Allosteric Enzymes

Enzymes are biological catalysts. They accelerate the rates of reactions in cells without being changed themselves during the process of reaction. Allosteric enzymes are a subset of enzymes that are involved in the control and regulation of biological processes. The control of processes is essential to biological systems: A cell that divides out of control has undergone a kind of transformation (cancerous or precancerous) that can threaten an entire organism. There are many processes occurring inside the cell at any one time, and they must coexist in proper balance. This balance is where allosteric enzymes come into play.

Allosteric enzymes have the ability to respond to several different conditions in their environments. Every enzyme contains an active site, the location on the enzyme where it catalyzes its specific reaction. Allosteric enzymes contain a second type of site, called an allosteric site. The allosteric site, through its binding of a nonsubstrate molecule, influences (enhances or impairs) the activity of the enzyme. The word "allosteric" is derived from two Greek words: allos, meaning other, and stereos, meaning site. Another important feature of allosteric enzymes is that they consist of multiple polypeptide chains, with multiple active and allosteric sites.

Whereas substrates bind to the active sites of enzymes, other nonsubstrate molecules (allosteric modulators) bind to the allosteric sites. The significance and role of the allosteric site is well illustrated by the example of the enzyme aspartate transcarbamoylase, which catalyzes reaction 1.

This is the first step of the pathway that leads to the formation of cytosine , a building block for DNA synthesis . The form of cytosine that is used to synthesize DNA (and RNA ) is the molecule cytidine triphosphate (CTP). When intracellular CTP concentrations are high, CTP molecules bind more often to the allosteric sites on aspartate transcarbamoylase molecules, causing a change in the shape of the enzyme that slows reaction 1 down markedly. Thus, CTP is an allosteric inhibitor of this enzyme.

There are times in the life of a cell when it needs to make more N-carbamoylaspartate (and eventually more CTP), even when concentrations of CTP are high to begin with. This is particularly true just prior to cell division, because the cell is at this point rapidly synthesizing DNA. Under these conditions, a high concentration of adenosine triphosphate (ATP) , another building block of DNA, will be present in the cell. ATP can also bind to the allosteric site on aspartate transcarbamoylase, but, unlike CTP, it acts as an enzyme activator, overcoming the inhibitory effect of CTP and thereby leading to the synthesis of more CTP, when CTP concentrations are already high.

see also Enzymes; Inhibitors; Proteins.

Paul A. Craig

Bibliography

Berg, Jeremy M.; Tymoczko, John L.; and Stryer, Lubert (2002). Biochemistry, 5th edition. New York: Freeman.

Horton, H. Robert; Moran, Laurence A.; Ochs, Raymond S.; Rawn, J. David; and Scrimgeour, K. Gray (2002). Principles of Biochemistry, 3rd edition. Upper Saddle River, NJ: Prentice Hall.

Voet, Donald; Voet, Judith G.; and Pratt, Charlotte (1999). Fundamentals of Biochemistry. New York: Wiley.