Question

Pala is an inhibitor at low concentration it increases Atcase activity why? how does this observation support a concerted allosteric mechanism for Atcase

Answer 1

ATCase is the abrreviated form for aspartate transcarbamoylase. It is an important enzyme as it catalyses the first step in biosynthesis of pyrimidines, which ultimately forms the nitrogenous bases (T, C and U) of nucleic acids. This enzyme mainly existsin two forms- R-form which is the active form of the enzyme and T-form which is the inactive form of the enzyme. PALA stands for N-Phosphoacetyl-L-Aspartate, which is a potent inhibitor as it bindes to the enzyme and blocks it's activity. This happens because PALA mimics as the substrate analog; however, at low concentrations it tends to increase the reaction velocity. When PALA binds to ATCase in low concentrations, it converts or switches the inactive T-form of the enzyme into active R-form. When an enzyme is bound to PALA then it has fewer catalytic sites as compared with the enzyme where no PALA molecule is bounded. R-state of the enzyme will be having higher affinity towards the substrate molecules and continues the metabolic process.

In the Conserted allostery model of APCase enzyme, an enzyme exists in 2 conformations (as stated above in the answer; R and T forms). The distinguishing characteristic feature of concerted model is such that, that the conformation of all the subunits of the enzyme changes simultaneously. So, in absence/presence of substrate or ligand (example: PALA) , then the equilibrium favors either of the 2 forms.

To provide an evidence for the concerted allostery, ATCase has 3 catalytic subunit wihich forms a trimer and 2 regulatory subunits which forms a dimer. Active site of the enzymeis formed at an interface , that is, catalytic chains provides 3 active sites for the enzyme. So, when PALA binds, catalytic trimer tends to move away by 10 angstorm distance and the reglatory dimer rotates by 15 degrees. Thus, this causes the opening up the enzyme surface upon binding of the PALA molecule.

However, though PALA changes the conformation to R form (active) but the enzyme cannot catalyze the reaction further. Thus, it is clear that it changes the enzyme into R form but, allosterically does not allow the enzyme to continue it's function. This observation defines the concerted allosteric mechanism.