cytosine alone is not able to inhibit whilethe cytidine and deoxycytidine due to the presence of sugar exhibit a small amount of inhibition. Furthermore,There is a large amount of inhibition shown by CMP, CDP and CTP, which indicates that inhibition is due to the presence of a large number of phosphate groups. On the other hand the UTP cannot inhibit the enzyme, as it indicates that the amino group is essential for inhibition. Finally the ATP and dATP function as stimulators of the enzyme in the above mentioned case.
The signifcance of metabolites is that iIn the above question the CTP functions acts an inhibitor as it is the product of the pathway. Moreove,the High concentrations of CTP produced is used to signal the enzyme that CTP is not needed any longer in the process.Furthermore,the ATP is used to stimulate the pathway as it is necessity that high concentration of purines should be able to match to a high concentration of pyrimidines for the process DNA and RNA synthesis to take place effectively.
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