Biochemistry Lecture No. 7: Enzyme Catalysis
Thursday September 20 , 2012
-The immune response is triggered by foreign macromolecules (like proteins and carbohydrates), which
are known as antigens. Immunoglobulins are soluble proteins that recognize and tag antigens for
destruction. These soluble antibodies are found in the blood, lymph and other bodily fluids and are
made by B lymphocytes. There are multiple types of immunoglobulins (such as A, D, E, G, and M), but
immunoglobulin G (IgG) is the main form found in the blood.
Immunoglobulin G (IgG) Tetramers:
-There are billions of different IgGs that can be manufactured by an individual. Different IgGs have
similar sequences, except in certain regions: variable regions (allow antibodies to bind to many different
antigens) and hypervariable regions (allow antibodies to have high specificity for certain antigens). Both
the heavy and light chains (2 of each to form a tetramer) of the antibody contribute to its specificity for
-Outside of the variable and hypervariable regions are constant regions which are relatively the same for
each immunoglobulin G. Disulphide bonds are rich in the centre area of the antibody and hold together
the heavy and light chains as well as both heavy chains.
Immunoglobulin G Domains:
-By observing the immunoglobulin’s secondary structure, we see that the light chain is made up of β-
sheets (2 sets in total) in both the variable and constant domains. The disulphide bonds within those β-
sheets are what are holding them tightly together. The heavy chains share a similar structure. At the
very end of each variable chain are loops (unstructured regions of the polypeptide) where the antigen
will bind. These loops are made up of different amino acids, whereby the centre tends to be very
-The variability of the polypeptide sequence in these loops is a key feature of the molecule as it is what
allows for antibody’s specificity, in terms of binding to antigens.
Functional Segments Of Immunoglobulin G:
-If we proteolyse (use a protease upon) immunoglobulin G, we get three molecular regions: 2 Fragment
antigen binding (Fab) pieces (regions that bind antigens and recognize them) and 1 Fragment
crystallized (Fc) piece (the region that has the functional part “effector,” which activates a compliment
pathway to the antigen it has recognized). This portion of the antibody binds to phagocytes, mast cells,
basophils, eosinophils, and natural killer cells. These two parts of the molecule have different functions,
their terms help describe these differences. Immunoglobulin G Antigen Binding:
-Antigen-antibody binding is quite specific. There is a high level of surface complementarity (or how
tightly the antigen binds to the antibody) achieved. The very tips of IgG molecules are where this very
specific interaction occurs.
The Production & Use Of Antibodies:
-Antibodies are critical tool for the lab as there is an enormous use for antibodies (even though they are
expensive) as they are an important part of most biomedical research. Antibodies are created by
injecting a given antigen into an animal (usually a rabbit or goat) and then harvesting the produced
antibodies after the animal’s immune response.
-Immunoglobulin G’s extreme specificity makes it a valuable tool in: laboratory experiments (the
identification of molecules in mixtures) like an immunoblot, clinical diagnosis of disease like HIV, clinical
therapy like antivenins. Antibodies are one of the newer ways of treating disease, by knocking out
proteins that are foreign to the body.
-Known as biological catalysts, enzymes perform nearly all chemical transformations in