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Allosteric Enzymes

Allosteric Enzymes

Allosteric enzymes are a class of biocatalysts with properties mimicking those of an enzyme. Yet they differ from normal enzymes due to the lack of normal Michaelis-Menten kinetic behaviour. The allosteric enzymes move in a sigmoid motion. They change their structural properties during their interaction with the effector, which alters the allosteric enzyme’s affinity for bonding at the ligand binding site. An allosteric enzyme’s activity is dependent on the presence of its effector. On the other hand, the allosteric enzyme's kinetics relies on the effector's quantity.

Table of contents

  • What are Allosteric Enzymes?
  • Properties of Allosteric Enzymes
  • How Enzymes Control Cellular Activities?
  • Regulation Mechanism of Allosteric Enzymes
  • Models Based on Allosteric Enzymes Regulation
  • Examples of Allosteric Enzyme
  • Practice Questions
  • Frequently Asked Questions

What are Allosteric Enzymes?

Allosteric enzymes are naturally protein and are colloidal in nature. They are specific in their actions and alter the rate of reaction. Some enzymes have additional sites called allosteric sites. Each site has its distinct properties. As named, allosteric enzymes have multiple allosteric sites.

Monod and Jacob, the two Nobel laureates, gave the term allosteric. They have illustrated the word to determine that an enzyme site that differs from the primary binding site may still affect the enzyme activity.

An allosteric enzyme is necessary to control our body's biological functions, like cell division and metabolism. These can act as rate-determining steps for numerous pathways and processes. Allosteric enzymes act as a regulatory enzyme that governs the overall rate of the metabolic pathway.

Generic Allosteric Enzyme

Generic Allosteric Enzyme

Source: A Level Chemistry

Properties of Allosteric Enzymes

Allosteric enzymes have crucial numerous properties. They are as follows

  • They possess multiple allosteric sites for binding a diverse range of modulators and effectors. 
  • Allosteric sites differ from an enzyme’s active site as they have specific substrate-binding sites.
  • The effectors or modulators can either be positive or negative, which will control the activity of an enzyme.
  • If the effector is positive, allosteric activity raises, and the allosteric site is called an activator site.
  • Enzyme activity is reduced when the allosteric modulator is negative. In this case, the allosteric site is called the inhibitor site.
  • Alteration in enzyme activity occurs when it binds to the allosteric site and is thus called Cooperative binding.
  • When we compare the reaction rate with the substrate concentration, an allosteric enzyme represents a graph forming an S-shaped curve.
  • The same substrate or different compound modulator-based enzymes are referred to as heterotrophic or homotropic enzymes.
  • Some enzymes can possess more than one modulator.
  • Enzymes that have numerous modulators have distinct binding sites on each enzyme.

How Enzymes Control Cellular Activities?

The key components to regulate cellular activities are Enzymes. The metabolic pathway involves interconnected reactions to complete a single cellular task. Each response requires to proceed in order and be catalysed by a particular enzyme that specifically reacts with its substrate only.

Generally, the product of the interaction serves as the substrate for the subsequent response. As a result, the enzyme activity and its abundance will have an impact on the rates of metabolites or metabolic flux. Thus, affecting the overall cellular processes. One chemical reaction of the pathway governs the rate of the entire metabolic pathway and is termed a rate-limiting reaction. It is well known as the rate-determination step.

Regulation Mechanism of Allosteric Enzymes

We can control allosteric enzymes based on types, that is, one for substrate and the other for effector molecules or modulators. The two types of allosteric regulations are the following

1. Homotropic Regulation

Substrate molecules act as an effector in homotropic regulation. They are primarily enzyme-activating and are called cooperativity. For example, the binding of oxygen to haemoglobin.

2. Heterotropic Regulation

The effector and the substrate are distinct from one another in heterotropic regulation. For example, the binding of carbon dioxide to haemoglobin.

