Immunoglobulins – structure, distribution and functions

IMMUNOGLOBULINS –
STRUCTURE,
DISTRIBUTION
AND
FUNCTIONS Aishwarya Babu
M.Sc. Biotechnology
Semester 3
1

Introduction
• Immunoglobulins function as antibodies, the antigen binding proteins present on the B-cell
membrane and secreted by plasma cells.
• Membrane-bound antibody confers antigenic specificity on B cells;
• Antigen-specific proliferation of B-cell clones depends on the interaction of membrane antibody
with antigen.
• Secreted antibodies circulate in the blood, effectors of humoral immunity.
• Most antigens are complex and contain many different antigenic determinants, and the immune
system responds by producing antibodies to several epitopes on the antigen.
2

• The γ-globulin fraction was identified as
containing serum antibodies, which were
called immunoglobulins, to distinguish
them from any other proteins that might be
contained in the - γ globulin fraction.
• IgG, the main class of antibody molecules,
is mostly found in the - γ globulin fraction,
significant amounts of it and other
important classes of antibody molecules are
found in the α and the β fractions of serum.
3

Basic Structure of
immunoglobulins
• 4 peptide chains.
• 2 identical light (L) chains, polypeptides of about 25,000
MW, and two identical heavy (H) chains, larger
polypeptides of 50,000 MW or more.
• Each light chain is bound to a heavy chain by a disulphide
bond, and by such noncovalent interactions to form a
heterodimer (H-L).
• Noncovalent interactions and disulphide bridges link the 2
identical heavy and light (H-L) chain combinations to each
other to form the basic four-chain (H-L)2 antibody
structure, a dimer of dimers.
4

• The first 110 or so amino acids of the amino-terminal region of a light or heavy chain varies greatly
among antibodies of different specificity.
• These segments of highly variable sequence are calledV regions:VL in light chains andVH in heavy.
• Most of the differences among antibodies fall within areas of theV regions called complementarity-
determining regions (CDRs).
• CDRs, on both light and heavy chains constitute the antigen binding site of the antibody molecule.
• The regions of relatively constant sequence beyond the variable regions have been dubbed C regions,
CL on the light chain and CH on the heavy chain.
• Antibodies are glycoproteins; with few exceptions, the sites of attachment for carbohydrates are
restricted to the constant region.
5

• Digestion of IgG with the enzyme papain produced 3
fragments, two of which were identical fragments
and a third, quite different.
• The 2 identical fragments had antigen-binding
activity and were called Fab fragments (“fragment,
antigen binding”).
• The other fragment had no antigen binding activity
at all and it was found to crystallize during cold
storage. So called the Fc fragment (“fragment,
crystallisable”).
• Pepsin digestion generated a single 100,000MW
fragment composed of two Fab-like fragments
designated the F(ab`)2 fragment, which binds
antigen.
• Mercaptoethanol reduction and alkylation
irreversibly cleaves disulphide bonds.
6

• Fab fragment consists of portions of a heavy and a light chain.
• Fc contains only heavy-chain components.
• The IgG molecule consists of two identical H chains and two identical L chains, which are linked by
disulphide bridges.
• Papain cleaves just above the interchain disulphide bonds linking the heavy chains.
• Pepsin cleaves just below these bonds, so that the two proteolytic enzymes generate different
digestion products.
• Mercaptoethanol reduction and alkylation allow separation of the individual heavy and light
chains.
7

Light chain sequencing
• The amino-terminal half of the chain, consisting of 100–110 amino acids, was found to vary and
were calledVariable (V) region.
• The carboxyl-terminal half of the molecule, called the constant (C) region, had 2 basic amino acid
sequences.This led to the recognition that there were 2 light chain types, kappa (κ) and lambda
(λ).
• In humans, 60% of the light chains are kappa and 40% are lambda.
• In mice, 95% of the light chains are kappa and only 5% are lambda.
• A single antibody molecule contains only one light chain type, either or ,never both.
• The amino acid sequences of light chains show minor differences that are used to classify light
chains into subtypes. 8

Heavy chain sequencing
• The amino-terminal part of the chain, consisting of 100–110 amino acids, showed great sequence
variation among heavy chains and was called the variable (V) region.
• 5 basic sequence patterns, corresponding to five different heavy-chain constant (C) regions
(μ,δ,γ,ε, and α).
• Each of these five different heavy chains is called an isotype.
• Length of C regions is app 330 amino acids for δ, γ, and α, and 440 amino acids for μ and ε.
• Heavy chains determine the class of that antibody: IgM, IgG, IgA, IgD, or IgE.
• Each class can have either κ or λ light chains. 9

• Minor differences in the amino acid sequences of the α
and γ heavy chains led to further classification of the
heavy chains into subclasses.
• In humans, there are two subclasses of α heavy
chains—1 and 2—(and thus two subclasses,IgA1 and
IgA2)—and four subclasses of γ heavy chains: 1, 2, 3,
and 4 (therefore four subclasses, IgG1, IgG2, IgG3, and
IgG4).
• In mice, there are 4 subclasses of γ heavy chains (1, 2a,
2b, and 3).
10

