Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Pepsin is an enzyme that acts in the stomach for protein digestion. Which of these is most likely the main role of pepsin in this metabolic reaction?
to speed up the rate of reaction
Protein digestion in the stomach.