The classic example for
any starch such as this
is using an enzyme such as amylase to
start breaking down the bonds of starch.
So let’s talk about
starch in more detail.
Starch has two
different bonds to it.
One is 1,4 bonds.
And the other is 1,6 bonds.
Amylase, which you can see here as
the protein with the meat cleavers,
is only able to break
down 1,4 bonds.
And where does
amylase come from?
You produce a little bit of
it in the salivary glands,
but the majority of it and the necessary
components of amylase are
produced in the pancreas.
However, even with all the different
enzymes that are involved
with digestion of carbohydrates,
some carbohydrates cannot
be digested in humans.
Thos involve cellulose, hemicellulose,
lignin, gums, and pectin.
These are dietary fibers.
Some of them are soluble and
some of them are insoluble,
but you can’t break them down into
smaller substrates to absorb.
So then, they would just be
passed through the GI system.
Now, there are some animals that undergo
different types of fermentation in the gut
that are able to utilize dietary
fibers, but we as humans are not.
Carbohydrates need to be broken down though
even if they are taken in as small sugars
such as lactose, sucrose,
isomaltose, so forth,
need to be broken down into
monosaccharides for absorption.
Lactose is a great example
for this, that’s milk sugar,
broken down by a lactase into
a glucose and a galactose.
And it’s only that glucose or
galactose that will be absorbed.
However, to undergo that absorption,
we also need to have
sodium gradient present
and that allows us to move
through the glucose molecule
into the enterocyte from
the intestinal lumen
There are other brush border enzymes
that a person can talk through and
we’ll get to those in the next slide.
Those are glucoamylase,
Protein digestion is a bit more complex
because you utilize more than just
amylase and the disaccharidases.
You have a cohort
of protein enzymes.
Those protein enzymes will be
broken down depending upon
if that protein is a
neutral or basic protein.
Chymotrypsin, elastase, trypsin,
help to form both neutral oligopeptides
and basic oligopeptides.
The chymotrypsis and elastase allow
for the neutral peptide breakdown
from proteins and trypsin from
proteins to basic oligopeptides.
Now, we have two forms of
carboxypeptidase that help us then
further break down
And that is carboxypeptidase
A and carboxypeptidase B.
A allows us to break down the neutral
oligopeptides into a neutral amino acids
and very short
Basic oligopeptides are broken
down by carboxypeptidase B
into both basic amino acids
as well as short peptides.
You need to be either an
amino acid or short peptide
for you to be able to
reuptake into the enterocyte
and then eventually
into the blood.
If we talk through these various peptides,
we can classify them in a
little bit better way.
And that is chymotrypsin is initially
secreted as chymotrypsinogen.
And that is from the pancreas.
However, to activate it, it
needs to have an endopeptidase
to break down a component of it
to make it into chymotrypsin.
Pepsinogen is broken down and activated
into pepsin, also by endopeptidases.
Carboxypeptidases A and
B are both broken down
into their active forms
Proelastase and trypsinogen are
also broken down by endopeptidases
and those are then in terms of their active
form, elastase and finally, trypsin.
These are the series of enzymes, which
are necessary in protein digestion.
Now, once you have a substance
such as pepsinogen,
how in the world do you activate it?
Well, you can activate it in
a number of different ways.
Pepsinogen is activated by a low pH.
And pepsinogen again is secreted
by chief cells in the stomach,
And then luckily, that’s
also where acid is produced
and therefore, they are coupled
together to be activated.
Trypsinogen is activated
by these endopepsidases
as well as other
All the other brush border
pepsidases are activated by trypsin.
So, now we can move
on to fat digestion.
Fat digestion involves
some new enzymes.
One is lipase.
Lipase can be produced either lingually in
the salivary glands or from the pancreas.
Lipases are necessary to break
down triglycerides into
both the glycerol backbone
as well as fatty acids.
Cholesterol requires a
And what this does is break down cholesterol
esters into cholesterol and fatty acids.
And finally phospholipase A2 will
allow us to break down phospholipids
into things like fatty acids.
So here, we have the
primary enzymes of lipase,
and phospholipase A2.
But there are a couple of other
items that are necessary
to help out this lipid digestion.
Those are bile salts, because
bile salts help emulsify the fat.
Colipase is necessary to bind with lipase
to undergo triglyceride digestion.
Cholesterol esterases and phospholipase
A2 round out fat digestion
for cholesterol and