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Allosteric Enzymes

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  • Last Updated : 09 Sep, 2022
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All forms of living organisms contain the type of protein known as enzymes. They serve as catalysts in living organisms and are mostly secreted by a source. They primarily control several biochemical processes that take place within living things. They are mostly made by fungi, bacteria, plants, and mammals. As they transform the substrate into various molecules known as products by acting as a catalyst. Enzymes are necessary for the catalyzing of nearly all metabolic reactions. More than 5,000 metabolic events take place inside the body thanks to enzymes.

Allosteric Enzymes

Allosteric Enzyme

 

It is a type of enzyme that, when it binds to an effector, such as an allosteric modulator, can alter its structural ensemble, changing the binding affinity at a different ligand binding site. They are crucial to many biological functions. The effector binds to a particular location known as the allosteric site. The effector can attach to the protein primarily through this location, which causes conformational changes influencing protein dynamics.

The term “allosteric” mostly refers to an allosteric molecule whose regulatory site is physically separate from its active site.

Properties of Allosteric Enzymes

Allosteric enzymes differ from other enzymes due to a number of distinctive characteristics. Some of these qualities include:

  1. One allosteric enzyme deviates from Michaelis-Menten’s Kinetic theory. The reason for this is because they have several active sites, and each of these active sites has a cooperativity property, meaning that when one active site binds to an enzyme, it might affect the binding of other active sites. The graph of allosteric enzymes at this other impacted site has a sigmoidal slope.
  2. They primarily exhibit a feature known as substrate concentration. Example: The majority of enzymes are found in the R state when the substrate concentration is high. T, however, is the preferred state when there is a lack of substrate present. Therefore, we can conclude that the equilibrium of both the T and R states relies on the substrate concentration.
  3. Allosteric enzymes can be controlled by other substances.
  4. They have the ability to react to a variety of situations that would otherwise increase biological processes.
  5. Allosteric enzymes have activators that make them more active, whereas inhibitors make them less active.
  6. Effectors are what bind molecules together; they can be either inhibitors or activators.
  7. Effectors alter the conformational properties of enzymes when they bind to compounds.
  8. Since enzymes are biological catalysts, they speed up the reaction.
  9. An enzyme molecule may contain several allosteric sites.
  10. They are able to react to many circumstances that affect biological processes.
  11. The molecule that binds is known as an effector, and it can either be an inhibitor or an activator.
  12. The conformation of the enzyme will change upon contacting the effector molecule.
  13. An enzyme’s activity is raised by its activator, whereas its activity is diminished after binding by its inhibitor.
  14. The velocity vs. substrate concentration graph of allosteric enzymes shows an S-curve rather than the more typical hyperbolic shape.

Allosteric Mechanism Regulation

On the basis of kinds, one for substrate and the other for effector molecules, we can control allosteric enzymes.

There are two kinds of allosteric regulation:

  1. Homotropic Regulation: Substrate molecules also serve as effectors in this sort of control. They are mostly known as cooperativity and enzyme activation. The binding of oxygen to hemoglobin is an example of homotropic control.
  2. Heterotropic Regulation: Substrate and effector are distinct in this type of regulation. The binding of carbon dioxide (CO2) to hemoglobin is an illustration of heterotropic control.

Based on the aforementioned function of the regulator, there are two different types of regulation: activators and inhibitors.

  • Allosteric Inhibition: In this mechanism, inhibitors attach to proteins, causing conformational changes at all protein active sites that lower enzyme activity.
  • Allosteric Activation: Enzymatic activity increases as a result of the activator’s binding to the protein in this process, which improves the functionality of the active sites.

Models Based on Allosteric Enzymes Regulation

The mechanism of allosteric enzymes has been the subject of a great deal of modeling; some of these models are included below:

  1. Simple Sequential Model: Koshland proposed this concept. In this model, the enzymes’ conformation changes from T (tensed) to R as a result of the binding of substrates (relaxed). According to the induced fit theory, their substrate binds.
  2. Concerted or Symmetry Model: Monad put forth this model. According to this hypothesis, all of the enzyme’s subunits change at the same time. For instance, Tyrosyl tRNA synthetase inhibits the binding of additional substrates when it binds to one substrate.

Allosteric Inhibitor

  1. An allosteric inhibitor is a substance that interacts with the enzyme at an allosteric location.
  2. The position of the allosteric site and the active site are dissimilar.
  3. The 3-dimensional geometry of the enzyme is altered by the inhibitor.
  4. Non-competitive inhibition is a type of allosteric inhibition.

Example of allosteric inhibitor:

  • A common allosteric inhibitor is ATP (adenosine triphosphate).
  • The glycolysis enzyme is called phosphofructokinase. ADP (adenosine diphosphate) is converted into ATP.
  • ATP acts as an allosteric inhibitor when its concentration in the system is too high.
  • ADP is converted more slowly when ATP and phosphofructokinase mix. ATP is preventing unneeded self-generation in this way.

