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Protein Digestion and Absorption

Protein Digestion and Absorption

Initial Orientation

  • Oral cavity: Not significant for protein digestion.

  • Esophagus: A conduit.

  • Stomach: Important for protein digestion.

  • Duodenum: First part of the small intestine; receives secretions from the pancreas, liver, and gallbladder.

  • Jejunum: Part of the small intestine with brush border cells that release enzymes.

  • Enterocytes: Cells lining the small intestine that absorb the smallest protein components into the bloodstream via the portal vein to the liver.

Protein Breakdown

  • Proteins are large molecules that need to be broken down into smaller units.

  • Proteins are broken down into polypeptides, then oligopeptides, and finally into individual amino acids.

  • Amino acids are the monomers that the body can absorb.

Protein Structure

  • Proteins are strings of amino acids folded into a three-dimensional structure.

  • One end has a carboxyl end, and the other has an amine end.

  • Amino acids have different characteristics (acidic, basic, positive, negative, hydrophilic, hydrophobic) that cause them to fold in specific ways.

Digestion in the Stomach

  • Proteins enter the stomach through the esophagus.

  • Gastric pits in the stomach contain chief cells and parietal cells.

Chief Cells

  • Release pepsinogen (inactive protease enzyme).

  • Pepsinogen is inactive to prevent it from digesting the stomach itself.

Parietal Cells

  • Release hydrochloric acid ((HCl)).

  • (HCl) creates an acidic environment (pH 1-3) in the stomach.

Role of (HCl)

  • Denatures proteins, unraveling their three-dimensional structure into a more linear form.

  • Activates pepsinogen by cleaving off its "ogen" to form pepsin (active protease).

Pepsin

  • Pepsin acts as molecular scissors, chopping the denatured protein into smaller polypeptides.

Duodenum and Pancreatic Juices

  • The acidic contents of the stomach (peptides, fats, acid) enter the duodenum.

  • Enteroendocrine cells in the duodenum release cholecystokinin (CCK).

Cholecystokinin (CCK)

  • Stimulates the gallbladder to contract (important for fat digestion).

  • Stimulates the pancreas to release pancreatic juices.

Pancreatic Juices

  • Contain inactive proteases:

    • Trypsinogen

    • Chymotrypsinogen

    • Proelastase

    • Procarboxypeptidase

Activation of Pancreatic Proteases

  • Enterokinase, produced by cells in the duodenum, activates trypsinogen into trypsin.

  • Trypsin then activates chymotrypsinogen, proelastase, and procarboxypeptidase into their active forms (chymotrypsin, elastase, and carboxypeptidase).

Function of Activated Proteases

  • Trypsin, chymotrypsin, and elastase cut amino acid bonds inside the protein, producing dipeptides and tripeptides.

  • Carboxypeptidase chops off the amino acids at the ends of the protein, producing individual amino acids.

Jejunum and Brush Border Cells

  • Brush border cells in the jejunum produce dipeptidase and aminopeptidase.

  • Dipeptidase chops dipeptides into individual amino acids.

  • Aminopeptidase chops off amino acids from the ends of tripeptides, allowing dipeptidase to complete the digestion.

Absorption into Enterocytes

Sodium-Potassium Pump

  • Maintains a sodium gradient by pumping sodium out of the cell.

  • Uses ATP to pump 3 sodium ions out and 2 potassium ions in.

  • Creates a low sodium concentration inside the cell, encouraging sodium to enter.

Sodium Glucose Transporter (SGLT)

  • Transports sodium along with glucose, galactose, and amino acids into the cell.

  • Harnesses the sodium gradient to bring in amino acids.

Other Transport Mechanisms

  • Dipeptides and tripeptides can be transported into the cell along with hydrogen ions.

  • Individual amino acids can piggyback on this hydrogen ion transport.

  • Peptidases inside the enterocyte break down these dipeptides and tripeptides into individual amino acids.

Endocytosis/Phagocytosis

  • Larger peptides can be engulfed by the cell membrane through endocytosis.

  • The engulfed peptides are then broken down by peptidases inside the cell.

Transport into the Bloodstream

  • Individual amino acids are transported from the enterocyte into the bloodstream via facilitated diffusion.

  • Specific channels facilitate the movement of amino acids down their concentration gradient.

Summary of Protein Digestion and Absorption

  • Oral Cavity: Negligible protein digestion.

  • Esophagus: Conduit.

  • Stomach:

    • Chief cells release pepsinogen.

    • Parietal cells release hydrochloric acid.

    • Hydrochloric acid denatures proteins and activates pepsinogen to pepsin.

    • Pepsin breaks down proteins into smaller polypeptides.

  • Duodenum:

    • Enteroendocrine cells release CCK, stimulating the pancreas.

    • Pancreas releases trypsinogen, chymotrypsinogen, proelastase, and procarboxypeptidase.

    • Enterokinase activates trypsinogen to trypsin, which activates the other proteases.

    • Trypsin, chymotrypsin, and elastase break internal amino acid bonds.

    • Carboxypeptidase chops off terminal amino acids.

  • Jejunum:

    • Brush border cells release dipeptidase and aminopeptidase.

    • Dipeptidase and aminopeptidase break down dipeptides and tripeptides into individual amino acids.

  • Absorption:

    • Amino acids are absorbed into enterocytes via SGLT (with sodium), hydrogen ion transport, and endocytosis.

    • Peptidases inside enterocytes break down any remaining peptides.

    • Amino acids move into the bloodstream via facilitated diffusion and are transported to the liver.