Dietary Proteins and Digestion

Dietary Proteins

• Dietary proteins come from animal or vegetable sources
• Principal animal sources include milk, dairy products, meat, fish, liver, and eggs.
• Principal vegetable sources include cereals, pulses, peas, beans, and nuts.
• Dietary protein intake is roughly 50-100 g/day.
• Endogenous protein (digestive enzymes and worn-out cells) is about 28 g daily.
• Protein loss through feces is minimal, approximately 5-10 g/day.

Digestion in the Mouth

• No proteolytic enzymes are present in saliva.
• Chewing breaks down food into smaller pieces ("bolus").
• The bolus moves to the stomach to interact with gastric juice.

Protein Degradation

• Proteins are broken down by hydrolases.
• Hydrolases are enzymes classified as peptidases, cleaving peptide bonds.
• Endopeptidases (proteases) attack internal peptide bonds, and produce fragments
• Exopeptidases act on terminal amino acids

Digestion in the Stomach

• Gastric juice contains proteolytic enzymes (e.g., pepsin, renin, gastricin).
• Hydrochloric acid (HCl) is also present in gastric juice.
• HCl lowers stomach pH to less than 2
• HCl denatures proteins, making them more susceptible to proteases
• Pepsin works optimally in highly acidic conditions (pH 1.6-2.5)
• Pepsin is a proteinase that hydrolyses peptide bonds, forming smaller fragments
• Rennin is present in infants and children; converting milk protein casein to paracaseinate, which pepsin digests more easily.
• Gastricin is a protease active at a pH of 3 to 4.
• Gelatinase hydrolyzes gelatin in an acidic medium

Digestion in the Duodenum

• Food from the stomach enters the duodenum, where it mixes with pancreatic juice.
• Pancreatic juice contains proteases that are secreted as proenzymes (zymogens).
• Cholecystokinin and secretin stimulate release of pancreatic juice.
• Pancreatic proteases include trypsin, chymotrypsin, carboxypeptidases, and elastases.
• Trypsin, a proteinase, is secreted as trypsinogen (inactive form).
• Enterokinase (enteropeptidase) activate trypsinogen to trypsin.
• Trypsin is essential for activating other pancreatic proenzymes.

Digestion in the Small Intestine

• Intestinal juice contains proteolytic enzymes (enterokinase, prolidase, aminopeptidases, dipeptidases, tripeptidases).
• Enterokinase converts trypsinogen to trypsin.
• Prolidase acts on proline containing peptides.
• Aminopeptidases, dipeptidases, and tripeptidases help break down peptides into smaller fragments.

Absorption of Amino Acids

• Amino acids are absorbed into the bloodstream from the small intestine's ileum and distal jejunum.
• Some oligopeptides (dipeptides and tripeptides) are also absorbed, but then further broken down.
• L-amino acids and L-peptides are absorbed faster than their D-isomers.
• They typically enter the bloodstream passively via special transporters.

How amino acids reach the liver

• Amino acids and digested products are absorbed from the intestines.
• They are carried by the portal vein to the liver for further use in the body.

Overview of Protein Digestion

• Food proteins enter the digestive tract.
• Pepsin in the stomach breaks down proteins into smaller polypeptides.
• In the small intestine, proteases and other enzymes continue breaking them down into amino acids.
• Small fragments of peptides are also broken down into amino acids.
• Amino acids are absorbed into the bloodstream and used by the body.

Enzyme Specificity

• Enzymes have specificity, cleaving only certain peptide bonds.
• Pepsin is active at low pHs, preferring peptide bonds with aromatic amino acids.
• Trypsin prefers peptide bonds with Lys or Arg at the carboxyl terminal part of peptide bond.
• Chymotrypsin works well on peptide bonds with aromatic amino acids on the carboxyl terminus.

This quiz covers the sources of dietary proteins, including both animal and vegetable origins, and discusses their daily intake. It also examines the process of digestion in the mouth and stomach, including the role of different enzymes in protein degradation. Test your knowledge on these essential topics!