What is the defining characteristic of an allosteric protein?
A. They contain two ligand-binding sites, one that activates the protein when a ligand binds, and the other that inactivates the protein when the same ligand binds.
B. They contain only one ligand-binding site, but because the specificity is low, many different ligands can bind to it.
C. They contain two polypeptide chains, with each being an exact mirror image of the other.
D. They contain more than one ligand-binding site, and noncovalent binding of a ligand to one site alters the shape of other ligand-binding sites.
E. They contain no binding sites of their own, but act by modulating the activity of other proteins.
Answer: D
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