COOPERATIVITY AND ALLOSTERISM

Dr. Tijani A. S. Learning Objectives

At the end of this topic, the student should be acquainted with:

 Enzyme Cooperativity

 Description of Allosterism

 Models of Allosterism and Kinetics of Allosteric Enzymes

Cooperativity

Cooperativity is a phenomenon displayed by enzymes or receptors that have multiple binding sites.

Cooperativity involves an interconversion of the monomer subunits between a tense form (less active subunits) and a relaxed form (more active subunits). Cooperativity can be positive or negative.

Positive cooperativity: ligand binding at one site increases the affinity of the other sites. E. g binding of oxygen to hemoglobin

Negative cooperativity the affinity of other sites is decreased by ligand binding to the first site.

This concept of transmitted structural changes in the protein, resulting in long- distance communication between sites, has been termed allostery, and enzymes that display these effects are known as allosteric enzymes Cooperativity The first description of cooperative binding to a multi-site protein was developed by A.V. Hill

The Sigmoid Plot and the Hill Equation

Non-allosteric enzyme V0 = Vmax [S] . Hyperbolic plot Km + [S] . MM Kinetics

Ý = [L]n = [L]n n n n Allosteric enzyme Kd + [L] K a + [L] ………..Hill Eqn. . Sigmoidal plot . Non MM kinetics n = Hill coefficient (sometimes written as nH ) Value gives a measure of cooperativity n = 1: No cooperativity, graph is hyperbolic n > 1: + ve Cooperativity, sigmoidal curve n < 1: - ve Cooperativity, sigmoidal curve

Hill Plot

Hill plot" is obtained by plotting log Ý vs log [L] . 1- Ý

Hill plot of the Hill equation in red, showing the slope of the curve being the Hill coefficient and the intercept with the x-axis providing the apparent dissociation constant. The green line shows the non- cooperative curve.

Molecular Model Of Allosterism

 Allostery is the process by which remote sites of a system are energetically coupled to elicit a functional response and enzymes showing this property are termed allosteric enzymes.

 Allosterism describes the change in the affinity for binding of a ligand or substrate that is caused by the binding of another ligand away from the active site,

 These allosteric binding sites are very often where the enzymatic activity is controlled. Ligands that bind to the allosteric binding sites may be allosteric activators or allosteric inhibitors.

 A ligand is a molecule that binds to a binding a binding site on a macromolecule, such as an enzyme or a receptor.

Model of Allosterism

Many allosteric effects can be explained by:  The concerted MWC model put forth by Monod, Wyman, and Changeux,

 The sequential model described by Koshland, Nemethy, and Filmer.

 Both postulate that protein subunits exist in one of two conformations, tensed (T) or relaxed (R), and

 Relaxed subunits bind substrate more readily than those in the tense state.

 The two models differ most in their assumptions about subunit interaction and the preexistence of both states. The Concerted Model

Proposed by Monod, Wyman, and Changeux in 1965, Their model makes the following assumptions:  Proteins are oligomers; that is, they contain two or more subunits.

 Each protein molecule can exist in either of two states, called T (tense) and R (relaxed), which are in equilibrium.

 In the absence of substrate molecules, the T state is favored. When substrate molecules are bound to the enzyme, the equilibrium gradually shifts to the R state, which has a higher affinity for the ligand.

 All binding sites in each state are equivalent and have an identical dissociation

constant for the binding of ligands (KT for the T state and KR for the R state). The Concerted Model

Schematic representation of the concerted model. The squares represent the tense state; the circles represent the relaxed state. The Sequential Model

Proposed by Koshland-Némethy-Filmer (KNF) assumes that  Each subunit can exist in one of two conformations: active or inactive.

 Ligand binding to one subunit would induce an immediate conformational change of that subunit from the inactive to the active conformation, a mechanism described as "induced fit".

 While such an induced fit converts a subunit from the tensed state to relaxed state, it does not propagate the conformational change to adjacent subunits. Instead, substrate-binding at one subunit only slightly alters the structure of other subunits so that their binding sites are more receptive to substrate.

 Cooperativity, according to the KNF model, would arise from interactions between the subunits, the strength of which varies depending on the relative conformations of the subunits involved. The Sequential Model

Schematic representation the sequential model. The squares represent the tense state; the circles represent the relaxed state. The binding of a ligand to a subunit changes the conformation of that subunit (from T to R). This transition increases the affinity of the remaining subunits for the ligand. References  Adair GS (1925). "'The hemoglobin system. IV. The oxygen dissociation curve of hemoglobin". J Biol Chem. 63: 529–545.

 Hill AV (1910). "The possible effects of the aggregation of the molecules of haemoglobin on its dissociation curves". J Physiol. 40: iv–vii.

 Hilser VJ, Wrabl JO, Motlagh HN (2012). "Structural and energetic basis of allostery". Annual Review of Biophysics. 41: 585–609. doi:10.1146/annurev-biophys-050511- 102319. PMC 3935618. PMID 22577828.

 Motlagh HN, Wrabl JO, Li J, Hilser VJ (April 2014). "The ensemble nature of allostery". Nature. 508 (7496): 331- 339. doi:10.1038/nature13001. PMC 4224315. PMID 24740064.