332
Antibody Molecules, Genetic Engineering of
V
H
C
H
1
C
H
2
C
H
3
C
L
V
L
C
L
C
H
1
C
H
2
C
H
3
S
S
S
S
S
S
S
S
Fc
Fab
Fv
V
L
C
L
Light-chain genes
V
H
C
H
2
C
H
1
Heavy-chain genes
C
H
3
(a)
(b)
Hinge
V
L
V
H
Hinge
Fig. 1
(a) Diagram of an immunoglobulin G
(IgG) molecule (the most abundant antibody in
serum) and the active fragments that can be
derived from it. The antibody molecule is divided
into discrete functional domains: two domains
constitute the light chain (V
L
and C
L
), while four
domains make up the heavy chain (V
H
,C
H
1,
C
H
2, and C
H
3). The variable region domains
make the antibody binding site and are
designated as the Fv region. The effector
functions of the antibody are properties of the
constant region domains. The carbohydrate
units (black circles) present within the C
H
2
domains contribute to the functional properties
of the antibody. The hinge region provides
flexibility to the antibody molecule, facilitating
antigen binding and some effector functions. The
enzyme papain cleaves the antibody into two Fab
fragments containing the antigen binding sites
and an Fc fragment responsible for the effector
functions. (b) Genes that encode the heavy and
light chains. In the genes, each domain is
encoded by a discrete exon (indicated by boxes)
separated by intervening sequences (introns)
indicated by the line; the intervening sequences
are present in the primary transcript but are
removed from the mature mRNA by splicing.
Both heavy and light chains contain hydrophobic
leader sequences (indicated by the black exon)
necessary for their secretion. This leader
sequence is present in the newly synthesized
heavy and light chains but is cleaved from them
after they enter the endoplasmic reticulum and,
therefore, is not present in the mature chains.
the C
L
region. The constant region of the
heavy chain is further divided into three
structural domains stabilized by intrachain
disulFde bonds: C
H
1, C
H
2, C
H
3(±
ig
.1
)
.
The C
H
3 domain of the heavy chain repre-
sents its carboxy-terminus. The domain
structure of the antibodies is very im-
portant for genetic engineering because
it facilitates protein engineering, allowing
the exchange between molecules of func-
tional domains carrying antigen-binding
activities (±abs or ±vs) or effector func-
tions (±c).
The hinge region, a segment of heavy
chain between the C
H
1andC
H
2domains,
provides flexibility in the molecule. Papain
digestion of IgG yields two ±ab fragments
and one ±c fragment. The ±ab region binds
antigen, while the ±c region mediates
effector functions such as complement
activation, antibody-dependent cellular cy-
totoxicity (ADCC), and placental transmis-
sion. All antibodies are glycoproteins, and
the carbohydrate present in the constant
region has been shown to be essential for
many of its effector functions.
1.2
Classes and Subclasses of Antibodies
The constant region of the heavy chain
determines the class or isotype of the
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