Biology Reference
In-Depth Information
The light chains are covalently linked to the heavy chains by disulflde or S-
S bonds. Several S-S bonds in the hinge regions of two heavy chains link
them together. Since the hinge regions are relative exposed to the solvent,
enzymes, such as papain or pepsin, can cut the peptide backbones (Porter,
1959). After such digestion, the antibody molecule is broken up into two
Fab fragments and one Fc fragment. The Fc portion is the relatively
constant end of the antibody molecule capable of binding special Fc
receptors on macrophages and other cells able to engulf foreign antigens.
The Fab portions of antibodies will bind specific antigens. The specificity
resides in the amino acid sequences of the V
L
and V
H
regions collectively
known as the F
v
fragment of the Fab portion of the antibody molecule.
Therefore, to understand the adaptive aspect of our immune system, we need
to determine a large number of amino acid sequences of the variable regions
of the light and heavy chains of antibodies. The relative simplicity of
isolating Bence Jones proteins from urine samples of multiple myeloma
patients provided a huge experimental resource of such sequence studies.
Bence Jones proteins had been used as a diagnostic method for multiple
myeloma. In the late 1960's and early 1970's these proteins have provided
vast amount of experimental data to our understanding of the intricacies of
our antibody system.
Within, five years, around eighty partial or complete amino acid sequences
of Bence Jones proteins from human patients were determined. A couple
mouse sequences were also available. The mouse model for human multiple
myeloma was developed by Potter (1968) by injecting mineral oil into the
peritoneal cavity of some inbred strains. Their V
L
regions consisted of about
107 amino acid residues as noted by Hilschmann and Craig (1965). It was
immediately clear that we could not have enough genetic material to code
for millions or billions of different variable region sequences separately.
Furthermore, one of the central problems was how different antibodies could
have the ability to bind specific antigens.
In order to study this problem, these sequences were aligned, by positioning
the two invariant Cys residues forming an internal S-S bond at fixed
locations numbered 23 and 88 from the N-terminal. An invariant Trp
residue was fixed at position 35, and two invariant Gly residues at positions
99 and 101. Some sequences were longer, and gaps were introduced
between positions 27 and 28, and between 95 and 96 (Figure 1-1). This
scheme is the Kabat numbering system for the variable regions of antibody
light chains (Kabat
et al
., 1991).
