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.