Enzymes responsible for the digestion of carbohydrates, proteins, and lipids and the steps of digestion of macromolecules

1. Digestion of Carbohydrates, Proteins and Lipids
Bilal Sami Al-Moshaigah

2. Learning Objectives
Digestion of carbohydrates, proteins and lipids
List the various enzymes participating in their digestion
Mention the specific role of these enzymes

3. Carbohydrates digestion
The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen
This digestion is rapid and is catalyzed by enzymes known as glycoside hydrolases (glycosidases)
that hydrolyze glycosidic bonds
Salivary α-amylase
Pancreatic α-amylase
Intestinal disaccharidases
Salivary gland
Pancreas
Small intestine

4. Salivary α-amylase
Salivary α-amylase acts briefly on dietary starch and glycogen, hydrolyzing random α(1→4) bonds
Optimum pH is 6.7
α–amylase can not attack α-1-6 linkage or α–1-4 linkage close to α-1-6 branch point
Partially?
Chloride ions

5. Pancreatic amylase
α-amylase enzyme is produced by pancreas and acts in small intestine
Optimum pH is 7.1
Acts on dietary starch and glycogen , hydrolysing them into maltose ,
isomaltose and other
Chloride ions

6. Intestinal disaccharidases
The final digestive processes occur primarily at the mucosal lining of the duodenum and upper jejunum
Include the action of several disaccharidases
Isomaltase cleaves the α(1→6) bond in isomaltose
Maltase cleaves the α(1→4) bond in maltose and maltotriose producing glucose
Sucrase cleaves the α(1→2) bond in sucrose, producing glucose and fructose
Lactase (β-galactosidase) cleaves the β(1→4) bond in lactose, producing
galactose and glucose ?

7. Cellulose
Cellulose contains β(1-4) bonds between glucose molecules
In humans, there is no β (1-4) glucosidase that can
digest such bonds So cellulose passes as such in stool
Function
Helps water retention during the passage of food along the intestine
Producing larger and softer feces Preventing constipation
?

8. Proteins digestion
Dietary proteins are chemically long chains of amino acids bound together by peptide linkages
Proteins are hydrolyzed into amino acids ,by proteolytic enzymes that produced
by three different organs: the stomach, the pancreas, and the small intestine
Proteins must be hydrolyzed to yield di- and tripeptides as well as individual
amino acids, which can be absorbed

9. Stomach
The digestion of proteins begins in the stomach, which secretes gastric juice
Hydrochloric acid (HCL) (pH 2–3)
Secreted by the parietal cells of the stomach, functions to kill some bacteria and to denature proteins
Proenzyme pepsinogen
This acid-stable endopeptidase is secreted by the chief cells of the stomach as an inactive zymogen
Pepsin
In the presence of HCl
Releases polypeptides and a few free amino acids from dietary proteins
?
pH optimum of 2
Gastrin hormone

11. Intestine
Secretin hormone
Stimulates the release of pancreatic juice rich in bicarbonate to neutralize gastric secretion
Optimum environment for
the enzymes
CCK hormone
Stimulates the release of pancreatic zymogens like : trypsinogen and chymotrypsinogen
Enteropeptidase (enterokinase)
Initiates activation of pancreatic zymogens through activation of trypsinogen into active trypsin
Has autocatalytic activity
Activates the remaining pancreatic zymogens including itself
The entry of acidic chyme into small intestine stimulates the release of :

13. Pancreas Enzymes include : trypsin, chymotrypsin, carboxypeptidase, elastase and collagenase
Trypsin is an endopeptidase hydrolyzing the peptide bonds formed by basic amino acids e.g. lysine and arginine
Acts as activator for all other inactive pancreatic enzymes
Chymotrypsin
It is endopeptidase hydrolyzing the peptide bonds formed by
aromatic amino acids e.g. tryptophan, tyrosine, and phenylalanine
Optimum pH: 8
Optimum pH: 8
Secreted as inactive proenzyme chymotrypsinogen which is
activated by trypsin enzyme
Trypsin