Based on the aforementioned functions of the regulator, two distinct types of regulation are exhibited. They are

3. Allosteric Inhibition

When inhibitors bind with proteins, they induce conformational changes at all active sites of the protein, further decreasing the enzyme activity.

4. Allosteric Activation

When the activator attaches to the protein, it increases the function of the active sites. Thus, it leads to enhanced enzymatic activity.

Models Based on Allosteric Enzymes Regulation

Many models have been proposed for the regulation of allosteric enzymes. Some are as follows

1. Simple Sequential Model

In this model, the conformation of an enzyme changes to R (relaxed) from T (tense) due to the binding of the substrates. A researcher named Koshland proposed the model stating that their substrate binds are per induced fit theory.

2. Concerted or Symmetry Model

The model was introduced by Monad, stating that subunits of enzymes undergo a simultaneous change. For example, in this binding of one substrate, Tyrosyl tRNA synthetase prevents the binding of other substrates.

Examples of Allosteric Enzymes

Several allosteric enzymes support diverse biochemical processes occurring throughout our body system. Some of the following are popular allosteric names mentioned as follows:

1. Aspartate Transcarbamoylase

Pyrimidine is biosynthesised by Aspartate Transcarbomoylase (ATCase). The end product cytidine triphosphate (CTP) likewise acts as an inhibitor. A purine nucleotide initiates the feedback regulation adenosine triphosphate (ATP) process, and a high concentration of ATP may disable the inhibition of CTP.

Therefore, pyrimidine nucleotide synthesis is ensured in the significantly high concentration of purine nucleotides.

2. Glucokinase

Glucokinase plays a crucial role in maintaining glucose homeostasis. It converts glucose to glucose-6-phosphate. They also increase the glycogen synthesis in the liver. Furthermore, it also detects the glucose concentration to trigger insulin release from pancreatic beta cells.

Since glucokinase possesses low activity for glucose, it acts only when a high concentration of glucose in the liver needs to be transformed into glycogen. Glucokinase regulatory proteins govern the activity of glucokinase.

3. Acetyl-CoA Carboxylase

The process of lipogenesis is regulated by acetyl-CoA carboxylase. Citrate activates Acetyl-CoA carboxylase. On the other hand, the enzyme is inhibited by long-chain acyl-CoA molecules. Palmitoyl-CoA exhibits itself as an example of negative feedback inhibition by product. Furthermore, phosphorylation and dephosphorylation of acetyl-CoA carboxylase are controlled by hormones, including adrenaline and glucagon.

Practice Questions

Q1. What catalyses the biosynthesis of pyrimidine?

A. Purine
B. Acetyl-CoA
C. ATCase
D. Glucose

Answer: C. ATCase

Aspartate Transcarbamyoylase (ATCase) catalyses the biosynthesis of pyrimidine.

Q2. Which enzyme governs the lipogenesis process?

A. Purine
B. Acetyl-CoA carboxylase
C. ATCase
D. Glucokinase

Answer:  B. Acetyl-CoA

The process of lipogenesis is controlled by the enzyme Acetyl-CoA Carboxylase

Q3. Effector can be

A. Activator
B. Inhibitor
C. All of the above
D. None of the above

Answer:  C. All of the above

The binding of molecules is known as an effector, which can either be an activator or an inhibitor.

Frequently Asked Questions

Q1. What is the role of phosphofructokinase?
Answer: 
 Phosphofructokinase (PFK) is an allosteric enzyme that derives the process of glycolysis by converting fructose 6-phosphate to fructose 1,6-bisphosphate along with ATP hydrolysis. Thus, AMP is the allosteric activator among multiple other compounds. 

Q2. State the purpose of isocitrate dehydrogenase.
Answer: The enzyme is important for Krebs’ cycle, where they catalyse oxidative decarboxylation in the presence of NADP reduction. 

Q3. What are enzymes?
Answer:  Enzymes are proteins that accelerate chemical reactions by catalysing them without getting themselves altered during the cellular process in living organisms.

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