Allosteric Regulation
* Definition: Allosteric regulation is a way to control enzyme activity by binding molecules (called *allosteric effectors*) to sites on the enzyme that are different from the active site. This binding changes the enzyme's shape, altering its activity.
* Types of Allosteric Effectors:
* Activators: Bind to the enzyme and increase its activity.
* Inhibitors: Bind to the enzyme and decrease its activity.
* Mechanism: The binding of an effector molecule to the allosteric site can cause a conformational change in the enzyme. This change can either make the active site more accessible to the substrate (activation) or less accessible (inhibition).
What You Would Usually Find in Allosteric Regulation:
1. Quaternary Structure: Allosteric enzymes often have multiple subunits (e.g., a dimer or tetramer). The interaction between these subunits allows for cooperative binding of substrates and effectors.
2. Multiple Binding Sites: Allosteric enzymes have separate active sites for substrate binding and allosteric sites for effector binding.
3. Sigmoidal Kinetics: When plotted on a graph of enzyme activity versus substrate concentration, allosteric enzymes show a sigmoidal (S-shaped) curve. This is because of the cooperative binding of substrate molecules.
4. Regulatory Sites: These sites are distinct from the active site and bind allosteric effectors.
5. Conformational Changes: The binding of allosteric effectors causes a change in the enzyme's shape, which can either increase or decrease the activity of the active site.
Examples of Allosteric Enzymes:
* Phosphofructokinase-1 (PFK-1) in glycolysis: PFK-1 is activated by ADP and inhibited by ATP and citrate.
* Hemoglobin: The binding of oxygen to one subunit of hemoglobin increases the affinity of the other subunits for oxygen, leading to cooperative binding.
Key Points:
* Allosteric regulation is a crucial mechanism for controlling metabolic pathways, ensuring that enzymes are active only when needed.
* By binding at allosteric sites, effector molecules can fine-tune enzyme activity in response to changes in cellular conditions.
