Final answer:
Allosteric proteins, including enzymes, may possess multiple allosteric sites. These sites control enzyme activity through conformational changes induced by effector molecule binding. The number of allosteric sites varies among proteins, with some having a single site and others having several.
Step-by-step explanation:
Allosteric proteins, such as enzymes and other ligand-binding systems like hemoglobin, have multiple functional sites. Allosteric enzymes, in particular, feature both an active site, where substrate binding and catalysis occurs, and at least one allosteric site for the regulation of enzyme activity. These allosteric sites are where effectors or modulator molecules bind, inducing a conformational change that affects the enzyme's affinity for its substrate. The allosteric control can be either positive or negative.
The number of allosteric sites can vary. Some proteins may have only a single allosteric site, while others may have multiple sites that interact with each other for highly-controlled reaction rates. As an example, the well-known oxygen-binding protein hemoglobin has four allosteric sites for oxygen, while myoglobin has a single site. Allosterically regulated enzymes, often composed of more than one polypeptide subunit, can exhibit changes across all their active sites when an allosteric effector binds.