4. Proteins Lecture 3

Digestion of Protein The goal of protein digestion is the hydrolysis of all peptide bonds to produce free amino acids. No chemical digestion of protein occurs in the mouth. (peptide bonds are amide bonds)

Digestion of Proteins (In Stomach) 1. HCl in the stomach (pH 1–2) functions to kill some bacteria and to denature dietary protein, thus making them more susceptible to hydrolysis by proteases. “Denaturation is a process in which proteins or nucleic acids lose their secondary or tertiary structure” Pepsin: is an acid stable enzymes that is secreted by serous cells of the stomach as an inactive enzyme (zymogen or pro enzyme) known as Pepsinogen. Pepsinogen contains extra amino acids in their sequences, which prevent them from being active. Pepsinogen is activated to Pepsin by HCl and also by their pepsin that have already been activated. Pepsin releases peptides and a few free amino acids from dietary protein.

Digestion of Proteins (In small Intestine) Next, the polypeptides enter the small intestine, where the pH is about 7–8. Most of the digestive enzymes in the small intestine are secreted by the pancreas in proenzyme (zymogen) forms. Peptides that reach the small intestine from the stomach will be further cleaved to oligopeptides and amino acids by the action of a group of pancreatic proteases including: Trypsin: cleave proteins into smaller peptides. chymotrypsin: cleave proteins into smaller peptides. Elastase: cleave proteins into smaller peptides. Carboxypeptidase: splits one amino acid at a time. Specificity: each of these enzymes has a different specificity for the amino acid R-group adjacent to the susceptible peptide bond. Release of zymogens: it is mediated by the secretion of cholecystokinin and secretin hormons of the digestive tract.

Pro-Carboxypeptidase Digestion of Proteins (In small Intestine) Activation of zymogens: Enterokinase (Enteropptidase) converts the pancreatic trypsinogen (inactive “zymogen” form of trypsin) to trypsin. Active trypsin subsequently converts other pancreatic zymogens into active proteases: Digestion of oligopeptides: occur at the brush border of enterocytes by different brush border enzymes. Aminopeptidase cleaves amino acids from the amino ends of oligopeptides (only free amino acids absorbed into blood). Dipeptidases and tripeptidases: that hydrolyze di- and tripeptides to amino acids. Trypsinogen Trypsin Enterokinase Chemotrypsinogen Chemorypsin Pro-Carboxypeptidase Carboxypeptidase Pro-elastase Elastase To be absorbed

Cleavage of dietary protein by proteases from the pancreas.

& Absorption of amino acids Further digestion & Absorption of amino acids Following digestion, amino acids and small peptides are taken into the enterocytes by Na+ -linked transport systems At least five brush border transport systems exist: 1. neutral amino acids (uncharged aliphatic and aromatic) 2. basic amino acids (Lys, Arg, Cys, Cys-Cys) 3. acidic amino acids (Asp, Glu) 4. imino acids (Pro), Hydroxyproline) 5. di- and tripeptides Inside enterocytes: di and tripeptides (from oligopeptides hydrolysis) are hydrolyzed by peptidases to amino acids (only free amino acids absorbed into blood). Free amino acids will then be diffused into the blood stream and carried into the liver via Na-

Summary Small peptides (Oligopptides) Dipeptides, tripeptides & free amino acids. Free amino acids. Aminopeptidase Dipeptidases Tripeptidase Trypsin, chymotrypsin, elastase, carboxypeptidases At the brush border of enterocytes In the enterocytes Peptidases Amino acids cross the basolateral membrane into blood stream .

Amino acid pool This pool is supplied by three sources: The amount of free amino acids distributed throughout the body is called AMINO ACID POOL. AMINO ACID POOL: This pool is supplied by three sources: 1- amino acids derived from the dietary protein 2- amino acids provided by the degradation of body proteins 3- synthesis of non-essential amino acids from simple intermediates of metabolism. In healthy, well fed individuals, the input to the amino acid pool is balanced by the output, that is, the amount of amino acids contained in the pool is constant. The amino acid pool is depleted by three routes: 1- synthesis of body protein 2-amino acids consumed as precursors of essential nitrogen-containing small molecules 3- conversion of amino acid to glucose, glycogen, fatty acids or CO2

Our bodies do not store nitrogen-containing compounds and ammonia because it is toxic to cells.

A Few Key Terms in Amino Acid Metabolism Transamination: This process is the first step of amino acid catabolism. The amino (-NH3+) group is removed. This reaction is reversible. Amino group from 1 is transferred to 2. An a-keto acid is a compound that has a C= O group on the a carbon. A class of enzymes known as “transaminase’s” work with several Different amino acids to do this task.

A Few Key Terms in Amino Acid Metabolism Oxidative De-amination: The amino group is removed as ammonia and is replaced by a Keto group. The ammonia later on enters the urea cycle. Amino group Keto group