Molecular complex

Molecular binding is an attractive interaction between two molecules that results in a stable association in which the molecules are in close proximity to each other.

In some cases, the associations can be quite strong—for example, the protein streptavidin and the vitamin biotin have a dissociation constant (reflecting the ratio between bound and free biotin) on the order of 10¹⁴—and so the reactions are effectively irreversible. The result of molecular binding is sometimes the formation of a molecular complex in which the attractive forces holding the components together are generally non-covalent, and thus are normally energetically weaker than covalent bonds.

Molecular binding occurs in biological complexes (e.g., between pairs or sets of proteins, or between a protein and a small molecule ligand it binds) and also in abiologic chemical systems, e.g. as in cases of coordination polymers and coordination networks such as metal-organic frameworks.

Molecular binding can be classified into the following types:

Bound molecules are sometimes called a "molecular complex"—the term generally refers to non-covalent associations. Non-covalent interactions can effectively become irreversible; for example, tight binding inhibitors of enzymes can have kinetics that closely resemble irreversible covalent inhibitors. Among the tightest known protein–protein complexes is that between the enzyme angiogenin and ribonuclease inhibitor; the dissociation constant for the human proteins is 5x10⁻¹⁶ mol/L. Another biological example is the binding protein streptavidin, which has extraordinarily high affinity for biotin (vitamin B7/H, dissociation constant, K_d ≈10⁻¹⁴ mol/L). In such cases, if the reaction conditions change (e.g., the protein moves into an environment where biotin concentrations are very low, or pH or ionic conditions are altered), the reverse reaction can be promoted. For example, the biotin-streptavidin interaction can be broken by incubating the complex in water at 70 °C, without damaging either molecule. An example of change in local concentration causing dissociation can be found in the Bohr effect, which describes the dissociation of ligands from hemoglobin in the lung versus peripheral tissues.

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