Protein Metabolism Overview

Protein Digestion Overview

• Dietary protein intake is 50-100 g/day
• Proteins are denatured during cooking which makes them easier to digest
• Protein absorption is highly efficient in healthy humans with minimal loss in feces (5-10 g/day)

Protein Roles in the Body

• Structural: Provides support and shape to tissues
• Catalytic: Acts as enzymes to catalyze biochemical reactions
• Transport: Carries molecules throughout the body
• Regulatory: Controls various cellular activities
• Protective: Protects against damage
• Homeostatic: Maintains internal balance
• Accumulating: Involved in storage

Protein Metabolism Dynamics

• Daily protein hydrolysis is ~400 g
• Daily protein synthesis is ~400 g
• Daily protein requirements are ~100 g
• Protein half-renewal time is 80 days

Protein Metabolism Index: Nitrogen Balance

• Positive nitrogen balance: Nitrogen gained exceeds lost (children)
• Negative nitrogen balance: Nitrogen lost exceeds gained (elderly, chronic diseases)
• Nitrogen balance/equilibrium: Nitrogen intake equals nitrogen output (healthy adults)

Protein Content in Food

• Various food items provide different amounts of protein, both plant-based and animal-based
• Tables detailing this are presented

Essential & Non-Essential Amino Acids

• Essential amino acids cannot be synthesized in the body and must be obtained from food
• Non-essential amino acids can be synthesized in the body
• A list of essential and non-essential amino acids is provided
• Important amino acids for muscle formation fall between 30-40% of the essential amino acids.

Food Products Lacking Essential Amino Acids

• Some plant proteins are deficient in certain essential amino acids
• Proteins of maize lack lysine
• Proteins of soybean lack tryptophan

Protein Digestion Processes

• Stomach:
  • Gastric juice contains hydrochloric acid (HCl) and pepsinogen
  • Parietal cells secrete HCl, lowering stomach pH (<2)
  • HCl denatures proteins and kills microorganisms
  • Optimum pH for pepsin activity is around 2
  • Pepsinogen is converted into pepsin by cleaving of 44 amino acids from the N-terminal end by HCl
  • Pepsin is an endopeptidase that catalyzes hydrolysis of peptide bonds formed by carboxyl groups of phenylalanine (Phe), tyrosine (Tyr), tryptophan (Trp), and methionine (Met).
  • Rennin (chymosin) is present in infants and children to convert milk protein casein into solid calcium paracaseinate.
• Small Intestine:
  • Denatured & partially hydrolyzed proteins
  • Protease activity
  • Various proteolytic enzymes (e.g. trypsin, chymotrypsin, elastase) for continued protein breakdown
  • Endopeptidases Act on peptide bonds inside the protein molecule; smaller & smaller units
  • Exopeptidases Act on the peptide bonds at the ends of the chain; carboxypeptidase and aminopeptidase
    • Carboxypeptidase acts on the carboxyl terminal end
    • Aminopeptidase acts on the amino terminal end
• Intestinal Lumen:
  • Aminopeptidases and dipeptidases within intestinal epithelial cells
  • Aminopeptidase is a non-specific exopeptidase that cleaves N-terminal amino acids one by one
• Digested Protein Absorption:
  • Amino acids enter the capillary blood in the villi and the hepatic portal vein to the liver

Pancreatic Digestion

• Optimal pH for pancreatic enzymes is around 8 provided by alkaline pancreatic juice
• Cholecystokinin and secretin stimulate pancreatic juice secretion & enzyme release
• Secretion in the stomach and pancreas are regulated by hormones (secretin)
• Pancreatic juice contains endopeptidases (trypsin, chymotrypsin, elastase) and exopeptidases (carboxypeptidases)
• Enzymes are secreted as inactive zymogens & stimulated to active form

Intestinal Digestion of Proteins

• Aminopeptidases and dipeptidases are on the luminal surface of intestinal epithelial cells
• Dipeptidases act on peptides
• Aminopeptidase is a non-specific exopeptidase (cleaves N-terminal amino acids) to form free-amino-acids

Amino Acid Absorption

• Absorption mainly in the small intestine which requires energy
• 5 different carrier systems for amino acids: Neutral, basic, immino, acidic, and beta-amino
• Na+ dependent active process; Na+ diffuses maintaining a gradient & driving active amino acid transport

Toxic Substance Formation

• Formation of toxic substances from amino acids in the colon from bacterial enzymes
• Hydroxylation of toxic substance in the liver, from indoxyl, skatole, & indoxyl hydroxysulfate

Diagnostic Value of Indican in Urine

• Serum and urine indican levels can indicate various conditions (e.g., intestinal obstruction, renal failure)

Overview of specific enzymes for protein digestion

• Pepsin
• Trypsin
• Chymotrypsin
• Elastase
• Carboxypeptidase A
• Carboxypeptidase B
• aminopeptidase
• Dipeptidases and tripeptidases