Digestion and absorption of protein Dr.Ashok Kumar j

Dr. Ashok Kumar J; IMC; MSU 1

Digestion and Absorption of ProteinsDr. Ashok Kumar J

International School of Medicine Management and Science University

Malaysia

Proteins

• Polymers of amino acids• Macromolecules• Contain about 16% nitrogen in addition to Carbon,

Hydrogen and Oxygen

2Dr. Ashok Kumar J; IMC; MSU

Digestion of proteins and absorption of amino acids


• Intake of dietary protein is in the range of 50-100 g/day • About 30-100 g/day of endogenous protein is derived form

the digestive enzymes and worn out cells of the digestive tract

• The digestion and absorption of proteins is very efficient in healthy humans, hence very little protein (about 5-10 g/day) is lost through feces

Amino acids are linked to each other by peptide linkage

Amino acid

Amino acid

Amino acid

Amino acid

Amino acid

Peptide bonds


The peptide bonds of the proteins must be hydrolyzed to give their constituent amino acids

(Digestion of protein)

Proteases (peptidases)

Amino acid

Amino acid

Amino acid

Amino acid

Amino acid

Peptide bonds

Amino acid

Amino acid

Amino acid

Amino acid

Amino acid



Peptidases(Hydrolases; Class 3)

• Endopeptidase : acts inside the core of protein, forms small peptide fragments

• Exopeptidase : acts from the amino terminal or carboxyl terminal ends of protein• Aminopeptidase - e.g : amino

peptidase• Carboxypeptidase - e,g :

carboxypeptidase

Proenzymes (zymogens)

• Peptidases are secreted in the inactive form

• When zymogens reach the site of action they are activated

Proteolytic enzymes of GIT are secreted by three different organs


STOMACH

PepsinRenin

Pancreas

TrypsinChymotrypsinElastaseCarboxypeptidase ACarboxypeptidase B

Intestine

AminopeptidaseDipeptidase

Digestion of protein begins in the stomach


Stomach secrets

• Strong mineral acid HCl

• Proenzyme pepsinogen

• Denatures proteins • Decreases the pH

(pH 2 to 3)• Activates pepsinogen• Kills some bacteria• Helps in the absorption

of Vitamin B12

Hydorchloric acid (HCl) Pepsinogen

Activated to pepsin by HCl Acid Stable endopeptidase Pepsinogen

Pepsinogen

Pepsin Autocatalysis

Pepsin


Pepsin acts on

Peptide bond formed by carboxyl group of

Phenylalanine, Tyrosine, Tryptophan, methionine

Converts proteins into proteoses and peptones

RENNIN

Also called chymosinActive in infantscurdling of milk

Absent in adult stomach

Converts Casein of milk into paracasein

It combines with calcium to form calcium paracaseinate


Activity of pepsin reduced

Pancreatic juice secretion stimulated by hormone

1. Cholecystokinin 2. Pancreozymin

Large polypeptides further cleaved to oligopeptides by

pancreatic peptidases

Digestion of proteins in the Intestine


Secreted as pro-enzymes

Optimum pH is 8

Pancreatic Peptidases

Carboxypeptidase ACarboxypeptidase B

(Exopeptidase)

TrypsinChymotrypsin

Elastase(Endopeptidases)(Serine proteases)


Trypsin(endopeptidase)

Trypsin

TrypsinogenENTEROPEPTIDASEPresent on the brush border surface

Trypsinogen (Zymogen)

• Hydrolyzes the peptide bond formed by the Basic amino acids (Arginine and Lysine )

• Acts on the zymogen forms of other pancreatic enzymes and activates them

[autocatalysis]


CHYMOTRYPSIN(Endopeptidase)

• Zymogen form – Chymotrypsinogen

• activated to chymotrypsin by trypsin

• Hydrolyzes the peptide bond formed by the carboxyl group of aromatic amino acids

Elastase(Endopeptidase)

• Zymogen form – Proelastase

• activated to elastase by trypsin

• Acts on peptide bonds formed by the amino acids like glycine, alanine, serine


Carboxypeptidase- Zinc containing - Exopeptidase- Zymogen form is procarboxypeptidase- Activated by trypsin- Cannot act on dipeptides

Carboxypeptidase A

acts on the Carboxy terminal peptide

bond connected to Tyrosine,

Phenylalanine or tryptophan

Carboxypeptidase B

acts on the carboxy terminal peptide

bond connected to Arginine, lysine


• Present on the luminal surface of the intestinal mucosa • Is an exopeptidase. • Acts on the amino terminal peptide bond • Release free amino acidAmino

peptidase

• Present on the surface of the intestinal mucosal They act on dipeptides and release free amino acids

• Enterocytes take up some di and tripeptides• These peptides are hydrolyzed to amino acids by intracellular

dipeptidase

Di and Tripeptidases

Absorption of amino acids

• By Na+ dependent active transport system (Na+ amino acid cotransport)• An energy requiring process

•Transporter for acidic amino acids•Transporter for basic amino acids•Transporter for neutral amino acids•Transporter for imino acid •Transporter for β- amino acids

There are 5 different carriers for amino acids



• Intestines, kidney tubules and brain

• Absorption of neutral amino acids

• Also called Miester cycle• Requires the tripeptide

glutathione (GSH) and energy

• Catalysed by gamma glutamyl transferase

γ - glutamyl cycle

5-oxoprolinase deficiency leads to oxoprolinuria


Absorption of intact proteins and polypeptides

• Short period, immediately after birth, the small intestine of infants can absorb intact proteins and polypeptide by endocytosis or pinocytosis

• Intact proteins and polypeptides are not absorbed by the adult intestine

• Macromolecular absorption in certain individuals appears to be responsible for antibody formation that often causes food allergy.


Defects in the intestinal amino acid transport systems are seen in inborn errors of metabolism

• Hartnup’s disease• Iminoglycinuria• Cystinuria : [dibasic amino acids, ornithine, arginine, and lysine

(represented as “COAL”)]• Oasthouse syndrome

Acute pancreatitis: • Premature activation of trypsinogen inside the pancreas itself will

result in the autodigestion of pancreatic cells


• Celiac disease (celiac sprue) is a disease of malabsorption resulting from immune-mediated

damage to the small intestine in response to ingestion of gluten (or gliadin produced from gluten), a protein found in wheat and barley


Thank you

Ref:


Female flower of the hop plant dried as a part of brewing process in oast house; Characteristic smell resembles the smell of the urine in Oasthouse syndrome

Health & Medicine

Digestion and absorption of protein Dr.Ashok Kumar j