a catalyst is a molecule that speeds-up (catalyzes) reactions without getting used up or permanently changed in the process. Biological catalysts are called enzymes and they’re typically proteins, RNA, or protein/RNA combos. An important feature of catalysts is that they can’t change what reaction happens, and they can’t make any reaction they want happen – instead, they can only make it easier for reactions that already could happen to actually happen (by lowering the activation energy barrier). You can think of it kinda like the “you can lead a horse to water but you can’t make them drink” saying – catalysts can bring molecules together, hold them in the optimal form to react, provide alternative reaction mechanisms, etc. but they can’t make that reaction happen. Another important thing about catalysts is that they catalyze the reaction in both directions – so a protease can speed up breaking of peptide bonds and making of peptide bonds but the breaking is waaaaay more favorable, so the reaction is virtually only forward. Finally, catalysts must be able to catalyze the reaction over and over and over without getting used up. Sometimes catalysts are temporarily changed (such as by having a reaction intermediate stuck to them) but they are then restored to their original state.
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