Proteases are enzymes which digest proteins into amino acids by breaking the peptide bonds which hold them together. There are many different types which play crucial roles in various metabolic pathways. For example pepsin which is an important enzyme in digestion. It is found in the stomach and so works optimally at the pH produced by stomach acid, which is between pH 1-5-3.0 (at pH 1.5 pepsin exhibits around 90% of maximal activity). Further down the digestive tract other proteases help digest food such as trypsin in the duodenum.