894
UNIT 4
Maintenance of the Body
23
Digestion of Proteins
Proteins digested in the GI tract include not only dietary proteins
(typically about 125 g per day), but also 15–25 g of enzyme pro-
teins secreted into the GI tract lumen by its various glands and
(probably) an equal amount of protein derived from sloughed and
disintegrating mucosal cells. Healthy individuals digest much of
this protein all the way to its
amino acid
monomers.
Protein digestion begins in the stomach when pepsinogen
secreted by the chief cells is activated to
pepsin
(actually a group
of protein-digesting enzymes) (Figure 23.32). Pepsin functions
optimally in the acidic pH range found in the stomach: 1.5–
2.5. It preferentially cleaves bonds involving the amino acids
tyrosine and phenylalanine, breaking the proteins down into
polypeptides and free amino acids.
Pepsin, which hydrolyzes 10–15% of ingested protein, is in-
activated by the high pH in the duodenum, so its proteolytic
activity is restricted to the stomach.
Rennin
(the enzyme that
coagulates milk protein) is not produced in adults.
Because the colon does not secrete digestive enzymes, diges-
tion
officially ends
in the small intestine. As noted earlier, how-
ever, resident colon bacteria do break down and metabolize the
residual complex carbohydrates and some proteins further, add-
ing much to their own nutrition but essentially nothing to ours.
Homeostatic Imbalance
23.16
In some people, intestinal lactase is present at birth but then be-
comes deficient due to genetic factors. When people with
lactose
intolerance
consume lactose, the undigested disaccharides cre-
ate osmotic gradients that prevent water from being absorbed
in the intestines and also pull water from the interstitial space
into the intestines. Te result is diarrhea. Bacterial metabolism
of the undigested solutes produces large amounts of gas that
result in bloating, flatulence, and cramping pain. For the most
part, the solution to this problem is simple—add lactase enzyme
“drops” to your milk or take a lactase tablet before consuming
milk products.
Absorptive
epithelial
cell
Apical
membrane
(microvilli)
Amino
acid
carrier
Capillary
Lumen
of intestine
Pancreatic
proteases
Amino acids
of protein
fragments
Brush
border
enzymes
Na
u
Na
u
1
Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxy-
peptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
2
The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side.
3
The amino acids leave the villus
epithelial cell by facilitated
diffusion and enter the capillary
via intercellular clefts.
Figure 23.33
Protein digestion and absorption in the small intestine.
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