Immunoglobulin Fine Structure
• Structure is determined by the primary, secondary, tertiary,
and quaternary organization of the protein.
• 10 structure - the amino acid sequence, accounts for the
variable and constant regions of the heavy and light chains.
• 20 - formed by folding of the extended polypeptide chain back
and forth upon itself into an antiparallel β pleated sheet .
• The chains are then folded into a 30 of compact globular
domains, which are connected to neighbouring domains.
• Globular domains of adjacent heavy and light polypeptide
chains interact in the quaternary structure, forming functional
domains.
• Enable the molecule to specifically bind antigen and, to
perform a number of biological effector functions.
11

Immunoglobulin domains
• Both heavy and light chains contain Immunoglobulin domains.
• Each domain contains 110 amino acids; an intra-chain disulphide bond forms a loop of about 60
amino acids.
• Light chains contains: one variable domain (VL) and one constant domain (CL) while heavy chain
contains one variable domain (VH) and either three or four constant domain (CH).
• Each domain is folded into a characteristic immunoglobulin fold.
• Contains “sandwich” of two antiparallel β pleated sheets, which are connected by loops of various
length.
• The two β pleated sheets within a fold are stabilized by hydrogen bonds that connect the NH2
groups in one strand to COOH group in the adjacent strand.
12

• The basic structure of the
immunoglobulin fold contributes to
the quaternary structure of
immunoglobulins by facilitating
noncovalent interactions between
domains across the faces of the β
sheets.
14

Variable region domains
• Sequence variability is concentrated in several hypervariable
(HV) regions.
• Both κ and λ light chains and heavy chains possess three
hypervariable regions, constituting 15%-20% of variable region
domain.
• Each hypervariable region is relatively short, consists of 6-10
amino acids.
• Hypervariable regions are also called complementarity
determining regions (CDRs), as these regions are antigen
binding sites of the antibody molecule and are
complementary to the structure of the antigen epitope.
• The remaining part of the variable region of both heavy (VH)
and light (VL) chain exhibits less variation and are called as
framework regions (FRS).
15

Constant region domains
• The CH1 and CL domains serve to extend the Fab arms of the antibody molecule.
• Facilitating interaction with antigen and increasing the maximum rotation of the Fab arms.
• Constant-region domains help to hold theVH andVL domains together by virtue of the
interchain disulphide bond between them.
• CH1 and CL domains may contribute to antibody diversity by allowing more random
associations betweenVH andVL domains.
• Random rearrangements of the Ig genes generate uniqueVH andVL sequences for the heavy
and light chains expressed by each B lymphocyte;
• Association of theVH andVL sequences then generates a unique antigen-binding site.
16

Hinge region
• The γ,δ, and α heavy chains contain an extended
peptide sequence between the CH1 and CH2 domains.
• The hinge region, is rich in proline residues and is
flexible, giving IgG, IgD, and IgA segmental flexibility.
• Two prominent amino acids in the hinge region are
proline and cysteine.
• Vulnerable to cleavage by proteolytic enzymes.
17

• The number of interchain disulfide bonds in the hinge
region varies considerably among different classes of
antibodies and between species.
• μ and ε chains lack a hinge region, they have an
additional domain of 110 amino acids (CH2/CH2) that has
hinge like features.
18

Effector functions mediated by
Immunoglobulin constant domain
• Variable region of the antibody is responsible for specific binding with antigen.
• The constant region of heavy chain is responsible for invoking effector response of the humoral
immunity through interaction with other proteins, lymphoid cells, basophils, eosinophils, mast
cells etc.
• Few of the major effector responses are:
Opsonization,
Activation of complement system,
Antibody dependent cell mediated cytotoxicity (ADCC). 19

Antibody Classes and
Biological Activities
21

Immunoglobulin G
(IgG)
• Most abundant class in serum.
• Constitutes about 80% of the total serum immunoglobulin.
• Monomer consisting of 2γ heavy chains and 2 κ or 2 λ light
chains.
• 4 human IgG subclasses, numbered according to their
decreasing average serum concentrations: IgG1,IgG2,IgG3,and
IgG4.
• The four IgG subclasses are encoded by different germ-lineCH
genes, whose DNA sequences are 90%–95% homologous.
22

General structure of the
4 subclasses of human
IgG • IgG1,IgG3, & IgG4 readily cross the placenta and
play an important role in protecting the
developing foetus.
• IgG3 - most effective complement activator,
followed by IgG1;
• IgG2 is less efficient, and IgG4 is not able to
activate complement at all.
• IgG1 & IgG3 bind with high affinity to Fc receptors
on phagocytic cells and thus mediate
opsonization.
• IgG4 - intermediate affinity for Fc receptors, and
IgG2 has an extremely low affinity. 23

Immunoglobulin M
(IgM)
• Accounts for 5%–10% of the total serum immunoglobulin.
• Monomeric IgM, expressed as membrane-bound
antibody on B cells.
• Secreted by plasma cells as a pentamer.
• 5 monomer units are held together by disulphide bonds
that link their carboxyl-terminal heavy chain domains and
their C3/C3 domains.
• 5 monomer subunits arranged with their Fc regions in the
centre of the pentamer and the 10 antigen-binding sites
on the periphery of the molecule.
24

