Structure of Allosteric Enzymes
Allosteric enzymes possess a complex spatial organization. They are typically oligomeric proteins whose structure features two types of functional sites that are spatially separated from each other:
- Active (catalytic) site — the site of direct substrate binding where the chemical reaction takes place.
- Allosteric site — the regulatory region to which the effector molecule binds non-covalently.
Often, these sites are located on different subunits. The regulatory protomer carries the allosteric site, while the catalytic protomer contains the active site. A single enzyme may have multiple allosteric sites: some specific for activators and others for inhibitors.
Mechanism of Action and Cooperativity
Allosteric regulation is fundamentally based on the property of cooperativity. The binding of an effector to the regulatory site induces a cooperative conformational change (spatial structure alteration) across all enzyme subunits. This leads to an alteration in the shape of the active site and the enzyme's affinity for the substrate.
- Action of an inhibitor (negative effector): The inhibitor binds to the allosteric site and induces structural adjustments that reduce the affinity of the active site for the substrate. The enzyme transitions into an inactive form, and the catalytic reaction rate drops.
- Action of an activator (positive effector): The activator induces changes that increase substrate affinity. The enzyme becomes activated, and the reaction rate increases.
Feedback Regulation
Allosteric enzymes are located at key control points in metabolic pathways: they catalyze initial, slowest steps or reactions at branch points of pathways. This is essential for controlling the entire cascade of transformations.
Most commonly in the body, regulation occurs via negative feedback (feedback inhibition). Its core principle is that when an end product accumulates in excess, it acts as an allosteric inhibitor for enzymes at the beginning of the chain. Concurrently, initial or intermediate substances can act as activators, accelerating substrate processing. This combination allows for extremely fine and rapid tuning of metabolic rates.
Regulation Example: Glucose Catabolism
A classic example of allosteric control is the glucose breakdown pathway, where ATP serves as the end product.
If ATP molecules are not consumed and accumulate, they signal an energy surplus. ATP acts as a negative feedback inhibitor, preventing excessive energy production when it is not required.
| Target Enzyme | Regulator | Effect |
|---|---|---|
| Phosphofructokinase | ATP (end product) | Inhibition |
| Pyruvate kinase | ATP (end product) | Inhibition |
| Pyruvate kinase | Fructose-1,6-bisphosphate (intermediate metabolite) | Activation |
Conversely, the accumulation of the intermediate metabolite fructose-1,6-bisphosphate signals pyruvate kinase that there is an excess of substrate from the previous step, accelerating the conversion of phosphoenolpyruvate to pyruvate.