English written summary of all the lectures in the course HNH30706 - Food Digestion: Nutrient breakdown & absorption in Wageningen University. Including pictures of the lectures and detailed descriptions.
Food digestion: Nutrient Breakdown and absorption
Molecular aspects of: protein digestion and absorption
This lecture will be
mostly about absorption
and protein quality;
digestibility and
evaluation systems.
Dietary proteins are from different sources. Such as meat, dairy, legumes, nuts, etc.
Also, endogenous proteins occur in the body:
- Salivary and gastric secretions
- Pancreatic and bile secretions
- Small intestinal secretions
- Sloughed epithelial cells
- Mucus
- Microbial protein
At some state they will be digested and absorbed.
There are a whole bunch of enzymes present in the GI-tract
as listed in the table below. These enzymes are not secreted
in the active form but need to be activated. It’s important to
realize that these enzymes have specific points where they
will cleave; specific amino acids.
,But…
- Enzyme digestion is not exactly straight forward
o Specificity for a cleavage site
o But not all cleavage sites on a protein are always utilized by an enzyme
o Peptide composition from hydrolysis is difficult to predict. It depends on
selectivity of enzyme towards cleavage site
- Spontaneous non-enzymatic breakdown
The absorption is done by
enterocytes. There is
absorption of free amino
acids but also di- and
tripeptides. These are
absorbed by PEPT1, a
protein on the apical
membrane of the
enterocyte.
PEPT1 can transport di-
and tripeptides. It has a
very broad substrate
specifity; over 400
dipeptides and over 8000 tripeptides.
It’s a secondary active transport,
which means that is costs energy to
transport the di- and tripeptides but
it’s indirectly needed. This means the
PEPT1 doesn’t need the energy itself.
Transport is driven by protein uptake;
there needs to be a gradient and for
this the sodium/proton pump is used.
ATP is needed to export the sodium
out of the cell, enabling the other
forms of transport as can be seen in
the picture as well.
,Apart of this, there are amino acids transporters. There are many different transporters
and the transporters have:
- Stereospecificity à L-amino acids
- Substrate specificity à multiple amino acids
- Overlap in specificity à multiple transporters per amino acid
Mechanism:
- Facilitated diffusion
- Secondary active transport
In the intestine amino acid transport system can be present at the apical membrane but
also at the basolateral membrane. Remember that there are a lot of transporters.
Underneath, a picture is shown with all kinds of different transporters. Don’t learn them
by heart, just know there are many forms.
In some herniated diseases, transporters don’t function as they are supposed to do. 2
diseases will be discussed in this lecture: Hartnup disorder and Cystinuria.
The + signs show where the receptors are present in the body, but this don’t mean that
the symptoms of the disease are present at the specific locations mentioned there.
, The Hartnup disorder:
- Defective intestinal transport: B0AT1 (neutral AA)
- Effect: poor nutrition results I in pellagra-like skin changes, cerebellar ataxia and
psychiatric abnormalities
- Treatment: sometimes only a high protein diet
The essential amino acid is not absorbed, how come the people are still alive? Since the
body can’t make the essential amino acids itself. You need to retrieve them from the diet.
The PEPT1 transporter is not defect, so tryptophan can be absorbed via di- and tri-
peptides. Even though you don’t have the specific transporter for tryptophan.
Another inherited disease is Cystinuria:
- Defective intestinal transport rBAT/b0, +AT(cationic AA)
- Effect: kidney stones made of cysteine (transporter also present in kidneys for
reabsorption)
- Treatment: multiple options to prevent kidney stones
Again, these people can still absorb the essential amino acids. In this disorder the most
problems are present in the kidneys, causing kidney stones. Since the cysteine is not
reabsorbed in the kidneys, these will result in kidney stones.
After these AA, di- and tripeptides are
absorbed, the free AA can first pass
metabolism. This means that they won’t
reach the circulation and is used in the
gut itself. Glutamine, glutamate and
aspartate are oxidized. Also, transport
over basolateral membrane can take
place so the free amino acids are put into
the body circulation.
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