Many of the biochemical reactions in a living cell can go both ways. For example, mammalian cells both catabolize and synthesize glucose. The rates at which these reactions occur must be regulated (Enzyme regulation); otherwise, energy is wasted by what is called a futile cycle carrying out opposing reactions at high rates with no net substrate flow in either direction.
Remember that the Second Law of Thermodynamics states that entropy increases in a favored reaction; entropy is wasted energy in that it can’t be used to carry out work. Sometimes an enzyme that uses ATP as a substrate to transfer phosphate to another molecule can hydrolyze ATP to ADP and inorganic phosphate in the absence of the other substrate. This kind of reaction would obviously consume the cell’s energy without doing useful work.
Allostery and Enzyme regulation
In biochemistry, allosteric regulation is the regulation of an enzyme or other protein by binding an effector molecule at the protein’s allosteric site (that is, a site other than the protein’s active site). Effectors that enhance the protein’s activity are referred to as allosteric activators, whereas those that decrease the protein’s activity are called allosteric inhibitors.
The term allostery comes from the Greek allos, “other”, and stereos, “solid (object)”, in reference to the fact that the regulatory site of an allosteric protein is physically distinct from its active site. Allosteric enzyme regulation is a natural example of control loops, such as feedback from downstream products or feedforward from upstream substrates. Long-range allostery is especially important in cell signaling.
Models of Allosteric Enzyme regulation
Most allosteric effects can be explained by the concerted MWC model put forth by Monod, Wyman, and Changeux, or by the sequential model described by Koshland, Nemethy, and Filmer. Both postulate that enzyme subunits exist in one of two conformations – Tensed (T) or Relaxed (R), and that relaxed subunits bind substrate more readily than those in the tense state. The two models differ most in their assumptions about subunit interaction and the preexistence of both states.
- The concerted model of allostery also referred to as the symmetry model or MWC model, postulates that enzyme subunits are connected in such a way that a conformational change in one subunit is necessarily conferred to all other subunits. Thus, all subunits must exist in the same conformation.
- The model further holds that, in the absence of any ligand (substrate or otherwise), the equilibrium favors one of the conformational states, T or R.
- The equilibrium can be shifted to the R or T state through the binding of one ligand (the allosteric effector or ligand) to a site that is different from the active site (the allosteric site).
- The morpheein model of allosteric regulation is a dissociative concerted model.
The sequential model of allosteric regulation holds that subunits are not connected in such a way that a conformational change in one induces a similar change in the others. Thus, all enzyme subunits do not necessitate the same conformation. Moreover, the sequential model dictates that molecules of substrate bind via an induced fit protocol.
In general, when a subunit randomly collides with a molecule of substrate, the active site, in essence, forms a glove around its substrate. While such an induced fit converts a subunit from the tensed state to relaxed state, it does not propagate the conformational change to adjacent subunits. Instead, substrate-binding at one subunit only slightly alters the structure of other subunits so that their binding sites are more receptive to the substrate.
- subunits need not exist in the same conformation
- molecules of substrate bind via induced-fit protocol
- Conformational changes are not propagated to all subunits
- substrate-binding causes increased substrate affinity in adjacent subunits
- This is also part of Enzyme regulation.
- Positive allosteric modulation (sometimes called an allosteric activation) occurs when the binding of one ligand enhances the attraction between substrate molecules and other binding sites.
- An example is the binding of oxygen molecules to hemoglobin, where oxygen is effectively both the substrate and the effector.
- The allosteric, or “other”, the site is the active site of an adjoining protein subunit.
- The binding of oxygen to one subunit induces a conformational change in that subunit that interacts with the remaining active sites to enhance their oxygen affinity.
- Negative allosteric modulation (also known as allosteric inhibition) occurs when the binding of one ligand decreases the affinity for the substrate at other active sites. For example, when 2,3-BPG binds to an allosteric site on hemoglobin, the affinity for oxygen of all subunits decreases. This is when a regulator is absent from the binding site.
- Another example is strychnine, a convulsant poison, which acts as an allosteric inhibitor of the glycine receptor. Glycine is a major post-synaptic inhibitory neurotransmitter in mammalian spinal cord and brain stem.
- Strychnine acts as a separate binding site on the glycine receptor in an allosteric manner; i.e., its binding lowers the affinity of the glycine receptor for glycine.
- Thus, strychnine inhibits the action of an inhibitory transmitter, leading to convulsions.
- Another instance in which negative allosteric modulation can be seen is between ATP and the enzyme Phosphofructokinase within the negative feedback loop that regulates glycolysis. Phosphofructokinase (generally referred to as PFK) is an enzyme that catalyzes the third step of glycolysis: the phosphorylation of Fructose-6-phosphate into Fructose 1,6-bisphosphate.
- PFK can be allosterically inhibited by high levels of ATP within the cell. When ATP levels are high, ATP will bind to an allosteric site on phosphofructokinase, causing a change in the enzyme’s three-dimensional shape.
- This change causes its affinity for substrate (fructose-6-phosphate and ATP) at the active site to decrease, and the enzyme is deemed inactive.
- This causes glycolysis to cease when ATP levels are high, thus conserving the body’s glucose and maintaining balanced levels of cellular ATP.
- In this way, ATP serves as a negative allosteric modulator for PFK, despite the fact that it is also a substrate of the enzyme.
Types of allosteric regulation
- A homotropic allosteric modulator is a substrate for its target enzyme, as well as a regulatory molecule of the enzyme’s activity.
- It is typically an activator of the enzyme.
- For example, O2 is a homotropic allosteric modulator of hemoglobin.
- A heterotropic allosteric modulator is a regulatory molecule that is not also the enzyme’s substrate.
- It may be either an activator or an inhibitor of the enzyme.
- For example, H+, CO2, and 2,3-bisphosphoglycerate are heterotropic allosteric modulators of hemoglobin
Some allosteric proteins can be regulated by both their substrates and other molecules. Such proteins are capable of both homotropic and heterotropic interactions.