14. Pancreas Enzymes include : trypsin, chymotrypsin, carboxypeptidase, elastase and collagenase
Carboxypeptidase
Secreted as inactive pro-carboxypeptidase and activated by trypsin enzyme
Exopeptidase
It hydrolyzes peptide bonds adjacent to free –COOH group of the polypeptide
chain releasing each time a single free amino acid
Elastase Endopeptidase
Secreted as inactive proelastase and activated by trypsin.
It hydrolyzes peptide bonds formed by small amino acids e.g. glycine, alanine and serine
Collagenase: It catalyzes the hydrolysis of collagen.
optimum pH is 7.4
optimum pH is 7.4

15. Intestine
Aminopeptidase Exopeptidase
hydrolyzes the peptide bonds adjacent to the free –NH2
group of the polypeptide chain releasing each time a
single free amino acid.
Dipeptidase It completes the digestion of dipeptides
Tripeptidase It acts on tripeptide

16. Fat digestion
Adult man ingests about 60-150 g of lipids/day of which > 90% is normally Triglycerides, the remainder of
dietary lipids is made of cholesterol, cholesteryl esters, Phospholipids and Free Fatty acids
Triglycerides (TG)
Digested by a group of enzymes. These are Lingual, gastric, pancreatic and intestinal lipase enzymes
Lingual Lipase
Secreted by Ebner’s glands on the dorsal surface of the tongue
Because foods remain for a short time in the mouth, digestion of TG by this enzyme is minimal
Gastric Lipase
Optimum pH for gastric lipase is 4-6 ? Infants stomach
(pH:5)
Active at low pH and continues functioning in stomach
It acts on mother milk fat that contains TGs consisting
of short or medium chain fatty acids.
Optimum pH is 4-6

17. Fat
Triglycerides (TG)
Pancreatic Lipase
It is secreted into the intestine and prefers long chain FAs
It is the most important lipase in TGs digestion
It digests the primary ester bonds (position 1 and 3), hydrolysing them into fatty acids and 2-monoacylglycerols
Fate ?

18. 2-monoacylglycerols
Absorbed as 2-monoacylglycerols Converted into 1-monoacylglycerols
72% 28%
Absorbed as 1-
monoacylglycerols
Hydrolyzed into glycerol
and fatty acids
6% 22%
Intestinal lipase
Isomerase
Fate ?
Absorbed

19. Cholesterol & Cholesterol ester
Cholesterol itself undergoes no digestion and absorbed as such
Cholesterol ester are digested by pancreatic cholesterol esterase into cholesterol and free fatty acids
Enhanced in the presence of bile acid
Most dietary cholesterol is present in the free (nonesterified) form, with 10%–15% present in the esterified form

20. Phospholipids
Glycerophospholipids is hydrolyzed by pancreatic enzyme phospholipase A2
Remove FA in position 2 to form lysophospholipids + Free fatty acid
lysophospholipase
Glycerol, FA and nitrogenous base
Hydrolyzes the glycerol part leaving
phospharylated nitrogenous base
Separates the phosphate part from the base
(position 1)

21. Summary
The principal sites of dietary carbohydrate digestion are the mouth and intestinal lumen
Cellulose helps water retention during the passage of food along the intestine
Proteins are hydrolyzed in three different regions: stomach, pancreas, and small intestine
Trypsin Acts as activator for all other inactive pancreatic enzymes
Pancreatic lipase is the most important lipase in TGs digestion

22. Any Question

23. Reference
Denise Ferrier (2017) Lippincott Illustrated Reviews: Biochemistry, 7th edn., US:
Wolters Kluwer Health.
David Lee Nelson, Michael M. Cox (2013) Lehninger Principles of Biochemistry, 6th
edn., New York, United States: W.H.Freeman & Co Ltd.
Jan Koolman, Klaus-Heinrich Röhm ( 2005) Color atlas of biochemistry, 2nd edn.,
Stuttgart, Germany: Thieme