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Biochemistry Aspects of Gastrointestinal Tract

Digestion and Absorption of Proteins

Proteins are broken down by peptidases. Peptidase is divided into:

• Endopeptidases (proteases), which attack internal bonds and liberate large peptide fragments.

• Exopeptidases, which cleave off one amino acid at a time from either the C! (carboxypeptidases) or 

the  NH 2 termin"s (aminopeptidases).

Endopeptidases are important for an initial breakdown of long polypeptides into smaller prod"cts, which

can then be attacked more efficiently by exopeptidases. #he final prod"cts are free amino acids and di and

tripeptides.

$s we know most en%ymes can&t work optimally or it can be destroyed in an extreme p!. Pepsins are

"ni'"e in that they are acid stable. $ctive pepsin is generated from the proen%yme pepsinogen by the

removal of amino acids from the  NH 2  termin"s.

Cleavage between resid"es and of pepsinogen occ"rs as either an intramolec"lar reaction

(a"toactivation) below p! or by active pepsin (a"tocatalysis). #he liberated peptide from the  NH 2

termin"s remains bo"nd to pepsin and acts as *pepsin. #he ma+or prod"cts of pepsin action are large peptide

fragments and some free amino acids.

Pancreatic +"ice is rich in proen%ymes of endopeptidases and carboxypeptidases, which are activated after 

they reach the l"men of the small intestine. Enteropeptidase, a protease prod"ced by d"odenal epithelial

cells, activates pancreatic trypsinogen to trypsin by scission of a hexapeptide from the  NH 2   termin"s.

#rypsin in t"rn a"tocatalytically activates more trypsinogen to trypsin and also acts on the other 

 proen%ymes.

Polypeptides generated from ingested proteins are degraded within the small intestinal l"men by

carboxypeptidases $ and . #he pancreatic carboxypeptidases are2+¿

Zn¿  metalloen%ymes and possess a

different type of catalytic mechanism than the carboxyl or serine peptidases. #he combined action of 

 pancreatic peptidases res"lts in the formation of free amino acids and small peptides.

Digestion and Absorption of Carbohydrates

-tarch, a ma+or n"trient, is a plant polysaccharide with a molec"lar mass of more than // k0a. 1t consists

of a mixt"re of linear chains of gl"cose molec"les linked by 23, gl"cosidic bonds (amylose) and of 

 branched chains with branch points made "p by 23,4 linkages (amylopectin).

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!ydrated starch and glycogen are attacked by the endosaccharidase, 23amylase present in saliva and

 pancreatic +"ice. $mylase is specific for internal 23, gl"cosidic bonds5 23,4 bonds are not attacked, nor 

are 23, bonds of gl"cose "nits that serve as branch points. #he prod"cts of the digestion by a amylase are

mainly the disaccharide maltose, the trisaccharide maltotriose, and so called a limit dextrins containing on

average eight gl"cose "nits with one or more 23,4 gl"cosidic bonds.

6ost of the s"rface oligosaccharidases are exoen%ymes that cleave off one monosaccharide at a time. #he

capacity of the 23gl"cosidases is normally m"ch greater than that needed for completion of the digestion of 

starch.

Digestion and Absorption of Lipid

#riacylglycerols constit"te more than 7/8 of the dietary fat. #he rest is made "p of phospholipids,

cholesterol, cholesterol esters, and free fatty acids. 0igestion of lipids is initiated in the stomach by an acid3

stable lipase, most of which is tho"ght to originate from glands at the back of the tong"e. !owever, the rate

of hydrolysis is slow beca"se the ingested triacylglycerols form a separate lipid phase with a limited water9 

lipid interface. #he lipase adsorbs to that interface and converts triacylglycerols into fatty acids and

diacylglycerols.

#he ma+or en%yme for triacylglycerol hydrolysis is the pancreatic lipase. #his en%yme is specific for esters

in the 23position of glycerol and prefers long chain fatty acids with more than ten carbon atoms. !ydrolysis

 by the pancreatic en%yme also occ"rs at the water9lipid interface of em"lsion droplets. #he prod"cts are

free fatty acids and 3monoacylglycerols.

 Devlin’s Textbook of Biochemistry with Clinical Correlations, 4th edition