Video Transcript
What is the best description of an
antibody? (A) The antigen-binding site is in
the light chain and the constant region is in the heavy chain. (B) The amino acid sequence and 3D
shape of all antibodies are the same. (C) It is composed of two subunits,
one heavy chain and one light chain. (D) The molecule is a globular
protein composed of four different polypeptide chains. (E) It is a globular protein with a
quaternary structure that includes two types of polypeptides.
This question asks us about the
structure of antibodies, which play a role in the specific immune response. Let’s review what an antibody does
and how its structure contributes to its function to answer this question.
Antibodies are part of the humoral
immune response, and they are produced by specific white blood cells called B
lymphocytes. In order to do their job,
antibodies will travel throughout the body and survey tissues for foreign substances
like pathogens. Antibodies can block pathogens from
infecting cells by binding them and marking them for their destruction. This tags the pathogen so other
cells can recognize them and eliminate them by phagocytosis. So how do antibodies recognize and
bind to pathogens? Let’s look at this interaction more
closely.
Here we can see the antibody on the
right bound to structures on the pathogen on the left. The structures on the pathogen that
the antibody is binding to are called antigens. An antigen is usually something
foreign that can trigger an immune response. Here, the antigen is highlighted in
black. The antibody has a complementary
antigen-binding site that is specific for the structure of the antigen. We can see that this particular
antibody has two antigen-binding sites.
Each antibody is made up of four
polypeptide chains. The two in blue are the same and
called the light chains, and the two in orange are the same and called the heavy
chain. The combination of the heavy and
light chains is what makes the antigen-binding site that has a specific shape that’s
complementary to the antigen.
The part of the heavy and light
chain that contributes to the antigen-binding site are called the variable region
because they can differ between antibodies. The rest of the heavy and light
chains are called the constant region because they’re the same between
antibodies.
These chains are held together by
disulfide bridges. These disulfide bridges hold the
four polypeptides in place to form this quaternary protein structure. We often draw an antibody as a
Y-shaped structure. But in reality, it’s more rounded
and is sometimes called globular in its structure.
Now that we’ve learned more about
antibodies and their structure, let’s return to our question and choose the best
answer.
Answer choice (A) is incorrect
because the antigen-binding site is a combination of the heavy and light chains in a
region called the variable region. The rest of the heavy and light
chains are the constant region.
Answer choice (B) is incorrect
because not all antibodies have the same amino acid sequence. The variable region of the heavy
and light chains will differ to allow for the development of different antibodies
that can bind to different antigens.
Answer choice (C) is incorrect
because antibodies have two heavy chains and two light chains for a total of four
subunits, not two.
Answer choice (D) is incorrect
because although an antibody does have four polypeptide chains in total, only two
are different, because the heavy and light chains are the same.
The option that best describes an
antibody is given by answer choice (E). It is a globular protein with a
quaternary structure that includes two types of polypeptides.
