What is Protein Metabolism?
Protein metabolism covers how the body breaks down dietary and cellular proteins into amino acids, extracts their nitrogen when they're used for energy, and safely disposes of the resulting toxic ammonia through the urea cycle.
Protein metabolism is the process of digesting proteins into amino acids, removing their amino groups by transamination and deamination, and converting the toxic ammonia produced into safe, excretable urea via the liver's urea cycle.
- 1.Transamination — An amino group is transferred from an amino acid to alpha-ketoglutarate, forming glutamate, via transaminase enzymes.
- 2.Oxidative Deamination — Glutamate releases its amino group as free ammonia (NH3) in the liver mitochondria.
- 3.Urea Cycle — Ammonia combines with CO2 and aspartate through five enzymatic steps in the liver, consuming ATP, to form urea.
- 4.Excretion — Urea travels through the blood to the kidneys and is excreted in urine.
Step-by-step worked examples
In transamination, alanine transfers its amino group to alpha-ketoglutarate. What two products form?
Alanine (amino donor) + alpha-ketoglutarate (amino acceptor) The amino group moves to alpha-ketoglutarate Products: pyruvate (from alanine) and glutamate (from alpha-ketoglutarate)
One turn of the urea cycle uses 2 nitrogen atoms (from NH4+ and aspartate), 1 CO2, and consumes 3 ATP (4 high-energy phosphate bonds) to make 1 urea. How many ATP and nitrogen atoms are needed to make 5 urea molecules?
ATP needed = 5 × 3 = 15 ATP Nitrogen atoms needed = 5 × 2 = 10 nitrogen atoms Result: 15 ATP and 10 nitrogen atoms are used to produce 5 urea molecules
A protein of 250 amino acid residues is fully hydrolyzed during digestion. How many peptide bonds must be broken to release all free amino acids?
A chain of n residues has (n − 1) peptide bonds Bonds = 250 − 1 = 249 Result: 249 peptide bonds must be hydrolyzed
Flashcards
Quick quiz
Q1.What toxic byproduct does amino acid breakdown produce?
Q2.Which organ converts ammonia into urea?
Q3.What is the main function of transamination?
Q4.How is urea eliminated from the body?
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Common mistakes
Ammonia and urea are the same molecule. — Correct: Ammonia (NH3) is toxic and is converted by the liver's urea cycle into urea, a much safer molecule for excretion.
All amino acids are stored in the body like fat or glycogen. — Correct: The body has no dedicated amino acid storage form; excess amino acids are deaminated and their carbon skeletons used for energy or converted to fat/glucose.
The urea cycle occurs entirely in one cell compartment. — Correct: It spans both the mitochondria (first two steps) and the cytoplasm (remaining steps) of liver cells.
Deamination and transamination are the same reaction. — Correct: Transamination moves an amino group between molecules without releasing free ammonia; deamination releases free ammonia (NH3) directly.
FAQ
What is protein metabolism?
It's the breakdown of proteins into amino acids, removal of their nitrogen via transamination/deamination, and safe disposal of ammonia as urea.
What is the formula for the urea cycle?
2 NH3 (as ammonium and aspartate nitrogen) + CO2 + 3 ATP → urea + 2 ADP + AMP + 4 Pi (simplified), consuming 4 high-energy phosphate bonds per urea.
What are examples of protein metabolism?
Digesting dietary meat into amino acids, using excess amino acids for energy during fasting, and converting the resulting ammonia into urea for excretion.
How do you calculate ATP cost for producing multiple urea molecules?
Multiply the number of urea molecules by 3 ATP (or 4 high-energy phosphate bonds), since that's the cost of one turn of the urea cycle.