1,362
edits
Changes
Small Intestine - Anatomy & Physiology (view source)
Revision as of 09:50, 16 July 2008
, 09:50, 16 July 2008→Function
'''Protein Digestion and Absorption'''
'''Protein Digestion and Absorption'''
*Protein digestion begins in the [[#Stomach - Anatomy & Physiology - Histology|stomach]]
*Protein digestion begins in the [[Forestomach - Anatomy & Physiology|stomach]] where pepsin is secreted as a zymogen, pepsinogen. Pepsin is an endopeptidase and produces smaller polypeptides.
**Pepsin prefers to break the bonds of larger polypeptides, where there is a large hydrophobic amino acid on the N-terminal side.
*Protein digestion continues in the small intestine.
**There are three endopeptidases in the small intestine; trypsin; chymotrypsin; and elastase.
**They are all secreted as zymogens; inactive precursors.
***Trypsin is secreted as trypsinogen.
***Chymotrypsin is secreted as chymotrypsinogen.
***Elastase is secreted as proelastase.
****Trypsinogen is initially activated by enterokinase (activation involves the cleavage of 6 amino acids). Trypsinogen can then activate itself, and also chymotrypsin and elastase.
*The short polypeptides produced are further digested by exopeptidases which remove amino acids from the end of the polypeptide chain.
**There are two types of exopeptidase in the small intestine:
***'''Carboxypeptidases''' that split amino acids off the polypeptide from the C-terminus.
****Produced by the pancreas as zymogens that also become activated by trypsin.
***'''Aminopeptidases''' that split amino acids off the polypepetide from the N-terminus.
****Produced by the small intestine (not as zymogens).
*The final products of protein digestion are amino acids and some di-,tri- and oligo-peptidases.
*Amino acids are absorbed by four carrier proteins; each transports a different type of amino acid.
**Neutral amino acids e.g. ala, val, leu.
**Dibasic amino acids e.g. lys, arg, orn.
**Acidic amino acids e.g. glu, asp.
**Small amino acids inc. pro, hydroxyproline and gly.
***Amino acids within the same group compete with each other for the transport molecule.
***Amino acids are transported with sodium through a symport (like glucose), where amino acids travel up their concentration gradient and sodium travels down its concentration gradient.
*Di-,tri- and oligo- peptidases are absorbed by the γ glutamyl transfer cycle.
**γ Glutamyl transferase spans the enterocyte membrane and combines glutathione from the inside of the cell with a di-,tri- or oligo-peptide from the intestinal lumen forming a γ-glu-aa complex which is transported into the cell. The aas are then released and glutathione is recombined to enter the process again.
==Regulation & Control==
==Regulation & Control==