Now why would we want to cut a protein? Well, basically certain proteins are synthesized in their inactive or xiaomogen form. In order to actually activate those proteins, our enzymes called proteases must break certain peptide bonds in those proteins. Now many of the digestive enzymes in the stomach as well as in a small intestine undergo this process of proteolysis. Now, let's move on to the fifth type of posttranslational modification, known as nacetythylation. So, nacetylation is the transfer of an acetyl group from one molecule to an amino acid, or unto the polypeptide chain.