Name an allosteric inhibitor that inhibits Aspartate transcarbamoylase activity and is also an example of feedback inhibition.
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Remember that the ratio of ATP to CTP regulates ATCase activity.
High ATP activates the enzyme to match purine levels, whereas high CTP inhibits it to prevent pyrimidine overproduction.
Step 1: Understanding the Concept:
Aspartate transcarbamoylase (ATCase) is a highly regulated, multi-subunit enzyme that catalyzes the first committed step in the de novo biosynthesis of pyrimidine nucleotides.
This step involves the condensation of carbamoyl phosphate and L-aspartate to form N-carbamoylaspartate and inorganic phosphate.
Allosteric regulation in such metabolic pathways is critical for maintaining homeostatic levels of nucleotides, ensuring that resources are not wasted when the final products are abundant.
Feedback inhibition occurs when the ultimate end-product of a multi-step biosynthetic pathway binds to and inhibits an enzyme acting at an earlier, rate-limiting step. Step 2: Detailed Explanation:
In the de novo pyrimidine pathway, the end-product is cytidine triphosphate (CTP).
When CTP accumulates within the cell to high concentrations, it serves as a signal that the cellular pool of pyrimidines is sufficient for DNA and RNA synthesis.
CTP acts as an allosteric inhibitor by binding to the regulatory subunits of ATCase.
The ATCase enzyme is a dodecamer composed of six catalytic subunits and six regulatory subunits.
The binding of CTP to the regulatory polypeptide chains induces a significant conformational change throughout the entire enzyme complex.
This structural transition shifts the enzyme from its active R (relaxed) state to its inactive T (tense) state.
In the T-state, the affinity of the catalytic sites for the substrates, particularly aspartate, is significantly decreased, thereby slowing down the rate of pyrimidine synthesis.
Conversely, ATP acts as an allosteric activator of the same enzyme.
High ATP levels indicate abundant energy resources and a high concentration of purines.
To maintain the precise stoichiometric ratio between purines and pyrimidines required for nucleic acid replication, ATP binds to the regulatory subunits and shifts the equilibrium back to the active R-state.
Thus, CTP acts as a classic feedback inhibitor of ATCase. Step 3: Final Answer:
Therefore, CTP is the allosteric inhibitor that regulates aspartate transcarbamoylase via feedback inhibition.
