Enzyme Activity: Allosteric Regulation

Abstract Cells can respond to changes in their environment by altering the flow through special, regulated metabolic steps performed by allosteric enzymes. These enzymes adopt either an active or inactive conformation in response to binding positive or negative effectors. As the energy difference between the two conformations is normally modest, the binding of a small metabolite ligand is adequate to stabilise the conformation that binds it. Most allosteric enzymes are K‐type, denoting that the principal feature that is altered is their affinity for their substrate, measured as the K M constant. This enables them to become more or less active when the substrate concentration in the cell remains largely constant. A small, but important, subset of enzymes are V‐type, denoting that the important change is in their maximum activity, defined as V max . These normally inactive enzymes can increase their activity by up to a million‐fold. Key Concepts: All protein enzymes are sufficiently flexible to modestly alter their folded structure and experience two or more different conformational states under normal physiological conditions. Such conformational changes may modestly alter the position of amino acids around the catalytic site, thereby making the ability of the substrate to bind at this site more, or less, favourable. The change in the free energy for such different conformations is quite modest (normally in the range of 2–7 kcal mol −1 ). Then, if one of these conformations has an appropriately formed binding site for a regulatory ligand, the binding energy with such a ligand binding with simple noncovalent interactions (normally in the range of 2–7 kcal mol −1 ) is adequate to stabilise that particular conformation, and make it more abundant in the cell. For about a third of all enzymes there exist one or more cellular metabolite(s) that can bind to and stabilise either the more active or the less active conformation. Many allosteric enzymes may also be regulated by being covalently modified, via the attachment of some chemical group. Though more than 20 different types of such regulatory adducts have been observed, phosphorylation is most common, being used with almost half of all allosteric enzymes. Allosteric enzymes are defined as K‐type (approximately 30% of all enzymes) or V‐type (less than 1% of enzymes), depending on whether the major regulatory feature is their change in affinity ( K M ) or their change in maximum activity ( V max ). More than 90% of K‐type enzymes display positive cooperativity, as they are converted from a fairly inactive T conformation to the active R conformation. A fairly small subset of K‐type enzymes displays negative cooperativity, resulting when many of the catalytic sites, either in a single enzyme oligomer, or in the overall enzyme ensemble, have a normal affinity for the substrate, while the remaining sites have a much lower affinity. This feature greatly extends the concentration range over which the substrate can be used.