Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Antibodies Bind Tightly and Specifically to Antigen

The binding specificity of an antibody is determined by the amino acid residues in the variable domains of its heavy and light chains. Many residues in these domains are variable, but not equally so. Some, particularly those lining the antigen-binding site, are hypervariable— especially likely to differ. Specificity is conferred by chemical complementarity between the antigen and its specific binding site, in terms of shape and the location of charged, nonpolar, and hydrogen-bonding groups. For example, a binding site with a negatively charged group may bind an antigen with a positive charge in the com plementary position. In many instances, complementarity is achieved interactively as the structures of antigen and binding site are influenced by each other during the approach of the ligand. Conformational changes in the antibody and/or the antigen then occur that allow the complementary groups to interact fully. This is an ex ample of induced fit (Fig. 5–27).

FIGURE 5–27 Induced fit in the binding of an antigen to IgG. The molecule, shown in surface contour, is the Fab fragment of an IgG. The antigen bound by this IgG is a small peptide derived from HIV. Two residues from the heavy chain (blue) and one from the light chain (pink) are colored to provide visual points of reference. (a) View of the Fab fragment, looking down on the antigen-binding site (PDB ID1GGC). (b)The same view, but here the Fab fragment is in the “bound” conformation (PDB ID 1GGI); the antigen has been omitted from the image to provide an unobstructed view of the altered binding site. Note how the binding cavity has enlarged and several groups have shifted position. (c) The same view as in (b), but with the antigen in the binding site, pictured as a red stick structure.

A typical antibody-antigen interaction is quite strong, characterized by Kd values as low as 10^-10 M (re call that a lower Kd corresponds to a stronger binding interaction). The Kd reflects the energy derived from the various ionic, hydrogen-bonding, hydrophobic, and van der Waals interactions that stabilize the binding. The binding energy required to produce a Kd of 10^-10 M is about 65 kJ/mol. The complex of a peptide derived from HIV (a model antigen) and an Fab molecule, shown in Figure 5–27, il lustrates some of these properties. The changes in structure observed on antigen binding are particularly striking in this example.

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مواضيع ذات صلة

Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and Molecular Motors: -Additional Proteins Organize the Thin and Thick Filaments into Ordered Structures

Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and Molecular Motors: - The Major Proteins of Muscle Are Myosin and Actin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -The Antibody-Antigen Interaction Is the Basis for a Variety of Important Analytical Procedures

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -Antibodies Have Two Identical Antigen-Binding Sites

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -Self Is Distinguished from Nonself by the Display of Peptides on Cell Surfaces

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -The Immune Response Features a Specialized Array of Cells and Proteins

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Sickle-Cell Anemia Is a Molecular Disease of Hemoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Oxygen Binding to Hemoglobin Is Regulated by 2,3-Bisphosphoglycerate

Protein Denaturation and Folding: - Loss of Protein Structure Results in Loss of Function

Protein Tertiary and Quaternary Structures: -There Are Limits to the Size of Proteins

Protein Tertiary and Quaternary Structures: -Protein Quaternary Structures Range from Simple Dimers to Large Complexes

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