In this lecture, I will discuss four more functions
proteins carry out for the organisms and the
cells within them. These functions include
proteins functioning as antibodies in the
immune system, proteins and their role in
regulating gene expression, proteins acting
as enzymes to catalyze reactions, and finally
proteins acting to transport materials across
biological membranes and within organisms.
Now multicellular organisms have an immune
system that protects them against outsiding
invaders. The functional parts of the immune
system are antibodies, as seen in this slide.
Antibodies are proteins that are produced
by the adaptive immune system of a multicellular
organism. Now the antibodies are the functional
part of that adaptive immune system.
They recognize and bind to specific molecules,
and those specific molecules are called antigens
as seen on the screen above. The remarkable
thing about the adaptive immune system is
that antibodies can be created to recognize
things that the body never before encountered.
Now antibodies are present in both soluble
forms, meaning released freely, and also in
membrane-bound forms. The structure of antibodies
is shown on the screen, and in this slide
you can see that the antibody consists of
four polypeptide chains. Two long chains that
are called heavy chains that are the central
part of the Y, and the two heavy chains are
joined to each other at the bottom of the
Y by disulfide bonds. The antibody also consists
of two light chains that are joined to the
heavy chains at the upper part of the Y.
The light chains are joined to the heavy chains
also by disulfide bonds. Now as we examine
the antibody overall, we see that there are
different regions of the antibody that have
functions that they perform in the recognition.
For example, the bottom part of the antibody
or the bottom part of the Y as you see it,
is a region that’s called the constant region,
and in this region the sequence of amino acids
is relatively constant from one antibody to
another within a given type of antibody. By
contrast, the region above the constant regions
are called the variable regions and it's in
the variable regions where the binding site
for the antigens occurs, and note that the
constant regions are part of both the light
chain and of the heavy chain. The amazing thing
about antibodies is the diversity of antibodies.
It is estimated that the human body can produce
up to 1 trillion antibodies and that’s given
the fact that it has a genome of only about
7 billion base pairs. This remarkable diversity
of antibodies that the immune system can produce
is a result of shuffling, of both DNA sequences
and RNA sequences, to make this multitude
of different possible structures that can
be there. More structures translate to more
antigens that can be bound by an antibody.
It is true that immune cells are the only
cells in the human body that are different
from each other in terms of DNA sequence because
of the shuffling that occurs within them.