Protein Digestion, and Opium from Cows


The goal of protein digestion is breaking
proteins all the way down to the individual amino acids that they are made of. By doing this, as you recall, their original
function will be lost, which is not a problem because we eat protein only to get the pieces,
the amino acids that we will then use to build our own proteins. To break down proteins we do two things: denaturation
and enzymatic hydrolysis. Denaturation allows us to unfold proteins
so that they can then be more easily accessed by our digestive hydrolytic enzymes, which
will start breaking the peptide bonds to create smaller and smaller protein fragments, until
all the individual amino acids have been separated. Those amino acids will then be absorbed and
used by the cells in our body to build the proteins that we need. We could consider cooking to be the first
step of protein digestion. As you recall, exposure to heat causes protein
denaturation, thus making it easier to then complete protein digestion. By softening tissues, cooking also increases
the bioavailability of proteins from many foods. Not much happens in our mouth, and the first
real digestive step is in the stomach. Here, the strongly acidic environment completes
protein denaturation and unfolding. The proteolytic enzyme pepsin starts hydrolyzing
proteins. It is not a thorough job, it just starts cutting
protein chains here and there and so smaller protein fragments will be created from longer
chains. The real protein breakdown takes place in
the small intestine. Here, we have a family of proteolytic enzymes
from the pancreas, led by trypsin, which make these chains smaller and smaller until they
are small peptides of just a few amino acids. These small peptides will then reach the brush
border, the walls of our intestine, where we have the intestinal absorptive cells, the
enterocytes. Here, another family of proteolytic enzymes
will complete protein hydrolysis to the individual amino acids, and this can happen on the brush
border right before absorption, but also inside our intestinal absorptive cells. Indeed, we do have carriers to absorb small
peptides, two or three amino acids long, into our enterocytes, and once they are inside,
the enzymes will complete their breakdown to the individual amino acids before they
are released in the bloodstream. Notice that if we were to eat single hydrolyzed
amino acids, our digestion would be easier, but our absorption would be way less efficient
because we wouldn’t be able to use these carriers for di- and tripeptides on our brush
border. For this reason, supplements of individual
amino acids are better absorbed if they are actually made of a mix of single amino acids
and small peptides. Either way, once they are inside the intestinal
absorptive cells, our proteins have been broken all the way down to single individual amino
acids. These amino acids can now through the intestinal
wall into the bloodstream. Amino acids are water soluble so they can
go directly in the bloodstream by themselves. From the small intestine they take the portal
vein and go to the liver. The liver will sort them out depending on
what the body needs to do with these amino acids that were just absorbed: they can be
used for protein synthesis, or for energy production, and if we don’t immediately
need them for protein synthesis or energy production then, since we cannot store them,
we will convert them to either fat or glucose that can be stored. We will see all these different uses of protein
later on, but for now let’s just remember that it’s the liver that does this sorting
activity and decides where the amino acids go and what they should be used for. There are actually three amino acids, the
branched chain amino acids leucine isoleucine and valine that skip this liver step and go
straight to the muscle which will use them for energy production. We will also return on this later. There are some exceptions to the general rule
that with digestion we completely break protein down, lose their function and only absorb
the individual amino acids. Sometimes very small proteins or protein fragments,
can leak through the intestine in different ways and this especially happens in infants
younger than 5 months, their intestine is a little bit more permeable, and so these
small peptides can go through without being completely broken down. This has two main consequences, the first
one is allergies predisposition because when we have a whole protein or a piece of protein
inside our body coming directly from outside, it may be recognized as something foreign
and activate some kind of immune response which is the basis for possibly developing
an allergy. For this reason it is recommended that infants
until 5-6 months do not get those foods that contain proteins frequently associated with
allergies, such as nuts, fish, or strawberries. The other consequence is that if this protein
or protein fragment has a biological function, this function will be preserved, and so now
we have inside our body a protein that has a function but that we didn’t build ourselves. For some small peptides these can happen in
adults as well. For example, there are some casomorphins in
milk that have opiate-like activity and that are sometimes absorbed intact. Some people claim they cannot sleep without
drinking milk, and some researchers hypothesize that this happens because there are these
little peptides that have this calming activity.

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