It's possible you're thinking of these related concepts:
* Reactants: These are the molecules that enzymes act upon in a chemical reaction. They are also sometimes called "substrates."
* Transition state: This is a high-energy, unstable intermediate state that reactants must pass through to become products. Enzymes lower the activation energy needed to reach this state, thereby speeding up the reaction.
* Activation energy: This is the minimum amount of energy required for a reaction to occur. Enzymes lower this energy barrier.
If you can provide more context or the specific sentence you're trying to understand, I can help you clarify the meaning.
