Enzyme Regulation
Part II. Allosteric Regulation
Control of Enzyme Activity by Non-Covalent Modifiers is usually called allosteric regulation since the modifier binds to the enzyme at a site other than the active site but alters the shape of the active site. Allosteric is a word derived from two Greek words: 'allo' meaning other and 'steric' meaning place or site; so allosteric means other site and an 'allosteric enzyme' is one with two binding sites - one for the substrate and one for the allosteric modifier molecule, which is not changed by the enzyme so it is not a substrate. The molecule binding at the allosteric site is not called an inhibitor because it does not necessarily have to cause inhibition - so they are called modifiers. A negative allosteric modifier will cause the enzyme to have less activity, while a positive allosteric modifier will cause the enzyme to be more active. In order for allosteric regulation to work, the enzyme must be multimeric (ie. a dimer, trimer, tetramer etc.). The concept is easily illustrated using a dimer as the model system, but it applies equally well to higher order multimers such as trimers and tetramers, etc.
Figure 1. A model of an Allosteric Enzyme.
The allosteric enzyme is a dimer with identical subunits so it is called a homodimer (A-A, where 'A' represents one of the identical subunits). Each subunit has an active site, which are shown being located at the interface of the two subunits in the model in Fig. 1. The binding sites for the allosteric modifier are shown as being remote from the active site. Thus, the influence of the allosteric modifier must be transmitted through the framework of the enzyme in order to influence the activity of the enzyme - either positively or negatively.
Figure 2. General Example of the Regulation of an Enzyme by its Allosteric Modifier. In this model, the allosteric modifier is represented by 'M'.
First, the enzyme in absence of the positive allosteric modifier (M) has low activity and a high Km for the substrate. Then, when the positive allosteric modifier is present, the enzyme is altered in shape, usually both subunits change shape and the dimer takes on a new conformation. In the altered form of the enzyme with M bound, it will have a higher activity and a lower Km for substrate.
A special case of the positive allosteric modifier is when the substrate itself is the modifier. In this case, there is no 'M' or allosteric modifier site; the binding of the substrate causes the enzyme dimer to take on a new shape and be converted to a more active form. This is called 'Positive Cooperative'.
Figure 3. Model of a Dimer with Positive Cooperative when Substrate (S) binds.
When the first substrate molecule binds it makes it easier for the second substrate molecule to bind. Enzymes with positive cooperative do not have Michaelis-Menten kinetics!
Figure 4. Vo versus [S] plot for an enzyme with positive cooperative to binding of substrate.
Enzymes displaying positive cooperative have sigmodial kinetics reflecting the very large increase in Vo with increasing [S] over a very narrow range of [S]. Note the very small change in Vo at first as more substrate is added. Then at a certain level of [S], the enzyme population changes to the more active form with a higher affinity for substrate and more activity (ie more production of product). But eventually, the Vmax limit is reached since there are still a limited number of substrate binding sites, so the increase in Vo plateaus like it does for an M-M type enzyme. This is the reason, I taught you to always draw a Vo versus [S] plot as your first step in analysis of an enzyme's kinetics. If you are dealing with an enzyme with sigmodial kinetics, it will be much easier to detect using the Vo versus plot than the double reciprocal plot (plot of 1/Vo versus 1[S]). In fact, the double reciprocal plot will be difficult to construct and make a good graph for because the properties of the enzyme actually change in response to adding more substrate.
The advantage of sigmodial enzyme kinetics is that a large increase in catalytic rate results from only a very small increase in [S]. Logarithymic change in Vo for a linear change in [S].
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©Wilbur H. Campbell, 1995; wcampbel@mtu.edu