Let's talk about conformational stability
and how this relates to protein-folding
and denaturation.
So first, let's review a couple of terms
just to make sure we're all on the same page.
And first, let's start out with the term conformation.
And the term confirmation just refers
to a protein's folded, 3D structure,
or in other words, the active form of a protein.
And next we can review what the term denatured means
when you're talking about proteins.
And denatured proteins just refer
to proteins that have become unfolded, or inactive.
So all conformational stability is really talking about
are the various forces that help to keep
a protein folded in the right way.
And these various forces are the four different levels
of protein structure, and we can review those
briefly right here.
So recall that the primary structure of a protein
just refers to the actual sequence of amino acids
in that protein, and this is determined
by a protein's peptide bonds.
And then next, you have secondary structure,
which just refers to the local substructures in a protein.
And they are determined by backbone interactions
held together by hydrogen bonds.
Then you have tertiary structure,
which just talks about the overall 3D
structure of a single protein molecule.
And this is described by distant interactions between groups
within a single protein.
And these interactions are stabilized
by van der Waals interactions, hydrophobic packing,
and disulfide bonding, in addition to the same hydrogen
bonding that helps to determine secondary structure.
And then quaternary structure just
describes the different interactions
between individual protein subunits.
So you have the folded-up proteins
that then come together to assemble the completed,
overall protein.
And the interaction of these different protein subunits
are stabilized by the same kinds of bonds
that help to determine tertiary structure.
So all of these levels of protein structure
help to stabilize the folded-up, active confirmation
of a protein.
So why is it so important to know
about the different levels of protein structure
and how they contribute to conformational stability?
Well, like I said, a protein is only functional
when they are in their proper conformation,
in their proper 3D form.
And an improperly folded or degraded, denatured,
protein is inactive.
So in addition to the four levels of protein structure
that I just reviewed, there is also
another force that helps to stabilize
a protein's conformation.
And that force is called the solvation shell.
Now, the solvation shell is just a fancy way
of describing the layer of solvent that
is surrounding a protein.
So say I have a protein who has all these exterior
residues that are overall positively charged.
And picture this protein in the watery environment
of the interior of one of our cells,
then the solvation shell is going to be the layer of water
right next to this protein molecule.
And remember that water is a polar molecule,
so you have the electronegative oxygen atom
with a predominantly negative charge leaving
a positive charge over next to the hydrogen atoms.
The same is true for each of these water molecules.
So now, as you can see, the electronegative oxygen atoms
are stabilizing all the positively charged amino acid
residues on the exterior of this protein.
So as you can see, the conformational stability
of a protein depends not only on all
of these interactions that contribute
to primary, secondary, tertiary, and quaternary structure,
but also what sort of environment that protein is in.
And all of these interactions are
very crucial for keeping a protein folded properly,
so that it can do its job.
Now, what happens when things go wrong?
How does a protein become unfolded and thus inactive?
Well, remember that this is called denaturation.
And this can be done by changing a lot of different parameters
within a protein's environment, including
changing the temperature, the pH,
adding chemical denaturants, or even adding enzymes.
So let's start with what happens if you alter
the temperature around a protein.
And we can use the example of an egg
when we put it into a pot of boiling water,
because an egg, especially the white part, is full of protein,
and this pot of boiling water is representing heat.
And remember that heat is really just a form of energy.
So when you heat an egg, the proteins
gain energy and literally shake apart
the bonds between the parts of the amino acid chains.
And this causes the proteins to unfold.
So increased temperature destroys
the secondary, tertiary, and quaternary structure
of a protein, but the primary structure is still preserved.
So the take-away point is that when
you change the temperature of a protein by heating it up,
you destroy all of the different levels of protein structure
except for the primary structure.
So now, let's say you were to take an egg
and then add vinegar, which is really just an acid.
The acid in the vinegar will break all the ionic bonds
that contribute to tertiary and quaternary structure.
So the take-away point when you change the pH surrounding
a protein is that you have disruption of ionic bonds.
And if we think about this a little bit more deeply,
it kind of makes sense, because ionic bonds
are dependent upon the interaction
of positive and negative charges.
So when you add either an acid or a base, which
in the case of an acid is just like adding
a bunch of positive charges, you kind of
disrupt the balance between all these interactions
between the positive and negative charges
within the protein.
So now let's look at how chemicals denature proteins.
Chemical denaturants often disrupt the hydrogen bonding
within a protein.
And remember that hydrogen bonds contribute
to secondary, tertiary, all the way up to quaternary structure.
So all these levels of protein structure
will be disrupted if you add a chemical denaturant.
So let's take our same example of a protein with an egg,
and say, if you were 21 years or older,
you got your hands on some alcohol,
and you added this to the egg.
Then, all the hydrogen bonds would be broken up,
leaving you with just linear polypeptide chains.
And then finally, let's take our hard-boiled egg
from the temperature example, and let's eat it.
So here's my beautiful drawing of a person,
representing you, eating this hard boiled egg.
Once the egg enters our digestive tract
we have enzymes that break down the already denatured proteins
in the egg even further.
They take the linear polypeptide chain, whose primary structure
is still intact.
And they break the bonds between the individual amino acids,
the peptide bonds, so that we can absorb these amino acids
from our intestines into our bloodstream.
And then we can use them as building blocks
for our own protein synthesis.
And that's how enzymes can alter a protein's primary structure
and thus the protein's overall conformational stability.
So what did we learn?
Well, we learned that the conformational stability
refers to all the forces that keep a protein properly
folded in its active form.
And this includes all the different levels
of protein structure, as well as the salvation shell.
And we also learned that a protein
can be denatured into its inactive form
by changing a variety of factors in its environment,
including changing the temperature, the pH,
adding chemicals, or enzymes.