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gihep 11: digestion + absorption of protein + carbs (i) (Protein digestion…
gihep 11: digestion + absorption of protein + carbs (i)
Protein digestion
1) in mouth
mechanical (chewing)
saliva aids swallowing
2) in stomach
homogenises protein (makes all molecules equal in composition
HCl in gastric juice...
causes low pH, which denatures protein by opening its structure + making it linear so pepsin can get to core
activates pepsinogen (released by chief cells)
3) in SI
pancreatic juice
chymotrypsin
breaks bonds of hydrophobic AAs
trypsin
breaks bonds of basic AAs
elastase
breaks bonds of small AAs
carboxypeptidase A+B
metalloprotease (uses zinc)
A breaks bonds of hydrophobic AAs @ C terminus
B breaks bonds of basic AAs @ C terminus
released as proenzymes
trypsin activated by enteropeptidase
others activated by trypsin
AAs move to blood in low SI using transporters + ATP
90% of protein becomes AA monomers
4) in liver
AA catabolism produces toxic nitrogenous ammonia
gets converted to urea + excreted
Peptide absorption into enterocyte
more rapid than free AA absorption
most protein absorbed this way
coupled to H+ gradient
maintained by Na+/H+ exchange
Mechanisms of absorption
site = villus
simple columnar epithelium
catalytic domains of brush border enzymes protrude
mostly in proximal SI
rate of transport proportional to SA
types ...
passive
slow, down conc gradient
facilitated
uses membrane proteins in enterocyte
active
uses energy-dependent membrane proteins