What is the Difference Between Concerted and Sequential Model of Allosterism?

The concerted and sequential models of allosterism are two principal models that describe the cooperative behavior of proteins, particularly enzymes, in response to ligand binding. The main differences between these models are:

1. Subunit Interaction: In the concerted model, enzyme subunits are connected in such a way that a conformational change in one subunit is necessarily transmitted to all other subunits. In contrast, the sequential model postulates that subunits are not connected in such a way that a conformational change in one subunit induces a similar change in the others.
2. Functional and Structural Symmetry: The concerted model imposes functional and structural symmetry, meaning that all subunits of the oligomer share the same structure/state. In the sequential model, asymmetry is allowed in the partially liganded derivative(s).
3. Conformational Equilibrium and Population Selection: The concerted model postulates conformational equilibrium and population selection by the ligands, whereas the sequential model forbids this.
4. Assumptions about the Protein Molecule: The sequential model accounts for different species of the protein molecule, whereas the concerted model does not.

Both models describe allosteric behavior in proteins, but they differ in their assumptions about subunit interaction, conformational change, and species of the protein molecule. Most allosteric effects can be explained by either the concerted MWC model (Monod, Wyman, and Changeux) or the sequential model described by Koshland.

Comparative Table: Concerted vs Sequential Model of Allosterism

The following table compares the key differences between the concerted and sequential model of allosterism:

The concerted model, also known as the symmetry model or MWC model, postulates that enzyme subunits are connected in such a way that a conformational change in one subunit is simultaneously propagated to all other subunits. This model was introduced by Jacques Monod, Jeffries Wyman, and Jean-Pierre Changeux in 1965.

On the other hand, the sequential model assumes that each subunit of a multimeric protein can exist independently in an "on" or "off" conformation, meaning that the conformational changes occur sequentially in each subunit, which can exist in different states independently. This model was introduced by Daniel E. Koshland, György Nemethy, and James A. Filmer in 1966.