Non-enzymatic protein function | Chemical processes | MCAT | Khan Academy


So we’re going to be talking
about non-enzymatic protein function. And before we get into
what exactly that means, let’s just say we are playing
a game of Scattergories. And the next category
that comes up is protein, and you have to list all the
different types of protein that you know. And so you’re like,
oh, I totally got this. We’ve got beef and
pork and chicken. Right? Those are different types
of protein, and that’s true. But what about the
biochemistry type of protein, as in the large
biomolecules that are made up of amino acids? What are the different
types of proteins then? And so that’s a
little bit tougher. And so that’s the
point of what we’re going to be talking
about today is to talk about all the
different types of proteins and the different functions
that they perform. And so the key to understanding
proteins as we go forward is understanding one unique
characteristic about proteins. And that unique
characteristic is that they can bind
various biomolecules, and they bind
specifically and tightly. And so just keep
that concept in mind as we’re talking about all
the different functions of proteins, and that
will help you understand how they are able to perform
the vast array of functions that they do. So I like to think
of proteins as being in one of two main classes. There are the enzymatic
proteins, so enzymes, and then the
non-enzymatic proteins, or we’ll just call
them the non-enzymes. And so let’s back
up just a minute. What exactly are enzymes? What do they do? Enzymes are, in a
nutshell, little chemical reaction machines. They can catalyze all sorts
of chemical reactions, so they catalyze reactions
that help to sustain life. And so they are really the
workhorses of the cell, helping to build up and
break down things as needed. And by acting as
catalyst, enzymes can help to accelerate
the rate and specificity of these chemical reactions. And one good example of
this that you’re probably familiar with already
is DNA polymerase, which catalyzes the synthesis
of new strands of DNA. And probably even
more familiar to you are the enzymes in your saliva. So every time you eat,
there is one enzyme in particular called
amylase, which is responsible for breaking
down starch into sugars. So amylase is another
example of a protein that has enzymatic function. And just notice for a second
that both of these enzymes end in A-S-E, ase. And so in general,
if you see a protein and it has this
sort of ending, you should be thinking
to yourself, oh, I bet this is an enzyme,
an enzymatic protein. So that’s a good rule of thumb. Now, non-enzymatic
proteins, or non-enzymes, are all of those
proteins that carry out functions that require the
capacity to bind, but not necessarily to
catalyze a reaction. And so what are some
examples that we’ll be talking about of
non-enzymatic protein function? Well, there are proteins that
function as receptors or ion channels in a cell membrane,
and we’ll talk more about that. And then there are proteins
that are transport proteins. There also motor proteins. And then a special
class of proteins that function as an integral
part of the immune system, and those are called antibodies. And so we’ll go through
each of these examples of non-enzymatic protein
function one by one. So let’s give ourselves
a little bit more room to talk about those. Now, let me quickly
interject that it’s important to realize that not
all proteins are always either an enzyme or a non-enzyme. Oftentimes, they have
characteristics of both, and so just keep
that in mind as I’m going through and highlighting
the non-enzymatic properties in this video. OK, so starting with
receptors and ion channels. There are certain proteins that
exist in the membrane of a cell and function as either
receptors or ion channels. Now, receptors are
proteins that receive, or bind, a signaling molecule. So let’s draw a cell here. We’re going to draw the
membrane bilayer here. And so here would be my
exterior of the cell, and here’s the
interior of the cell. And within this
membrane bilayer, you can find a receptor protein. So we’ll draw that here. And this receptor protein will
bind a signaling molecule, also known as a ligand,
which then induces some sort of chemical
response in the cell. So one example of a receptor
protein and ligand pair is an insulin
receptor and insulin. So let’s say this
ligand is insulin, and this is the
insulin receptor. Now, insulin is a
hormone that’s released by the pancreas in response to
an increase in blood glucose levels. So let’s say there’s extra
glucose, because you just ate a piece of
pizza or something. So let’s say there’s an
increase in glucose around, and then insulin is
going to be released by the pancreas in response to
this increase in blood glucose. And then once it’s
released, it binds to its corresponding receptor
on certain cells, which leads to a cascade of
signals within the cell. And this allows it to then
absorb this excess glucose into the cell. And then likewise, you can
also have an ion channel that also spans the membrane bilayer. So let’s extend this
lipid membrane bilayer, and then we’ll draw an
ion channel protein here. And so likewise, this protein
spans the entire bilayer of the cell, and it acts as a
pore or a channel through which certain ions, say calcium,
can enter or exit the cell. So it can come in and come
out through the same channel. So next up are the
transport proteins, and now these proteins
are responsible for binding small molecules
and transporting them to other locations
in a multicellular organism like humans. And the trick with
these proteins is that they have to have a
high affinity for their ligand when the ligand is present
in high concentration. So at high concentration
of a ligand, you have high affinity of
the protein for that ligand, and at low concentration,
you have low affinity. And a great example
of this is hemoglobin. So hemoglobin is present
in red blood cells, and it picks up oxygen in the
lungs– so here are my lungs, it’s high in oxygen
in your lungs– and then delivers this
oxygen to tissues. And we’ll draw a tissue
here, say it’s muscle tissue, where its present in
low concentrations. And so this is a great example
of a transport protein at work. So next up are the
motor proteins, which include myosin, kinesin,
and dynein, all of which are capable of
generating great forces. And these proteins are really
crucial for cellular motility. And myosin specifically
is a protein responsible for generating
the forces exerted by contracting muscles. So every time you flex your
bicep like so, your myosin protein in your muscles are
contracting and generating that force. Now, kinesin and dynein
are motor proteins that are responsible for
intracellular transport. And then dynein
in particular also plays a role in the
motility of cilia, which are these little
extensions of a cell that project out. And mutations in a
particular dynein protein can lead to a rare disease
called primary ciliary dyskinesia. So in primary
ciliary dyskinesia, you can see there’s some
sort of dyskinesia or problem in movement for the cilia. And mutations in a
particular dynein protein lead to this rare
disease in which the action of the cilia
of the cells lining the respiratory tract
fail to function. And this leads to a decrease in
mucus clearance from the lungs, and therefore an
increase susceptibility to chronic infections like
pneumonia and bronchitis. So as you can see, these motor
proteins are really important. And then finally, our last
class of non-enzymatic proteins that we’ll be talking
about are the antibodies of the immune system. Now, antibodies are
protein components of the adaptive immune
system whose main function is to find foreign antigens and
target them for destruction. So in this case,
the antigen, which comes from any
foreign substance, say a virus or something like
that, is the antibodies ligand. So here’s an example
of an antibody, and then here is the antigen. And the antigen is really
just the antibodies particular ligand. So you can think of
antibodies as being like little red flags for
the body’s immune system letting us know that
hey, this thing is not supposed to be here. We need to get
rid of it somehow. And it’s important to know
that an antibody’s affinity for its target antigen
is extraordinarily high. So the affinity is
strong, really high. And so there you have it. You can see all the different
types of non-enzymatic roles or functions that
proteins can play, either as receptors or ion channels,
as transport proteins, motor proteins, and then highly
specific antibodies in our immune system.

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