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Regulation of Enzymes: Enzyme Induction and Repression, Allosteric Enzymes Regulation

Numerous enzymes are responsible for the multiplication and division of cells in microorganisms.

Enzyme Induction and Repression

Numerous enzymes are responsible for the multiplication and division of cells in microorganisms. Microbes need certain enzymes to establish an infection, from a health perspective. Certain enzymes remain active at all times. Constitutive enzymes are the ones that are active all the time. On the other hand, other enzymes are only active occasionally, when their product is needed. Inducible enzymes are those enzymes in which the activity is controlled by microorganisms such as bacteria. Survival of the microorganism depends on this ability. Such enzymes, if constantly active, could over-produce a compound, resulting in an energy drain for the microbe. As well as being able to respond quickly to whatever condition they are geared to, inducible enzymes must also behave predictably.

Induction and repression serve the twin ends of controlling activity and speeding up response. RNA polymerase, which binds to DNA, is responsible for both induction and repression of gene expression. In general, the RNA polymerase binds directly to the DNA sequence that encodes a protein. That region is referred to as the operator. Positioning the polymerase correctly is essential to advancing the molecule from one end of the DNA to the other so that information can be interpreted as it moves along. RNA polymerase binds to the operator region because of its tri-dimensional shape. An effector is a molecule that can affect the operation of an operator. A polymerase effector can alter the shape of the polymerase-binding region, allowing it to bind more efficiently and easily. Induction is the result of this process. Effectors, on the other hand, can attach to operators and alter their configuration, making it so that they are less effective in binding polymers. This is known as repression.

As a result of the presence of a specific molecule, enzymes are induced to be produced. Inducers are molecules like this. A compound that induces an enzyme is called an inducer molecule. An inducer molecule is combined with a repressor molecule in the induction process. An inducer can bind to a repressor and block its function, which is to bind to a specific region known as an operator. Operators act as catalysts to promote the transcription of genes into messenger RNA that acts as a template for protein synthesis. RNA polymerase is a molecule that binds to operators to initiate transcription. By binding to the repressor, the inducer prevents the repressor from suppressing transcription, allowing transcription of the gene encoding the inducible enzyme. The inducing molecule overrides the default mode of transcription, which is repression. Lactose (lac) operons, which operate based on induction, are very well characterized in bacteria.

Repression of enzymes occurs when repressor molecules inhibit the production of enzymes. Feedback inhibition is the typical mechanism of repression. An amino acid can function as a repressor molecule in a series of enzyme-catalyzed reactions if its end product is that amino acid. The repressor often combines with another molecule and blocks the operator's action. It is possible for repressors and polymerase to compete for binding sites on the operator. It is also possible for the repressor duo to bind directly to the polymerase to inhibit the subsequent binding of the operator region by causing the polymerase to change its shape. No matter what happens, the result is that transcription of that particular gene is halted. Genes that are blocked in enzyme repression typically function as receptors for the first enzyme involved in repressor synthesis. Therefore, repression inhibits the synthesis of all enzymes associated with the metabolic pathway. The bacteria are thus able to conserve energy. It would then be necessary to produce enzymes, which would involve a large metabolic cost, and would not play an essential role in cellular processes.

A blockage of an enzyme is usually the first step in a pathway that leads to repression. The repression/induction cycle is triggered by nutrient concentration, pH, or other conditions that affect the enzyme's function. As such, repression interferes with the production of all enzymes involved in the metabolic process. This saves energy for the bacteria. It would otherwise be necessary to make enzymes that would have no function in cellular processes, at a high metabolic cost. As effector levels change, and nutrient concentrations, pH levels, or other conditions that are relevant to a particular effector, repression and induction mechanisms tend to cycle back and forth.

Allosteric Enzyme Reaction

It is an allosteric enzyme that has a binding site other than the active site for effector molecules. The binding changes the catalytic properties of the enzyme because it changes its conformation. Effector molecules can be inhibitors or activators. A well-regulated biological system is present in all organisms. Various regulatory mechanisms operate in the body to monitor and adapt to changes in the inside and outside environment. Gene expression, cell division, hormone production, metabolism, and enzyme production are all regulated to ensure proper development and survival. Enzymes are regulated by allostery, in which binding at one site affects the binding at subsequent sites.

Properties

  • Biological catalysts speed up reactions through their action on enzymes
  • The active site of allosteric enzymes, as well as the substrate-binding site, have additional sites. C-subunit refers to the substrate-binding site, and R-subunit or regulatory subunit refers to the effector binding site.
  • Allosteric sites can occur at more than one position on an enzyme molecule
  • They can respond to multiple conditions, which impact how they react biologically
  • An effector is a binding molecule, which can be both inhibitory and activating
  • By binding an effector molecule to an enzyme, the enzyme's conformation is altered
  • Enzymes that are activated increase their activities, while enzymes that are inhibited decrease their activities after they are bound
  • S-curves are typical instead of hyperbolic curves for velocity vs substrate concentration graphs for allosteric enzymes.
Using substrate and effector molecules as examples, allosteric regulation can be divided into two types:

Homotropic regulation - Substrate molecules can also act as effectors here. A major component of this process is the activation of enzymes, also known as cooperativity, i.e., oxygen binds to hemoglobin.

Heterotopic regulation - The substrate is not the same as the effector. Activators or inhibitors of the enzyme can be used, e.g., CO2 binding to hemoglobin.

The two types of allosteric regulation are inhibition and activation, based on the action of the regulator.

Allosteric inhibition - If a protein complex of an enzyme is bound to an inhibitor, the enzyme's activity decreases due to conformational changes at all the active sites.

Allosteric activation - Activators bind to active sites and increase their function, which in turn increases substrate binding.

The mechanisms of regulation of allosteric enzymes have been proposed in two ways:

Simple Sequential Model

This model was provided by Koshland. As the substrate binds to the enzyme, there is a change in conformation from T (tense) to R (relaxed). A conformational change occurs as the substrate is absorbed. Changing the conformation of one subunit results in similar changes in another subunit. Likewise, inhibitors and activators bind cooperatively in the same way. When an inhibitor binds, the T form is favored, whereas when an activator binds, the R form is favored. Another subunit's conformation is modulated by the binding of one subunit. A sequential model could explain negative cooperativity between enzymes, such as the binding inhibition of another substrate by the binding of a substrate in tyrosyl tRNA synthetase.

Concerted and Symmetric Model

A change in an enzyme occurs simultaneously in all its subunits, according to this model. The subunits all exist either in the T or R form, which has a lower affinity for substrates. An inhibitor shifts the equilibrium of T ⇄ R, to the T form, whereas an activator shifts the equilibrium to the R form, which favors binding. Both activators and inhibitors work cooperatively in this regulation.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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