Enzyme Regulation - Allosteric Enzyme Regulation

Allosteric Regulative FeedbackA method of controlling metabolism is through allosteric regulative feedback. Allosteric enzymes are distinguished by their response to changes in substrate concentration in addition to their susceptibility to regulation by other molecules. Allosteric enzymes are multi-subunit proteins with one or more active site on each subunit.

Sigmoidal Curve If we plot V0 against [S] for an allosteric enzyme, it gives a sigmoidal curve rather than a hyperbolic curve predicted by Michealis-Menten kinetics. The curve has a steep section in the middle of the substrate concentration range. This reflects the rapid increase in enzyme velocity that occurs over a narrow range of substrate concentrations. Allosteric enzymes are sensitive to small changes in substrate concentration.

Cooperative BindingAllosteric enzymes are said to behave cooperatively. The binding of a substrate at one active site induces a conformational change in the protein that is conveyed to other active sites and altering their affinity for substrate molecules. Increased affinity causes a positive cooperativity producing a sigmoidal response. It can also cause a negative cooperativity where the substrate affects its own binding – this is known as homotropic.

Models of Allosteric RegulationThere are two models for allosteric enzymes and their effects in proteins; The concerted model and the sequential model. There are differences in the assumptions about subunit interaction and pre-existence of both states. Enzyme subunits can exist in a Tensed state or a Relaxed state. The relaxed state binds substrates more readily.

The Concerted ModelThe concerted model is also known as the MWC model. It suggests that allosteric proteins are oligomeric and exist in 2 conformation states – Tense or Relaxed. The states are in equilibrium, and the substrate binds to the relaxed state causing stabilisation and positive cooperativity. Positive cooperativity whereby the the enzyme subunits are connected in such a way that a conformational change in one subunit is conferred in all other subunits, thereby suggesting all subunits must exist in the same conformation.  In the absence of substrate, the Tense state is favoured. You cannot have a mix of R and T states therefore the model is concerted.

The Sequential ModelThe sequential model is also known as the Koshland Model. It suggests that the subunits are not connected in such a way that a conformational change in one induces a similar change on others. All enzyme subunits do not necessate the same conformation.  Substrate binding at one subunit alters the structures of other subunits so that their binding sites are more receptive to substrate. Most enzymes occupy this model. The enzymes are induced by the presence of a substrate. Regulation occurs through the different sites on the protein.

K and V systemAllosteric activators and inhibitors stabilise R and T states in 2 ways. They are known as the K system and the V system. In the V system, the inhibitor and activator bind to different states, the inhibitor binding stabilises the tense state and the activator binding stabilises the relaxed state. In the K system, the substrate has a different affinity for the two states, R & T. The states differ in catalytic activity.

Example of Allosteric RegulationAspartate Carbamoyltransferase ATCase – the enzyme ATCase is an allosteric enzyme, it catalyses the first step in the synthesis of pyrimidine – the formation of N-carbamoylaspartate. The binding of the two substrates aspartate and carbamoyl phosphate is cooperative. There is a sigmoidal curve.  ATCase has 6 regulatory subunits and 6 catalytic subunits. The enzyme is feedback inhibited by the end product of the pathway – cytosine triphosphate. This acts as an allosteric inhibitor. This binds to the regulator subunits and causes a decrease in the catalytic activity of ATCase by decreasing the affinity of the catalytic subunits for substrate molecules. ATP is an intermediate in the pathway. It acts as an allosteric activator enhancing the affinity of ATCase for its substrates and leading to an increase in activity. ATP competes with the same binding site on the regulatory subunit as CTP. High levels of ATP signal to the cell that energy is available for DNA replication, so ATCase is activated resulting in the synthesis of pyrimidine nucleotides.

Allosteric Enzyme Regulation