Enzyme Regulation - Allosteric Enzyme Regulation

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Allosteric Regulative Feedback

A 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 Binding

Allosteric 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 Regulation

There 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 Model

The 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 Model

The 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 system

Allosteric 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 Regulation

Aspartate 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