NUTR 470 1st Edition Lecture 20-Overview of Protein Digestion• Dietary proteins are, with very few exceptions, not absorbed. Rather, they must be digested into amino acids or di and tripeptides first.• Proteins are broken down to peptide fragments by pepsin in the stomach, and by pancreatic trypsin and chemotrypsin in the small intestine.-There are two main classes of proteolytic digestive enzymes (proteases). • Endopeptidases-hydrolyze peptide bonds between specific amino acids throughout the molecule.-Endopeptidases are the first enzymes to act,yielding a larger number of smaller fragments-Examples: • Pepsin in the gastric juice• Trypsin, chymotrypsin, and elastase secreted into small intestine by the pancreas.• Exopeptidases -catalyze the hydrolysis of peptide bonds, one at a time, from the ends of peptides.-Carboxypeptidases, secreted in the pancreatic juice, release amino acid from the free carboxyl terminal; -aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. -Dipeptidases in the brush border of intestinal mucosal cells catalyze the hydrolysis of dipeptides, which are not substrates for amino- and carboxypeptidases. -Protein Digestion• The proteases are secreted as inactive zymogens.-Inactive zymogens• The active site of the enzyme is masked by a small region of the peptide chain that is removed by hydrolysis of a specific peptide bond. -Activation of zymogensThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.• In the stomach, pepsinogen is activated to pepsin by gastric acid and by activated pepsin (autocatalysis).• In the small intestine, trypsinogen, is activated by enteropeptidase; trypsin can then activate chymotrypsinogen to chymotrypsin, proelastase to elastase, procarboxypeptidase to carboxypeptidase, and proaminopeptidase to aminopeptidase.• Place of protein digestions-Digestion typically begins in the stomach, and continues in the small intestine.-The proteases are secreted as inactive zymogens.-The proteolysis in the small intestine plays the major role, because the digestion and absorption of dietary protein is not impaired by total absence of pepsin. • The end product of digestion-The end product of the action of endopeptidases and exopeptidases is a mixture of free amino acids, dipeptides, tripeptides, and oligopeptdies.• Overview of Peptides and Amino Acids Absorption-Free amino acids enter the epithelium by active transport and leave it by facilitated diffusion.-Small amounts of intact proteins can enter interstitial fluid by endo- and exocytosis.-Peptides and Amino Acids Absorption-Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. • Absorption of free amino acids-Free amino acids and small peptides are absorbed by different mechanisms. • Free amino acids and small peptides are absorbed by different mechanisms. -Absorption of small peptides• A peptide transporter, with high affinity for di- and tri-peptides, is absorbing the small peptides.• Basic amino acids and phenylalanine are absorbed primarily through facilitated diffusion from the gut lumen to the blood• The basolateral membrane is more permeable to amino acids than is the brushborder membrane. • Diffusion is more important for the basolateral transport, especially for amino acids with hydrophobic side chains.-Relatively large peptides• Relatively large peptide may be absorbed intact, either by uptake into mucosal epithelial cells (transcellular) or by passing between epithelial cells (paracellular).• Many such peptides are large enough to stimulate antibody production – allergic reactions to foods. -Summary• Basic concepts of protein nutrition• DRIs for protein• Issues in protein nutrition• Absorption of peptides and amino acids• Protein digestion•