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Allosteric Control of Enzyme Activity

  • Enzymes

In this allosteric control of enzyme activity post we have briefly explained about properties, allosteric enzyme regulation mechanism, and allosteric enzyme regulation examples.

Allosteric Enzyme Regulation

Allosteric enzyme regulation have an additional binding site to effector molecules other than the active site, and this binding causes conformational changes that alter the enzyme’s catalytic properties. An activator or inhibitor can be the effector molecule.

Each of the biological systems is well-regulated. Our body has many regulatory measures regulating all processes and responding to environmental changes. Everything is controlled to ensure development and survival. Allostery refers to enzyme regulation where binding at one site can influence binding at other sites.

Allosteric Regulation of Enzymes

Allosteric enzyme regulation; Image modified from “Enzymes: Figure 4,” by OpenStax College, Biology, CC BY 3.0.

Properties

Enzymes are a chemical catalyst, which increases the rate of reaction. The active and substrate binding sites are not the only sites of allosteric enzymes.

The C-subunit is the substrate-binding site, while the effector binding site (or regulatory subunit) is called R-subunit. 

An enzyme molecule can have more than one allosteric site. They can respond to multiple environmental conditions that affect their biological reactions. 

An effector is a binding molecule that can act as an activator or inhibitor. The binding of the effector molecule changes the conformation of the enzyme Activator increases enzyme activity, while inhibitor decreases enzyme activity after binding.

Mechanism

There are two types of allosteric enzyme regulation on the basis of substrate and effector molecules:

Homotropic Regulation: Here, the substrate molecule acts also as an effector. This is mainly enzyme activation, also known as cooperativity (e.g., Binding oxygen to hemoglobin.

Heterotropic Regulation: When the substrate and effector differ. The enzyme’s effector can activate or inhibit it, e.g., Binding of CO 2 with hemoglobin.

On the basis of action performed by the regulator, allosteric enzyme regulation is of two types, inhibition and activation.

Allosteric inhibition: When an inhibitor binds with the enzyme, all active sites in the protein complex undergo conformational changes that cause the enzyme’s activity.

Allosteric activation: When an activator binds to a substrate, it increases its function and leads to increased binding of substrate molecules.

Simple Sequential Model

Koshland created this model. This model shows that substrate binding causes a change of conformation in enzymes from T (tensed) to R (relaxed). According to the induced fit theory, the substrate binds and conformational changes in one unit can cause similar changes in the other subunits. This is how cooperative binding occurs. In the same way activators and activators bind, the T-form is preferred when the inhibitor binds, and the R-form is favoured when the activator binds. Binding at one subunit can affect the conformation of others. The sequential model describes the negative cooperativity of enzymes, e.g., tyrosyl tRNA synthetase, where the binding substrate inhibits another substrate.

Symmetry Model

According to this concerted or symmetry model, there is a simultaneous change in all the subunits of an enzyme. All the subunits are either present in R form (active form) or T form (inactive state), having less affinity to a substrate. An inhibitor shifts the equilibrium of T ⇄ R towards T, and an activator shifts the equilibrium towards R form and favours the binding. It explains the cooperative regulation of activators as well as inhibitors.

Examples

Many allosteric enzymes are involved in biochemical pathways, allowing the system to be well controlled and modulated.

Glucokinase: It is an essential part of glucose homeostasis. It converts glucose into glucose-6-phosphate and increases glycogen synthesis in liver cells. It senses glucose concentration to release insulin from the pancreatic beta-cell. Because the glucokinase is low in affinity for glucose, it should not be activated when there is more glucose in the liver. This should cause increased glycogen production. Glucokinase regulatory protein regulates the activity of glucokinase.

Further Readings