Activation of Zymogens:
In the digestion of food a variety of enzymes are active starting
in the mouth with amylase. Several other enzymes are active in
the stomach, but most digestion occurs in the small intestine.
Chyme is the final, partially digested and acidic, product
of stomach digestion. The entry of chyme into the first part
of the small intestine, the duodenum, stimulates a hormone called
secretin to stimulate both the pancreas and the liver.
In response, the pancreas secretes the hormone insulin for
use in the metabolism of glucose by the cells. In addition, a
strongly alkaline pancreatic juice is secreted into the duodenum
to neutralize the acidic chyme. In addition, pancreatic juice
contains several enzymes including amylase (hydrolyzes starch)
and steapsin (hydrolyzes lipids); and zymogens including trypsinogen,
chymotrypsinogen, and procarboxypeptidase.
The entry of chyme into the duodenum also stimulates the secretion
of intestinal juice. The intestinal juice includes a number of
enzymes such as: lipase, sucrase, maltase, lactase, aminopeptidase,
dipeptidase, and enterokinase. Enterokinase is used to activate
the zymogen, trypsinogen, to trypsin.
trypsinogen + enterokinase ---> trypsin + octapeptide
The activation of trypsinogen to trypsin occurs as follows:
Enterokinase hydrolyzes the peptide bond between amino acid residues
15 and 16 liberating a small peptide. The new amino terminal
unit, 16, then interacts ionically with aspartic acid-194. This,
in turn, alters the orientation of the lysine-145 and opens the
active site on the enzyme.
Trypsin in turn is used to activate two other zymogens:
chymotrypsinogen + trypsin ---> chymotrypsin
procarboxypeptidase + trypsin ---> carboxypeptidase
The above three enzymes are all active in the hydrolysis of