Allosteric control , in enzymology, inhibition or activation of an enzyme by a small regulatory molecule that interacts at a site allosteric site other than the active site at which catalytic activity occurs. The interaction changes the shape of the enzyme so as to affect the formation at the active site of the usual complex between the enzyme and its substrate the compound upon which it acts to form a product. As a result, the ability of the enzyme to catalyze a reaction is modified. This is the basis of the so-called induced-fit theory , which states that the binding of a substrate or some other molecule to an enzyme causes a change in the shape of the enzyme so as to enhance or inhibit its activity.
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View AQA revision. View OCR revision. View Edexcel revision. View CIE revision. View WJEC revision. View Eduqas revision. You will remember that enzymes are classed as biological catalysts. That is, they help to accelerate the rate of a reaction, but remain unchanged during the entire process. These are called allosteric sites , and enzymes can have more than one. They are unique in that they have the ability to respond to multiple different conditions in their immediate environment.
Also, when allosteric enzymes are shown on a graph as velocity against substrate concentration, they show a sigmoid curve rather than the usual hyperparabolic curve. Allosteric sites are binding sites on the enzyme — they are different from the active site and the substrate binding site.
The molecule that binds to the allosteric site is called an effector it can also be called a modulator , and it regulates the activity of the enzyme it binds to.
The activity of the enzyme is increased when a positive allosteric effector binds to the allosteric site. This means that the activity of the enzyme is decreased when a negative allosteric effector binds to the allosteric site — they inhibit the enzyme.
Allosteric enzymes are larger and more complex than non-allosteric enzymes and often have many sub-units. Enzymes with more than one effector have different and specific binding sites for each one.
In most allosteric enzymes, the substrate binding site and the effector binding site are on different subunits. The substrate binding site is on the catalytic subunit — often referred to as the C subunit. The effector binding site is on the regulatory subunit — often referred to as the R subunit.
When an effector molecule at one binding site causes a conformational change it that subunit, a conformational change is then caused in the other subunits in the protein — this means that a huge portion of the binding energy of the effector is used to change the conformation of the whole protein complex.
This interaction between all of the subunits can be expressed using the Hill coefficient — this is also called a cooperativity coefficient. The larger the Hill coefficient cooperativity coefficient , the stronger the interactions between all of the subunits in the enzyme. This allows for sophisticated response patterns in activity, which can play a huge role in biological function.
Once the effector dissociates from the binding site, the enzyme is then able to revert back to its inactive or less active form. They can control the rates of highly important reactions, such as ATP production.
They are usually activators of the enzyme. The below image shows a homotropic allosteric effector. A good example of a homotropic allosteric effector is oxygen O 2 — it acts as an effector of haemoglobin in the human body.
A heterotropic allosteric effector is a regulatory molecule which is not also the substrate for the enzyme. It can either activate or inhibit the enzyme it binds to. The below image shows a heterotropic allosteric effector. Essential activators are allosteric activators that, without which, the enzyme activity would be so low it would be negligible.
For example, N-acetylglutamate is an essential activator for carbamoyl phosphate synthetase I. They are the exact opposite of enzyme inhibitors. The image below shows a generic allosteric enzyme. Properties of allosteric enzymes Allosteric sites are binding sites on the enzyme — they are different from the active site and the substrate binding site.
When an effector binds to an enzyme, it is called cooperative binding. Heterotropic Regulation A heterotropic allosteric effector is a regulatory molecule which is not also the substrate for the enzyme. Essential Activators Essential activators are allosteric activators that, without which, the enzyme activity would be so low it would be negligible.
17: Enzymes and Allosteric Regulation
In biochemistry , allosteric regulation or allosteric control is the regulation of an enzyme by binding an effector molecule at a site other than the enzyme's active site. The site to which the effector binds is termed the allosteric site or regulatory site. Allosteric sites allow effectors to bind to the protein, often resulting in a conformational change involving protein dynamics. Effectors that enhance the protein's activity are referred to as allosteric activators , whereas those that decrease the protein's activity are called allosteric inhibitors. Allosteric regulations are a natural example of control loops, such as feedback from downstream products or feedforward from upstream substrates. Long-range allostery is especially important in cell signaling.
Many enzymes do not demonstrate hyperbolic saturation kinetics, or typical Michaelis-Menten kinetics. Enzymes that display this non Michaelis-Menten behavior have common characteristics. A classic examples of allosterically regulated enzymes includes glycogen phosphorylase which breaks down intracellular glycogen reserves. Glycogen Phosphorylase. Another is aspartate transcarbamyolase, which catalyzes the first step in the synthesis of pyrimidine nucleotides.
D4. Allosteric Enzymes
Allosteric enzymes are an exception to the Michaelis-Menten model. Because they have more than two subunits and active sites, they do not obey the Michaelis-Menten kinetics but instead have sigmoidal kinetics. Since allosteric enzymes are cooperative , a sigmoidal plot of V 0 versus [S] results:. A sigmoidal plot has an S curve resulting from the combination of the T state and R state curves.
Allosteric enzymes are enzymes that change their conformational ensemble upon binding of an effector allosteric modulator which results in an apparent change in binding affinity at a different ligand binding site. This "action at a distance" through binding of one ligand affecting the binding of another at a distinctly different site, is the essence of the allosteric concept. Allostery plays a crucial role in many fundamental biological processes, including but not limited to cell signaling and the regulation of metabolism. Allosteric enzymes need not be oligomers as previously thought,  and in fact many systems have demonstrated allostery within single enzymes.