In this article, I briefly describe allosteric enzymes and their properties.
Enzymes
The highly specialized proteins are known as enzymes, which have tremendous catalytic power. They have high specificity for their substrates and play the role of reaction catalysts of biological systems. Enzymes possess a high capacity to accelerate chemical reactions and function in aqueous solutions under mild conditions of temperature and pressure. They have a specific three-dimensional structure, with molecular weight ranging from 12,000 to more than 1 million. Enzymes may or may not require a coenzyme and one or more metal ions other than their amino acid residues for their activity. A prosthetic group is a coenzyme or metal ion that is very tightly or covalently bound to the enzyme protein. A complete catalytically active enzyme with its bound coenzyme or metal ions is called a holoenzyme. The protein part of a holoenzyme is known as apoenzyme.
Inhibitors and activators
In enzymatic reactions substrates are converted into different molecules called products. Enzymes are necessary for almost all chemical reactions in a biological cell. Enzymes are selective for their substrates and accelerate only a few reactions. They work as catalysts, thus lowering the activation energy for a reaction and dramatically increasing the rate of the reaction. As a result products are formed faster, and reactions reach their equilibrium state more rapidly. Inhibitors are the molecules that decrease enzyme activity, whereas activators increase their activity. Many drugs and poisons work as enzyme inhibitors. The factors that affect the activity are temperature, pressure, chemical environment (e.g., pH), and the concentration of substrate.
What are allosteric enzymes
The majority of enzymes are simple enzymes that contain only one domain. However, many enzymes are composed of two or more domains known as allosteric enzymes. Most enzymes are designed to function at a constant rate. However, allosteric enzymes are sensitive to physiological controls. They, therefore, adjust their rate and determine the flux through the metabolic pathway that they control.
The enzymes regulated by binding specific ligands are known as allosteric enzymes. They can change into two or more structural shapes that vary in their ability to bind a substrate. They also vary in their ability to position a critical catalytic side chain, and therefore in their rate of catalysis. Allosteric enzymes are classified into two major groups, i.e., K-type enzymes and V-type enzymes. The K-type enzymes keep their maximum rate fairly constant and regulated by changing their affinity for a substrate. In contrast, the V-type enzymes do not demonstrate significant changes in affinity, but possess large changes in the maximum rate.
The binding of allosteric enzymes with modulators
Allosteric modulators or allosteric effectors are small metabolites or cofactors that regulate allosteric enzymes by a reversible and non-covalent binding. However, other enzymes are regulated by a reversible and covalent binding. Separate regulatory proteins stimulate or inhibit some enzymes through binding. Allosteric enzymes recognize and bind the specific cellular metabolites and change their rate in response to changing concentrations of the cellular metabolites. This enables such enzymes to be sensitive to some metabolic aspects of the cell. Allosteric regulatory enzymes act as pacemakers for their pathway because they respond by appropriately altering their activity. They regulate the pathway in which they function and are themselves regulated by the binding of physiological effectors.
Allosteric enzymes after binding with modulators, undergo conformational changes. The modulators may stimulate or inhibit the allosteric enzymes. Homotropic enzymes are the regulatory enzymes for which the substrate and modulator are identical. The enzymes which have substrates other than the modulator are known as heterotropic enzymes. Allosteric enzymes possess a bigger size and more complex configuration than non-allosteric enzymes. They have one or many regulatory sites other than the active site for binding with the modulator. They significantly differ in their properties when compared to non-regulatory enzymes. The active site of an enzyme is specific for its substrate, and each regulatory site is specific for its modulator.
Properties of allosteric enzymes
The special properties of allosteric enzymes allow them to adapt to various conditions in the environment and thus make them unique compared to other enzymes. The allosteric enzyme contains an allosteric site on top of its active site, which binds the substrate. It also contains many polypeptide chains with multiple active and allosteric sites.
The distinct properties of allosteric Enzymes make it different compared to other enzymes.
- Allosteric enzymes do not follow the Michaelis-Menten Kinetics. They have multiple active sites, which exhibit the property of cooperativity. It says the binding of one active site affects the affinity of other active sites on the enzyme.
- Substrate concentration influences these enzymes. Allosteric enzymes exist in two conformational forms, i.e., T (tight or taut) and R (relaxed) forms, which exist in equilibrium. Modulators and substrates can bind to the R form of the enzyme, whereas the inhibitors can bind to the T form only (figure 1). The T and R state equilibrium of the enzyme depends upon the substrate concentration. More enzymes are found in the R state at high substrate concentrations. The enzymes are present in the T state at low substrate concentration. When there is an insufficient amount of substrate, the enzymes are found in the T state.
- Allosteric Enzymes are regulated by other molecules. This can be seen when the molecules 2,3-BPG (2,3-Bisphosphoglycerate), and CO2 modulate the binding affinity of hemoglobin to oxygen. 2,3-BPG stabilizes the T-state and thus reduces the binding affinity of O2 to hemoglobin. Hemoglobin releases oxygen in CO2 rich areas in the body.
The working of the allosteric enzyme Aspartate transcarbamoylase (ATcase)
It is an allosteric enzyme that has 12 polypeptide chains organized into catalytic and regulatory subunits. The enzyme catalyzes the first step in the synthesis of pyrimidines. It catalyzes the condensation of aspartate and carbamoyl phosphate to form N-carbamoyl aspartate and orthophosphate. The enzyme catalyzes the reaction yielding cytidine triphosphate (CTP).
