Which factors determine the specificity of protein binding?

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Specific binding comes from a combination of shape and chemistry. The binding site has a three‑dimensional shape that matches the ligand, providing geometric complementarity so the molecule fits snugly. But the chemistry—the pattern of charged, polar, and hydrophobic groups lining the site—provides the actual interactions that stabilize the complex, such as hydrogen bonds, ionic contacts, and hydrophobic interactions. Together, the correct shape places the ligand so those chemical interactions can form, giving high affinity and selectivity. If you focus only on shape, you miss whether the chemistry can support stable interactions; if you focus only on chemistry, you miss whether the ligand can physically fit. Some proteins can adjust their shape slightly to improve fit, illustrating how both aspects work in concert to determine specificity.