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.