Protein Digestion

Protein Digestion

• Peptide bond hydrolysis is a key part of protein digestion

Protein digestion in the mouth

• There is no protein digestion in the mouth.

Protein digestion in the stomach

• Pepsin in the stomach breaks down protein into smaller polypeptides.

Activation of Pancreatic Zymogens

• Propeptide cleavage occurs in the duodenum.
• Enteropeptidase, also known as enterokinase, is located in the IEC membrane protein within the duodenal epithelium.
• Enteropeptidase converts trypsinogen to trypsin.
• Trypsin then converts chymotrypsinogen to chymotrypsin, proelastase to elastase, and procarboxypeptidase to carboxypeptidase.
• These enzymes originate from the pancreas and are free in solution.

Protein Digestion in the Small Intestine

• Trypsin, chymotrypsin, and elastase break down polypeptides into oligopeptides.
• Carboxypeptidase removes one amino acid at a time from the (-COOH) end of oligopeptides
• Brush border enzymes, which are transmembrane proteins, facilitate contact digestion.
• The final products of protein digestion are oligo-, tri-, and dipeptides, which are broken down into individual amino acids.

Amino Acid Absorption

• Several different Na+ / amino acid cotransporters are responsible for amino acid absorption
• Carboxypeptidase, aminopeptidase, and dipeptidase are key players
• This process occurs in the small intestine, specifically through the actions of brush border enzymes involving contact digestion

Lipid Digestion

• Begins with mixing in the stomach
• Lingual and gastric lipase start the process, breaking lipids into smaller droplets.
• In the duodenum, lipids combine with bile micelles.
• Micelles "collect" more lipids, allowing food lipids to diffuse into them.
• Lipase then breaks down Triglyceride into Monoglyceride + 2 FFAs
• Lumen of the small intestine is where pancreatic lipase inserts into micelle for this process
• Cholesterol needs no digestion

Fat Absorption

• Micelles contact the enterocyte membrane, allowing for simple and facilitated diffusion.
• Micelles then gather more fats.
• Carrier proteins facilitate the process for fatty acids and cholesterol.

Fat Absorption Inside Enterocytes

• Fatty acids and monoglycerides are reassembled into triglycerides.
• These are then repackaged into chylomicrons, coated with phospholipids and proteins, within the Golgi apparatus.
• Exocytosis of chylomicrons occurs
• They are then absorbed by the lacteal

General Absorption

• The stomach absorbs relatively little, mainly alcohol and some drugs.
• The small intestine is the primary site of absorption.
• Most absorption occurs before the ileum
• Ileum is important for bile salts and vitamin B12 absorption
• Re-absorption of water and electrolytes is crucial, with "water follows salt" being a key principle.
• The body ingests 2000 mL/day of water, while saliva contributes 1500 mL/day and gastric secretions add 2000 mL/day.
• Intestinal secretions account for 1500 mL/day, and pancreatic juices contribute another 1500 mL/day.
• Bile accounts for 500 mL/day
• The small intestine absorbs 8500 mL/day, while the colon absorbs 400 mL/day
• Results in 100 mL/day water excreted with feces

Large Intestine

• Contains resident micro-organisms known as intestinal flora.

Intestinal Flora

• Consists mostly of bacteria, including E. coli and many others.
• Around 100 trillion micro-organisms per person
• 10 times more than human cells
• Ferments some indigestible carbs, such as the soluble fiber pectin from fruit skins.
• Produces gases like CH4, CO2, and dimethyl sulfide, as well as short-chain fatty acids (SCFAs) like acetic, propionic, and butyric acids.
• Generates calories for colonocytes
• Exhibits anti-inflammatory and proliferative effects.

Intestinal Flora: Mutualism (Symbiosis)

• Produces SCFAs
• Synthesizes vitamins B & K
• Role in immunity through dialogue with immune cells, tolerance, and barrier function.

Digestion in the Large Intestine

• Breakdown occurs primarily by bacteria
• Absorption encompasses bacterial products like vitamins B and K and SCFAs, as well as water and electrolytes.
• The colon compacts waste, while the rectum is mainly for waste storage.

Large Intestine (L.I.) Features

• Teniae coli are longitudinal ribbons of muscle with muscularis externa, facilitating fecal movement.
• Haustra are pouches.

Large Intestine Mucosa

• Lacks villi but has crypts
• Contains lots of goblet cells.

Defecation

• Parasympathetic defecation reflex involves stretch receptors, relaxation of the internal sphincter
• Contraction of the rectum and sigmoid
• An intrinsic defecation reflex due to myenteric reflex.
• Process can be voluntary when controlling the external sphincter.