Various protonation states can exist depending on the pH and the nature of the catalyst and substrate. In homogeneous catalysis, acids like H₂SO₄ or H₃PO₄ often act as proton donors. In heterogeneous catalysis, solid acids such as zeolites or acidic metal oxides can provide protons. Enzymes, being biological catalysts, have specific amino acids like histidine, aspartic acid, and glutamic acid that can donate or accept protons, thus modulating their catalytic activity.
