A change in the shape of a protein, usually as a result of binding a substrate.

For example, take Hemoglobin, a protein in our red blood cells that binds oxygen to the heme groups of each or its four subunits (see quaternary structure). These subunits do not function independently, but rather once one oxygen is bound by a hemoglobin subunit, that subunit undergoes an allosteric change (conformational change that affetcs its 3d shape), and in the process of doing so pulls on the other subunits, forcing them into a shape that favors further binding of oxygen by the other subunits. Though each subunit changes shape upon binding, the most significant change is after two subunits have bound. This behavior is essential to the sigmoidal shape of the oxygen binding curve (partial pressure of oxygen vs a measure of bound oxygen). This is why hemoglobin exhibits the cooperative binding that allows it to readily bind oxygen at the lungs, but still release it all in the peripheral tissue (where there is lower partial pressure of O2). (This slightly more complex, but has been slimplified for the purpose of this example.)

See also allosteric inhibition and motor proteins.