Understand the Allosteric Enzyme and its Importance
The name allosteric has come from the 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 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 enzymes 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 this article for a detailed insight into 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 modulators 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 the regulatory subunit is called R subunit.
A large portion of the binding energy of the effector is utilised to alter the whole configuration of a 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 enzymes is primarily divided into two types
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.
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.
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.
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 increases 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.
Quiz
1. (True/False) Allosteric modulators can be either inhibitory or stimulatory.
Answer- True
2. (True/False) Conversion of L-Leucine to L-isoleucine takes place in Allosteric feedback inhibition.
Answer- False
3. Which of the following best describes the allosteric regulation of enzymes?
a) Regulation by the substrate molecule at the active site only
b) Regulation by binding of molecules at a site other than the active site
c) Inhibition of enzyme activity by increased temperature
d) Activation of enzymes by pH changes
Answer-- b) Regulation by binding of molecules at a site other than the active site
4. What is the role of an allosteric inhibitor?
a) It binds to the active site and increases the enzyme's activity
b) It binds to the allosteric site and decreases the enzyme's activity
c) It changes the substrate’s structure
d) It binds to the substrate and stabilizes the enzyme's active form
Answer- b) It binds to the allosteric site and decreases the enzyme's activity
5. Which model explains the binding of a substrate causing a conformational change that affects other subunits of the enzyme?
a) Induced fit model
b) Simple Sequential Model
c) Lock and key model
d) Symmetry model
Answer- b) Simple Sequential Model
6. Which of the following is a characteristic of homotropic regulation in allosteric enzymes?
a) The substrate acts as an effector molecule
b) The effector molecule is different from the substrate
c) The enzyme is activated by an inhibitor
d) There is no cooperative binding of substrates
Answer- a) The substrate acts as an effector molecule
7. In the concerted model of allosteric regulation, what happens when an inhibitor binds?
a) It shifts the enzyme to its active form
b) It shifts the enzyme to its inactive form
c) It has no effect on the enzyme
d) It increases the enzyme's affinity for its substrate
Answer- b) It shifts the enzyme to its inactive form
8. Which of the following enzymes is an example of allosteric regulation in humans?
a) DNA polymerase
b) Hemoglobin
c) Lactase
d) Amylase
Answer- b) Hemoglobin
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Conclusion
Understanding the key concepts of allosteric enzymes is crucial for NEET preparation. The regulation of enzyme activity through allosteric sites plays a significant role in various biological processes, and learning this topic will undoubtedly help you solve any related questions in the exam. As you move forward with your revision, remember to focus not only on your studies but also on maintaining a positive mindset and staying healthy. Mental and physical well-being are just as important as academic preparation, especially with the exam approaching. Keep up the hard work, stay motivated, and believe in your abilities. Best of luck in your NEET exam!
FAQs on Allosteric Enzyme
1. Which Site of an Enzyme is an Allosteric Site?
The inactive site of an enzyme is called the Allosteric site. Some drugs bind to this site and effectively change the shape of that enzyme.
2. What is the Importance of Allosteric Enzymes?
Allosteric enzymes play a vital role in cellular regulation. They catalyse the reactions in metabolic pathways as well as control the rate of those pathways.
3. What are the Examples of Irreversible Enzyme Inhibitors?
Some examples of irreversible enzyme inhibitors are aspirin, penicillin and afatinib.
4. Why is Enzyme Regulation Vital?
Regulation of enzyme activity is vital because it coordinates different metabolic reactions. It also takes part in homeostasis.
5. What is the difference between allosteric enzymes and regular enzymes?
Regular enzymes typically have just one active site where the substrate binds and the enzyme carries out its function. In contrast, allosteric enzymes have multiple allosteric sites, which can influence the enzyme's activity. These additional sites allow for more complex regulation, either activating or inhibiting the enzyme's function depending on the molecule that binds.
6. Can you give two examples of allosteric enzymes?
Allosteric enzymes are involved in various biochemical processes. Some examples include Aspartate Transcarbamoylase, which helps in the synthesis of pyrimidines, and Glucokinase, which regulates blood sugar levels by controlling glucose metabolism. Another example is Acetyl-CoA Carboxylase, which plays a role in fatty acid synthesis.
7. Why are allosteric enzymes important?
Allosteric enzymes are important because they help control the metabolism of cells. They work by having special molecules (activators or inhibitors) bind to a specific site on the enzyme. This binding causes changes in the enzyme’s shape, either increasing or decreasing its activity, which is essential for regulating various processes in the body.
