Allosteric Enzyme

An Overview of Allosteric Enzyme

The name allosteric has come from Greek word “allo” which means ‘other’. Likewise, allosteric enzymes are the enzymes that have an additional site which is also an active site. Several molecules in our body affect the enzyme regulation by increasing or decreasing their activities. 

This regulation is classified into different types like allosteric regulation, covalent and genetic modification, etc. Furthermore, allosteric regulation of enzyme works in a particular way and controls cellular activities. However, all enzymes do not possess sites for allosteric binding. Notably, this is a vital topic for your NEET.

Consequently, read on this article for a detailed insight about the topic!

Allosteric Enzymes Properties

Following are a few distinct features of Allosteric enzymes. 

• These enzymes are more complicated and larger than other enzymes and often contain several sub-units.  

• Allosteric sites are mainly binding sites of enzyme. Moreover, they differ from substrate binding sites and active sites.

• Effectors or modulator are the molecules which bind to the Allosteric site. They also control the activity of the enzymes they link to. 

• Allosteric effectors regulate the activity of enzymes. For example, activity increases when a positive Allosteric effector binds to an Allosteric site. Similarly, activity decreases when a negative effector does the same. 

• For most Allosteric enzymes, the substrate binding site and effector binding site are on different sub-units. 

• The substrate binding site that is on the catalytic subunit is called C subunit. On the other hand, the effector binding one on regulatory subunit is called R subunit. 

• A large portion of binding energy of the effector is utilised to alter the whole configuration of protein complex. 

• Allosteric enzymes can also switch between their inactive and active form. For that, they play a crucial role in controlling some significant reactions like ATP production. 

Types of Regulation

Allosteric regulation of enzyme is primarily divided into two types

1. Homotropic Regulation- It is a substrate for its target enzyme. Also, it is a regulatory molecule of the enzyme’s activity. It is typically an activator of an enzyme. For example, O2 and CO are Homotropic Allosteric modulators of haemoglobin. 

2. Heterotropic Regulation- This is a regulatory molecule that is not the enzyme’s substrate. It can either be an activator or an inhibitor of the enzyme. For example, CO2, H+ and 2, 3- biphosphoglycerate are the heterotropic Allosteric modulators of haemoglobin.   

Apart From That, Allosteric Regulation Acts in Two Different Ways.

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1. Allosteric Inhibition- The binding of an Allosteric inhibitor makes the configurative changes of an enzyme and its active site. Therefore the substrates are unable to bind. As a result, the reaction rates reduce. 

2. Allosteric Activation- Allosteric activators, on the contrary, effectively increase the reaction rates. 

Allosteric Enzymes Mechanism

Based on the Allosteric regulation of enzymes, the following models are proposed. These two models are used to elucidate the cooperative behaviour of haemoglobin. 

• Simple Sequential Model 

In the sequential model, the binding of the oxygen stimulates a conformational change in the polypeptide. It binds to this in twin and induces a conformational change in the nearby polypeptide chains. As the subunits change in conformation, their affinity to O2 increases. 

According to this model, there are immediate states between the T state and R state. This model shows that the R state is only achieved when all four sites are occupied. 

• Concerted Model 

According to this model, the haemoglobin either exists in T state or R state. The binding of the oxygen simply shifts the equilibrium between these two states. Nonetheless, this model states that affinity of the other homo sites only increase when the haemoglobin assumes the R state. 

In conclusion, it can be said that neither of these models can describe the nature of haemoglobin alone by themselves. 

Allosteric Enzymes Examples

The most prominent Allosteric enzyme example is glycogen phosphorylase. Apart from that, the other examples include glutamine synthetase, phosphofructokinase and aspartate transcarbamoylase (ATCase). 

• ATCase- It catalyses the first step of pyrimidine formation. N-carbamoyl aspartate then forms some types of pyrimidine-based nucleoside triphosphate like CTP. 

• Glutamine Synthetase- Glutamate and glutamine can be synthesised from alpha-ketoglutarate, an intermediate form of TCA cycle. 

•  Glycogen Phosphorylase- It controls glycogen metabolism. It also takes an active part in Glycolysis. 

• Phosphofructokinase- This is the most vital enzyme of Glycolysis. It works through Allosteric inhibition. It also regulates the hormones like insulin and glucagon in eukaryotes. 

Thus Allosteric regulation of enzymes takes part in several crucial biological reactions.

Pop Quiz

True/false

1. Allosteric modulators can be either inhibitory or stimulatory.

2. Conversion of L-Leucine to L-isoleucine takes place in Allosteric feedback inhibition. 

Answers: 1-True, 

                  2-False

Hopefully, by now, you are almost done with the preparation for NEET. Now, it is time to revise the critical chapters of Biology like Allosteric enzymes, reproduction, genetics and others. 

The essential aspects of Allosteric regulation of enzymes are discussed above, if you can keep those points in mind, answering any kind of questions will not be difficult for you. Along with the methodical study, being positive and staying healthy is equally important, especially when your exam is just around the corner. Therefore, take good care of your mental as well as physical health and keep up your hard work.