The enzyme activity is low for a high amount of the final product CTP. However, for low concentrations of the final product CTP, the enzymatic activity is high. The CTP molecule has an odd configuration or shape unlike the substrates representing the allosteric nature of the enzyme. Instead of binding to the active site, CTP binds to the allosteric site. Thus, CTP functions as an allosteric inhibitor, decreasing the enzymatic activity of the enzyme.
When the concentration of CTP remains high, and cells in the body need more enzymes, then ATP functions as an allosteric molecule, and attaches to the allosteric site. Thus, it functions as an enzyme activator, and enhances the activity of the enzyme. So, with high concentrations of CTP, ATP could enhance the enzyme activity. This example explains the benefits of allosteric control and the ability of allosteric enzymes to adapt to various conditions of the environment.
The working of the allosteric enzyme fructose 1,6-bisphosphatase
The enzyme fructose 1,6-bisphosphatase (FBPase) is critically involved in the control of gluconeogenesis and is used as a target for anti-diabetic drugs. In diabetic patients fructose 1,6-bisphosphatase fails to regulate gluconeogenesis, leading to altered blood sugar levels. The enzyme FBPase is a homotetramer, and it catalyzes the hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate. However, AMP (adenosine monophosphate) and fructose 2,6-bisphosphate allosterically inhibit the enzyme FBPase. But in FBPase, unlike ATCase, the catalytic and regulatory binding sites are on the same polypeptide chain.
The enzyme FBPase changes its conformation from R state to T state by rotating the two dimeric halves of the molecule concerning each other by about 17°. These quaternary changes are accompanied by alterations in the tertiary structure. The allosteric transition of the enzyme from the T state to the R state results in the formation of the catalytically competent active site.
Feedback inhibition by allosteric enzymes
The enzyme-catalyzed reactions involved in the biosynthesis of an amino acid are carried out in a particular sequence, which is known as a biochemical pathway. In such pathways, the product of one reaction becomes the substrate for the next reaction. It is unnecessary and futile for the cells to carry out the biochemical reactions if the end product is easily available in the environment. Therefore, cells can shut down a pathway at the time of no need.
The reaction occurs at a site on the enzyme other than the active site, which is called the allosteric site. The binding of the product to the allosteric site brings a conformational change to the enzyme, and it can no longer react with its substrate. As the reaction of the regulatory enzyme is slowed down, all subsequent enzymes operate at reduced rates as their substrates are depleted. As a result there is no substrate for subsequent steps in the pathway, and the final product is no longer synthesized. Thus, there is a balance established according to the cell’s need.
When the product is utilized or broken down, its concentration is decreased. The decrease in the product concentration relaxes the inhibitor, and the product formation resumes. Such enzymes, whose ability to catalyze a reaction depends upon molecules other than their substrates (the ones upon which they act to form a product), are said to be under allosteric control. This type of regulation is known as feedback inhibition (figure 2), in which the end product of the pathway reacts with the first enzyme that is unique to the pathway.
Conversion of L-threonine to L-isoleucine
An example of allosteric feedback inhibition is the bacterial enzyme system that catalyzes the conversion of L-threonine to L-isoleucine in five steps. In this system, enzyme-1, L-threonine deaminase catalyzes the reaction, which removes the amino group from L-threonine (figure 3). The enzyme L-threonine deaminase is strongly inhibited by the ultimate product of the five reactions. L-isoleucine is the inhibitor as other amino acids or related compounds do not inhibit the enzyme. Thus, in this way, the cell regulates the amount of isoleucine produced. When the concentration of isoleucine increases, the whole chain of reactions is shut down by the inhibition of the first reaction in the series.
Conclusion
Enzymes are highly specialized proteins with tremendous catalytic power. A complete catalytically active enzyme with its bound coenzyme or metal ions is called a holoenzyme. Enzymes work as catalysts, thus lowering the activation energy for a reaction and dramatically increasing the rate of the reaction. Inhibitors are the molecules that decrease enzyme activity, whereas activators increase their activity.
The enzymes composed of two or more domains are known as allosteric enzymes. They can change into two or more structural shapes that vary in their ability to bind a substrate. Allosteric enzymes after binding with modulators, undergo conformational changes. The modulators may stimulate or inhibit the allosteric enzymes. Allosteric enzymes possess a bigger size and more complex configuration than non-allosteric enzymes.
Allosteric enzymes do not follow the Michaelis-Menten Kinetics. They exist in two conformational forms, i.e., T (tight or taut) and R (relaxed) forms, which exist in equilibrium. Allosteric Enzymes are regulated by other molecules. Allosteric enzymes do a type of regulation known as feedback inhibition, where the end product of a biochemical pathway reacts with the first enzyme that is unique to the pathway. The conversion of L-threonine to L-isoleucine is an example of allosteric feedback inhibition.
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I, Swagatika Sahu (author of this website), have done my master’s in Biotechnology. I have around twelve years of experience in writing and believe that writing is a great way to share knowledge. I hope the articles on the website will help users in enhancing their intellect in Biotechnology.