Energetics and Products of Pepsin/HCl Protein Digestion What are the energetics of protein digestion during which the enzyme pepsin is "activated" (whatever that means) by HCl? I've looked and been unable to find anything like a chemical equation that includes an energy term. Of course, pepsin, being an enzyme, is not used up in that sought-for equation. Is HCl used up? If so, what are the products? How and in what form is the chlorine removed, assuming the HCl is consumed in the equation?

This is well-explained on the Wikipedia page for pepsin. You are misinterpreting the use of the word _activated_. The protein is secreted by chief cells in the gastric glands in the form of pepsinogen, an inactive pro- form which has an extra ~40 amino acids at its N-terminus (the propeptide). The propeptide binds at the catalytic site of the enzyme and keeps it inactive. At low pH (this is where the HCl comes in - HCl secreted by parietal cells of the gastric glands acidifies the stomach) the protein is able to cleave off its own propeptide, making it fully active.

!enter image description here