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Protein digestion
when we eat food body's digestive system break down protein into
individual amino acid , absorbed and used by cells to build other protein
and a few macromolecule such as DNA
Example of good dietary protein source is egg

Digestion from mouth to the stomach
Mechanical digestion teeth breaks large pieces of protein into smaller piece
and swallowed

Protein digestion

1

Salivary gland provide saliva to aid in swallowing and passage of partial
mashed food through the esophagus
Food enters stomach through esophageal sphincter
Stomach releases gastric juice containing HCL and enzyme pepsin
Enzyme initiate breakdown of protein
Acidity stomach facilitate the unfolding of proteins that retain 3
Dimensional structure after cooking and help break down protein aggregate
formed during cooking
Pepsin secreted by cell dismantles protein chain into smaller and smaller
fragment
Egg protein has a large globular molecule and chemical breaking require
time
Powerful mechanical stomach contraction churn partially digested protein
into a uniform mixture called chyme
Protein digestion is longer than carbs digestion
Eating high protein meal increases amount of digestion and food remains
longer make you feel full

Digestion from stomach to small intestine
Stomach empties chyme containing broken piece into small intestine
Majority of protein digestion occur in small intestine
Pancreatic cells secrete digestive juice contain enzyme that digest protein
are trypsin and chymotrypsin
Cell line small intestine release additional enzyme and finally break apart
small protein fragment into individual amino acids
Muscle contraction of small intestine mix and propel digested protein to
absorption site known as villi

Protein digestion

2

In the lower part of small intestine, amino acid are transported from
intestinal lumen through the intestinal cells to the cells
The movement of individual amino acid require special transport protein
and cellular energy molecule, ATP adenosine triphosphate
Amino acid in the blood, transported to the liver
Amino with other macronutrient liver is checkpoint for amino acid
distribution and further breakdown of amino acid
Amino acid contain Nitrogen , further catabolism of amino release nitrogen
contain ammonia
Ammonia is toxic to the body , liver transforms it into urea
urea molecule contain 2 nitrogen and is highly soluble

urea is good choice for transporting nitrogen out of the body

Protein digestion

3

Amino acid is building block that the body reserves , to synthesize other
protein
More than 90% protein ingested does not get broken down further than
amino acid monomer

Protein digestion

4



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