What is the term for a molecule that combines with enzymes ,undergoes a chemical reaction ,and changes into new substance?

substrate

Explanation:

A substrate is the term for a molecule which combines with enzymes when acted upon and changes into a new substance.

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the answer is lactic acid from anaerobic respiration. the acid in lactic acid is what creates sore and burning muscles. if you think about how acid burns that will give you a clue for questions like this.