Allosteric Regulation

Graphic showing allosteric inhibition of an enzyme
This diagram shows how an allosteric inhibitor (D) binds to an allosteric site (B) of a protein (E), changing the conformation of its active site (A) and inhibiting substrate (C) binding.

Core Concepts

In this article, we will learn about allosteric regulation, including allosteric inhibition and allosteric activation. We will look at examples of allosteric regulation in glycolysis.

Concepts Covered in Other Articles

Introduction: Why Do We Need Protein Regulation?

Proteins are important molecules in the cell. They carry out many essential functions to keep the cell running, and they provide structure for cells and organelles. However, proteins are not all working at their maximum capacity at all times. If they were, the huge amount of reactions proceeding would stress and overwhelm the cell. In order to maintain homeostasis, the cell regulates protein activity. This ensures that certain reactions and processes proceed only when necessary.

What is Allosteric Regulation?

Luckily, we have many methods of regulating protein activity and production in the cell. One regulation method is allosteric regulation. Allosteric regulators reversibly bind to their target proteins in a site other than their active site. These regulators can change their target proteins’ conformation and thus, function. Usually, they either change a protein’s affinity for its substrate or change the reaction velocity. They can also either harm or enhance the protein’s ability to interact with other proteins to carry out its function. Allosteric regulators can either activate or inhibit a protein’s activity.

There are two types of allosteric regulation- heterotropic and homotropic. Heterotropic allosteric regulators are any molecule that is not the enzyme’s substrate. Homotropic allosteric regulators are the enzyme’s substrate. They bind to the protein and increase its affinity for subsequent substrate binding. A great example of this is oxygen’s binding to hemoglobin.

While most enzymes follow Michaelis-Menten kinetics, enzymes with allosteric regulators do not. This is because binding of allosteric regulators affects the protein’s substrate binding activity. This is known as cooperative binding or cooperativity, and results in a sigmoidal or S-shaped reaction progress curve.

This graph shows a sigmoidal reaction curve, something you will see when an enzyme displays cooperativity. Allosteric regulation can affect a curve in this way.

Allosteric Regulation in Glycolysis

For examples of both allosteric activation and allosteric inhibition, we will look at two regulators of phosphofructokinase 1 (PFK-1). PFK-1 is an important enzyme in glycolysis, a component of cellular respiration. PFK-1 catalyzes the addition of a second phosphate group to fructose-6-phosphate (F6P), converting it into fructose 1,6-bisphosphate (F1,6BP). Regulators target this step because it has a high delta G. This means the conversion requires a large amount of energy to proceed.  PFK-1 has several regulators, including ATP and AMP.

This image shows the reaction carried out by PFK-1: The transformation of F6P to F1,6BP.

ATP and PFK-1: Allosteric Inhibition

Allosteric inhibition occurs when binding of an allosteric regulator to a protein inhibits its binding to the substrate. One example is ATP’s inhibition of phosphofructokinase 1 (PFK-1). The main purpose of glycolysis, or cellular respiration, is to produce ATP. Thus, when there is a high concentration of ATP in the cell, glycolysis can slow down. When ATP concentrations are high, ATP allosterically binds to PFK1 to prevent overproduction of ATP. Because ATP is also a substrate of PFK-1 and can bind to one of its active sites, this is an example of homotropic allosteric regulation.

AMP and PFK-1: Allosteric Activation

AMP acts as an allosteric activator of PFK-1. It can reverse ATP’s inhibition of PFK-1. When the cell becomes low on ATP, it will convert ADP to ATP as a last resort. This reaction has a side product of AMP. So, when AMP concentrations are high, it means the cell has resorted to using ADP for energy, and it needs more ATP. AMP binds to PFK-1 and reverses ATP’s inhibition, re-activating the enzyme. Because AMP is not a substrate of PFK-1, this is an example of heterotropic allosteric activation. When AMP binds PFK-1, glycolysis resumes and creates more ATP for the cell to use.