Protein Metabolism Overview

Study Notes

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)

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)

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

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.

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

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

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

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

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