Editing Protein (section)

==Digestion==

{{main|Proteolysis|Protein (nutrient)}}

[[File:Proteolysis scheme.svg|thumb|class=skin-invert-image|230px|Hydrolysis of protein.  X = HCl and heat for industrial proteolysis.  X = protease for biological proteolysis]]

In the absence of catalysts, proteins are slow to [[Hydrolysis|hydrolyze]].<ref name="Radzicka Wolfenden 1996">{{cite journal |last1=Radzicka |first1=Anna |last2=Wolfenden |first2=Richard |title=Rates of Uncatalyzed Peptide Bond Hydrolysis in Neutral Solution and the Transition State Affinities of Proteases |journal=Journal of the American Chemical Society |volume=118 |issue=26 |date=1 January 1996  |doi=10.1021/ja954077c |pages=6105–6109|bibcode=1996JAChS.118.6105R }}</ref> The breakdown of proteins to small peptides and amino acids ([[proteolysis]]) is a step in [[digestion]]; these breakdown products are then absorbed in the small intestine<!--[[jejunum]]-->.<ref name="Keller 2013">{{cite book |last=Keller |first=J. |title=Encyclopedia of Biological Chemistry |chapter=Gastrointestinal Digestion and Absorption |publisher=Elsevier |date=2013 |isbn=978-0-12-378631-9 |doi=10.1016/b978-0-12-378630-2.00106-7 |pages=354–359}}</ref> The hydrolysis of proteins relies on enzymes called [[protease]]s or peptidases.  Proteases, which are themselves proteins, come in several types according to the particular [[peptide bond]]s that they cleave as well as their tendency to cleave peptide bonds at the terminus of a protein (exopeptidases) vs peptide bonds at the interior of the protein (endopeptidases).<ref>{{cite journal |last=Oda |first=Kohei |title=New families of carboxyl peptidases: serine-carboxyl peptidases and glutamic peptidases  |journal=Journal of Biochemistry |year= 2012 |volume=151 |issue=1 |pages=13–25 |doi= 10.1093/jb/mvr129 |pmid= 22016395 |doi-access=free}}</ref>  [[Pepsin]] is an endopeptidase in the stomach.  Subsequent to the stomach, the pancreas secretes other proteases to complete the hydrolysis, these include [[trypsin]] and [[chymotrypsin]].<ref name="Switzar Giera 2013">{{cite journal |last1=Switzar |first1=Linda |last2=Giera |first2=Martin |last3=Niessen |first3=Wilfried M. A. |title=Protein Digestion: An Overview of the Available Techniques and Recent Developments |journal=Journal of Proteome Research |volume=12 |issue=3 |date=1 March 2013 |doi=10.1021/pr301201x |pages=1067–1077|pmid=23368288 }}</ref>

Protein hydrolysis is employed commercially as a means of producing amino acids from bulk sources of protein, such as blood meal, feathers, keratin.  Such materials are treated with hot [[hydrochloric acid]], which effects the hydrolysis of the peptide bonds.<ref>{{cite book |doi=10.1002/14356007.a02_057.pub2 |chapter=Amino Acids |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2007 |last1=Drauz |first1=Karlheinz |last2=Grayson |first2=Ian |last3=Kleemann |first3=Axel |last4=Krimmer |first4=Hans-Peter |last5=Leuchtenberger |first5=Wolfgang |last6=Weckbecker |first6=Christoph |isbn=978-3-527-30385-4 }}</ref>