Term allosteric was coined by two Nobel laureates – Jacob and Monad to identify the enzyme site which is different from the active site. Allosteric enzymes are those which contain an additional site, the allosteric site in addition to the active site. These are unique areas on the enzymes where an allosteric enzyme may have one or more than one allosteric sites. They non-competitively associate with molecules apart from the substrates and influence the activity of enzymes.
Characteristics of Allosteric enzyme
- Allosteric sites are binding sites different from an enzyme active site or substrate binding site
- Allosteric enzymes have one or more allosteric sites
- Modulator or effector are the molecules which bind to allosteric sites
- Effectors check the activity of enzymes and can be negative or positive
- The action of enzymes is enhanced when a positive allosteric effector holds to the allosteric site known as activator site
- Negative allosteric effector holds to the allosteric site known as inhibitor site. It inhibits the activity of enzymes which is referred to as cooperative binding
Model of Allosteric regulation
There are two main models which have been suggested to describe the mechanistic grounds of enzyme allostery.
- Sequential model
- Concerted model
Sequential model –
This model was put forward in 1966 by Koshland Jr. As per this theory, the allosteric enzyme can exist in only 2 conformational changes separately.
Concerted model –
In 1965, the model was put forward by Jacques Monod and his colleagues. As per them, allosteric enzymes may exist in still two conformations, relaxed and active or in an inactive form.
Types of Allosteric regulation
There are two types of allosteric regulation, they are –
Homotropic – these allosteric modulators are substrates for their target enzymes and the regulatory molecule of the activity of enzymes. They serve as activators to enzymes.
Heterotropic – these allosteric modulators are regulatory molecules that are not also the substrate of the enzyme. This can be either inhibitor or activator of the enzyme
This was a brief on allosteric enzymes. Another interesting topic which could be studied is cretinism.
Cretinism or congenital hypothyroidism, seen in newborns, is a severe deficiency of the thyroid hormone. It leads to impaired neurological functioning, physical deformations and stunted growth. This may be as a result of a difficulty with the thyroid gland of the baby. In some other cases, it can be inadequate iodine content in the mother’s diet during her pregnancy.
Some symptoms of cretinism are – stunted growth, lack of weight gain, lethargy, excessive sleep, hoarse voice, constipation, abnormal growth of bones, unusually large tongue, low muscle tone, pale skin, myxedema, goitre, umbilical hernia to name a few.
In newborns, this condition could be due to a poorly formed or missing or unusually small thyroid gland, genetic defect affecting the production of thyroid hormone, radioactive iodine for thyroid cancer during pregnancy, use of medicines disrupting thyroid hormone production and so on.
This was a brief on cretinism. Alternatively you can subscribe to BYJU’S YouTube Channel for related content.