Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Cooperative Ligand Binding Can Be Described

Cooperative binding of oxygen by hemoglobin was first analyzed by Archibald Hill in 1910. From this work came a general approach to the study of cooperative ligand binding to multisubunit proteins. For a protein with n binding sites, the equilibrium of Equation 5–1 becomes P+nL⇌PLn (5–12) and the expression for the association constant becomes

The expression for (see Eqn 5–8) is

Rearranging, then taking the log of both sides, yields

where Kd=[L]ⁿ 0.5. Equation 5–16 is the Hill equation, and a plot of log [θ/(1-θ)] versus log [L] is called a Hill plot. Based on the equation, the Hill plot should have a slope of n. However, the experimentally determined slope actually reflects not the number of binding sites but the degree of interaction between them. The slope of a Hill plot is therefore denoted by n_H, the Hill coefficient, which is a measure of the degree of cooperativity. If n_H equals 1, ligand binding is not cooperative, a situation that can arise even in a multisubunit protein if the subunits do not communicate. An n_H of greater than 1 indicates positive cooperativity in ligand binding. This is the situation observed in hemoglobin, in which the binding of one molecule of ligand facilitates the binding of others. The theoretical upper limit for nH is reached when n_H=n. In this case the binding would be completely cooperative: all binding sites on the protein would bind ligand simultaneously, and no protein molecules partially saturated with ligand would be present under any conditions. This limit is never reached in practice, and the measured value of nH is always less than the actual number of ligand-binding sites in the protein. An n_H of less than 1 indicates negative cooperativity, in which the binding of one molecule of ligand impedes the binding of others. Well-documented cases of negative cooperativity are rare. To adapt the Hill equation to the binding of oxygen to hemoglobin we must again substitute pO2for [L] and Pⁿ50 for K_d:

Hill plots for myoglobin and hemoglobin are given in Fig ure 5–14.

FIGURE 5–14 Hill plots for the binding of oxygen to myoglobin and hemoglobin. When n_H=1, there is no evident cooperativity. The maximum degree of cooperativity observed for hemoglobin corresponds approximately to n_H=3. Note that while this indicates a high level of cooperativity, n_H is less than n, the number of O₂-binding sites in hemoglobin. This is normal for a protein that exhibits allosteric binding behavior.

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مواضيع ذات صلة

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Hemoglobin Undergoes a Structural Change on Binding Oxygen

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Hemoglobin Subunits Are Structurally Similar to Myoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Oxygen Is Transported in Blood by Hemoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Protein Structure Affects How Ligands Bind

Membrane Dynamics:- Transbilayer Movement of Lipids Requires Catalysis

The Composition and Architecture of Membranes:- Lipids and Proteins Diffuse Laterally in the Bilayer

Membrane Dynamics:- Acyl Groups in the Bilayer Interior Are Ordered to Varying Degrees

The Composition and Architecture of Membranes:- Covalently Attached Lipids Anchor Some Membrane Proteins

The Composition and Architecture of Membranes:- The Topology of an Integral Membrane Protein Can Be Predicted from Its Sequence

The Composition and Architecture of Membranes: -Integral Proteins Are Held in the Membrane by Hydrophobic Interactions with Lipids

The Composition and Architecture of Membranes:- All Biological Membranes Share Some Fundamental Properties

The Composition and Architecture of Membranes:- Each Type of Membrane Has Characteristic Lipids and Proteins