9.6. Allostery[[Student version, January 17, 2003]] 331
Figure 9.10:(Metaphor.) Allosteric feedback control. (a)Anallosteric enzyme has an active site (left), at which
it catalyzes the assembly of some intermediate product from substrate. (b)When a control molecule binds to the
regulatory site (right), however, the active site beomes inactive. (c)Inasimplified version of a synthetic pathway,
an allosteric enzyme (top, in fedora) catalyzes the first step of the synthesis. Its product is the substrate for another
enzyme, and so on. Most of the final product goes off on its errands in the cell, but some of it also serves as the
control molecule for the initial enzyme. When the final product is present in sufficient concentration, it binds to the
regulatory site, turning off the first enzyme’s catalytic activity. Thus the final product acts as a messenger sent to
the first worker on an assembly line, saying “stop production.” [Cartoons by Bert Dodson, from Hoagland & Dodson,
1995.]
took the form of scarlet needles, while crystals formed with oxygen present were purple plates.
Moreover, crystals prepared without oxygen shattered upon exposure to air. (Crystals of myoglobin
showed no such alarming behavior.) The crystals’ loss of stability upon oxygenation suggested to
Haurowitz that hemoglobin undergoes ashape changeupon binding oxygen. Perutz’s detailed
structural maps of hemoglobin, obtained many years later, confirmed this interpretation: The
quaternary (highest-order) structure of hemoglobin changes in the oxygenated form.
Today dozens of allosteric proteins are known, and their structures are being probed by an ever-
widening array of techniques. For example, Figure 9.11 shows three-dimensional reconstructed
electron-micrographs of the motor protein kinesin, which will occupy our attention throughout
muchof Chapter 10. Each kinesin molecule is adimer;that is, it consists of two identical subunits.
Each subunit has a binding site that can recognize and bind a microtubule, and another site that
can bind the cellular energy-supply molecule ATP. The figure shows that one particular domain
of the molecule, theneck linker,has two definite, stable positions when no ATP is bound. When
ATP(or a similarly shaped molecule) binds to its binding site, however, the neck linker freezes
into a definite third position. Thus kinesin displays a mechanochemical coupling. The function
of this allosteric interaction is quite different from the one in hemoglobin. Instead of regulating
the storage of a small molecule, Chapter 10 will show how the mechanical motion induced by the
chemical event of ATP binding can be harnessed to create a single-molecule motor.
The observation of gross conformational changes upon binding suggests a simple interpretation
of allosteric interactions:
- The binding of a molecule to one site on a protein can deform the neighborhood of that site.
Forexample, the site’s original shape may not precisely fit the target molecule, but the free
energy gain of making a good fit may be sufficient to pull the binding site into tight contact. - Asmall deformation, of even a few hundredths of a nanometer, can be amplified by a lever-like
arrangement of the protein’s subunits, transmitted to other parts of the protein by mechan-