Kinetic Properties

  • A conformational transition between a high-activity, high-affinity “relaxed” or R state and a low-activity, low-affinity “tense” state is frequently used to explain the kinetic features of allosteric enzymes. Numerous well-known allosteric enzymes have been shown to include these structurally different enzyme variants.
  • However, it is not widely known how molecules change between the two states. The “concerted model” of Monod, Wyman, and Changeux and the “sequential model” of Koshland, Nemethy, and Filmer are the two main models that have been put up to explain this mechanism.
  • The protein is believed to have two “all-or-none” global states in the coordinated model. Positive cooperativity, in which the binding of one ligand boosts the enzyme’s capacity to bind to more ligands, is evidence in favor of this concept.
  • Negative cooperativity, which states that losing one ligand makes it simpler for the enzyme to lose additional, does not support the hypothesis.
  • There are numerous distinct global conformational/energy states in the sequential model. Every time an enzyme binds a ligand, it craves the opportunity to bind another one. This is because binding one ligand alters the enzyme’s ability to bind subsequent ligands more readily.
  • However, neither model completely accounts for allosteric binding. Our understanding of allostery may be improved by the recent integration of physical techniques (like x-ray crystallography and solution small angle x-ray scattering, or SAXS) and genetic techniques (like site-directed mutagenesis, or SDM).

Examples 

Numerous allosteric enzymes support diverse biochemical processes that take place throughout the body. Following are a few of the well-known allosteric names:

  • Glucokinase: 
    • Due to the conversion of glucose to glucose-6-phosphate and increased hepatic glycogen production, it is crucial for maintaining glucose homeostasis.
    • Additionally, it keeps the blood’s concentration of glucose stable.
    • Glucokinase regulatory proteins control their activity.
  • Aspartate Transcarbamoylase:
    • They mostly aid in pyrimidine’s production.
    • When the concentration of purine rises, they maintain the rate of pyrimidine synthesis.
  • Acetyl-CoA Carboxylase:
    • They control the lipogenesis process.
    • These enzymes are activated by citrate, and their activity is inhibited by the long-chain acyl-CoA molecule products.
    • It is governed by phosphorylation, which is managed by glucagon and epinephrine-like hormones.

An allosteric enzyme’s kinetics and activity are regulated by the presence of its effector and the concentration of the effector, respectively. Allosteric enzymes can switch their activity on or off due to this effector dependence, but it also provides a checkpoint for conserving feedback that modulates metabolic activity in response to cellular signals.

  • Phosphofructokinase (PFK): The primary enzyme for observing glycolysis and the biological process of producing energy from the breakdown of carbohydrate molecules is phosphofructokinase (PFK). Pyruvate and adenosine triphosphate, two highly energetic molecules, are produced during glycolysis (ATP).
  • Isocitrate dehydrogenase (IDH): Isocitrate dehydrogenase (IDH) catalyses the citric acid cycle’s main regulatory component. The regulatory pathway in aerobic metabolism, also referred to as the Krebs cycle or the tricarboxylic acid (TCA) cycle, obtains energy from the production of acetyl CoA and provides the building blocks for the biosynthesis of amino acids, heme, and nucleic acids. Two heterodimer sets make up human IDH, one of which serves as a catalytic subunit and the other as a regulatory subunit.
  • Aspartate transcarbamoylase (ATCase): The enzyme known as aspartate transcarbamoylase (ATCase) aids in the flow limit and committed step of pyrimidine biosynthesis. The process produces pyrimidine, a substance found in nucleic acids. Large catalytic and tiny regulatory subunits are both present in ATCase.

FAQs on Allosteric Enzymes

Question 1: What are Allosteric Enzymes?

Answer:

It is a type of enzyme that, when it binds to an effector, such as an allosteric modulator, can alter its structural ensemble, changing the binding affinity at a different ligand binding site. They are crucial to many biological functions. The effector binds to a particular location known as the allosteric site. The effector can attach to the protein primarily through this location, which causes conformational changes influencing protein dynamics. The term “allosteric” mostly refers to an allosteric molecule whose regulatory site is physically separate from its active site.

Question 2: What is the symmetry model in allosteric enzymes?

Answer:

The symmetry model, sometimes referred to as the Manod-Wyman-Changeux Concerted Model, is used to describe allosteric enzymes made up of dimers with individual catalytic sites. According to this hypothesis, the enzyme is present in R and T’s equilibrium and the two subunits are in the same condition. As the concentration of the substrate rises, the equilibrium shifts in favour of the R state because the ligand or substrate of an allosteric enzyme binds preferentially to the R state. One subunit’s conformational shift occurs simultaneously with another, preserving the enzyme’s symmetry. In other words, the symmetry model’s enzyme can only exist in the RR or TT states, not the RT state.

Question 3: How is an allosteric enzyme activated?

Answer:

Substances produced in the enzyme pathway can either activate or inhibit allosteric enzymes. These chemicals, referred to as modulators, can modify the conformation of allosteric enzymes to change their activity.

Question 4: What is the role of allosteric enzyme?

Answer:

Enzyme allosteric modulation is essential for managing cellular metabolism. When an activator or inhibitor molecule attaches to an enzyme’s regulatory site, it causes conformational or electrostatic changes that either increase or decrease the activity of the enzyme.

Question 5: What are the properties of allosteric enzymes?

Answer:

Multiple subunits (Quaternary Structure) and multiple active sites are characteristics of allosteric or regulatory enzymes. Allosteric enzymes have different 3D geometries for their active and inactive states. Multiple inhibitor or activator binding sites help allosteric enzymes transition between active and inactive forms.

Question 6: What factors affect the allosteric enzyme’s activity?

Answer:

The concentration of the substrate affects allosteric enzymes. For instance, more enzymes are discovered in the R state when the substrate concentration is high. When there is not enough substrate for the enzyme to bind to, the T state is preferred.


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