How are they functionally different? Does structure define whether they're catalytic or regulatory?
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Proteins are sometimes made up of several domains (subunits) with different functions. A catalytic subunit is one that catalyzes a chemical reaction in enzyme proteins. A regulatory subunit is one with binding sites that activate or inhibit the protein's activity. And yes structure is partly how they differ. The catalytic subunit is shaped to bind the substrate, the regulatory subunit to bind the regulating molecule(s). When the regulatory subunit binds a regulating molecule the subunit changes shape which triggers a shape change in the catalytic subunit causing it to be activated or inhibited.