Digestion of proteins Dr. Samah Kotb Lecturer of Biochemistry 2015 Cellular Biochemistry and metabolism 1 CLS 331

Dehydration synthesis N-terminus NH2 end C-terminus COOH end

Amino Acid + Amino Acid --> Dipeptide Amino Acid + Dipeptide --> Tripeptide A.A. + A.A. + …..+ Tripeptide --> Polypeptide

A protein consists of one or more polypeptide chains. Dr Samah Kotb Lecturer of Biochemistry

Globular Fibrous Dr Samah Kotb Lecturer of Biochemistry

Generally speaking, proteins do everything in the living cells. 1) Catalysis: Almost all chemical reactions in a living cell are catalyzed by protein enzymes. 2) Transport: Some proteins transports various substances (Hemoglobin). 3) Regulatory: Many hormones are proteins (insulin). Dr Samah Kotb Lecturer of Biochemistry

4) Contraction: Some have role in contractile mechanism ( actin & myosin) (muscle contraction). 5)Proteins of immune defense: Antibodies. 6) Structural Functions: giving structure and form to the human organism. Example: keratin, collagen. Dr Samah Kotb8

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Dr Samah Kotb Lecturer of Biochemistry Digestion & Absorption of proteins Enzymes involved in the Digestion of Proteins  Proteases are proteolytic digestive enzymes and are of two types: Endopeptidases and Exopeptidases.

Digestion & Absorption of proteins  Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, e.g, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas.

Digestion & Absorption of proteins  Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, from the ends of polypeptides.  Exopeptidase is further classified as an  A. Carboxypeptidase or  B. Aminopeptidase or  C. Dipeptidases

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 Carboxypeptidases, secreted in the pancreatic juice, release amino acids from the free carboxyl terminal. Dr Samah Kotb17

Dr Samah Kotb18 Aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptidases, secreted by the brush border of intestinal mucosal cells, hydrolyze dipeptides into their two component amino acids.

ACTIVATION OF PROTEASES Dr Samah Kotb19

Digestion & Absorption of proteins The proteases are secreted as inactive zymogens, the active site of the enzyme is masked by a small region of its peptide chain, which is removed by hydrolysis of a specific peptide bond.

Digestion & Absorption of proteins For example:  Pepsinogen is activated to pepsin by gastric acid and by activated pepsin.  In the small intestine, trypsinogen, the precursor of trypsin, is activated by enteropeptidase, which is secreted by the duodenal epithelial cells.

Digestion & Absorption of proteins  Trypsin can then activate chymotrypsinogen to chymotrypsin, proelastase to elastase, procarboxypeptidase to carboxypeptidase, and proaminopeptidase to aminopeptidase.

Pepsinogen pepsin gastric acid activated pepsin Trypsinogen Trypsin Enteropeptidase Chymotrypsinogen Chymotrypsin Trypsin Elastase Proelastase ProcarboxypeptidaseCarboxypeptidase Trypsin Aminopeptidase Proaminopeptidase Trypsin Dr Samah Kotb23

Mechanisms of Amino Acids Absorption Dr Samah Kotb24

Free amino acids & small peptides are absorbed by different mechanisms  The end product of the action of endopeptidases and exopeptidases is a mixture of free amino acids, di- and tripeptides, and oligopeptides, all of which are absorbed.

Free amino acids & small peptides are absorbed by different mechanisms  Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport.  There are several different amino acid transporters, with specificity for the nature of the amino acid side chain (large or small; neutral, acidic, or basic).

Absorption of protein

 The various amino acids carried by any one transporter compete with each other for absorption and tissue uptake. Absorption of protein

 Dipeptides and tripeptides enter the brush border of the intestinal mucosal cells, where they are hydrolyzed to free amino acids, which are then transported into the hepatic portal vein. Absorption of protein

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