• Each pentamer contains J (joining) chain, which is disulphide-bonded to the carboxyl-terminal cysteine
residue of two of the ten chains.
• J chain - required for polymerization of the monomers to form pentameric IgM;
• First immunoglobulin class produced in a primary response to an antigen.
• First immunoglobulin to be synthesized by the neonate.
• Serum IgM has a higher valency than the other isotypes.
• Can bind 10 small hapten molecules; however, because of steric hindrance, only 5 or fewer molecules of
larger antigens can be bound simultaneously.
25

• More efficient in binding antigens with many repeating epitopes such as viral particles and red
blood cells (RBCs).
• More efficient than IgG at activating complement.
• IgM does not diffuse well.
• Found in very low concentrations in the intercellular tissue fluids.
• The J-chain facilitates secretion at mucosal surfaces.
• IgM plays an important accessory role as a secretory immunoglobulin.
26

Immunoglobulin A
(IgA)
• Constitutes only 10%–15% of the total immunoglobulin in serum.
• Predominant immunoglobulin class in external secretions such as
breast milk, saliva, tears, and mucus of the bronchial,
genitourinary, and digestive tracts.
• In serum, IgA exists primarily as a monomer, but polymeric forms
are sometimes seen, all containing a J-chain polypeptide.
• Secretory IgA, consists of a dimer or tetramer, a J-chain
polypeptide, & a polypeptide chain.
27

• Secretory component is derived from the receptor that is responsible for transporting polymeric IgA
across cell membranes.
• J-chain polypeptide - facilitates the polymerization of both serum IgA and secretory IgA.
• The secretory component is a 70,000-MW polypeptide produced by epithelial cells of mucous
membranes.
• Consists of five immunoglobulin-like domains that bind to the Fc region domains of the IgA dimer.
• IgA-secreting plasma cells are concentrated along mucous membrane surfaces.
• Every day, a human secretes from 5 g to 15 g of secretory IgA into mucous secretions.
28

Secretory IgA
• IgA preferentially migrate to sub epithelial tissue.
• Secreted IgA binds tightly to a receptor for polymeric
immunoglobulin molecules.
• Poly-Ig receptor is expressed on the basolateral surface of most
mucosal epithelia and on glandular epithelia in the mammary,
salivary, and lacrimal glands.
• Receptor-IgA complex is transported across the epithelial barrier to
the lumen.
• Transport involves receptor-mediated endocytosis into coated pits
and directed transport of the vesicle across the epithelial cell to the
luminal membrane.
• Vesicle fuses with the plasma membrane.
• Poly-Ig receptor is then cleaved enzymatically from the membrane
and becomes the secretory component, which is bound to and
released together with polymeric IgA into the mucous secretions.
29

• Secretory component masks sites susceptible to protease cleavage in the hinge region of secretory IgA.
• Secretory IgA serves an important effector function at mucous membrane surfaces, which are the main
entry sites for most pathogenic organisms.
• Secretory IgA can cross-link large antigens with multiple epitopes.
• Binding of secretory IgA to bacterial and viral surface antigens prevents attachment of the pathogens
to the mucosal cells, thus inhibiting viral infection and bacterial colonization.
30

• Complexes of secretory IgA and antigen are easily entrapped in mucus and then eliminated by the
ciliated epithelial cells of the respiratory tract or by peristalsis of the gut.
• Secretory IgA - an important line of defence against bacteria such as Salmonella,Vibrio cholerae,
and Neisseria gonorrhoeae and viruses such as polio, influenza, and reovirus.
• Breast milk contains secretory IgA and many other molecules that help protect the new-born
against infection during the first month of life.
31

Immunoglobulin E (IgE)
• Extremely low average serum concentration (0.3 g/ml).
• Mediate the immediate hypersensitivity reactions that are
responsible for the symptoms of hay fever, asthma, hives, and
anaphylactic shock.
• IgE binds to Fc receptors on the membranes of blood basophils
and tissue mast cells.
33

• Cross-linkage of receptor bound IgE molecules by antigen
induces basophils and mast cells to translocate their granules
to the plasma membrane and release their contents to the
extracellular environment.
• Pharmacologically active mediators are released and give rise
to allergic manifestations.
34

Immunoglobulin D (IgD)
• Serum concentration of 30 microgram/ml.
• Constitutes about 0.2% of the total immunoglobulin in serum.
• IgD, together with IgM, is the major membrane bound
immunoglobulin expressed by mature B cells.
• Its role in the physiology of B cells is under investigation.
• No biological effector function has been identified for IgD.
35

• Circulating IgD can react with specific bacterial proteins, such as the IgD binding protein of
Moraxella catarrhalis , independent of the variable regions of the antibody.
• The binding of these bacterial proteins to the constant region of IgD results in B cell stimulation
and activation.
36

References
• Immunology – Janis kuby, 3rd Edition, 1997,W.H.Freeman and Company, NewYork.
• Immunobiology – C. Janeway and P.Travers, 6th Edition, 2006,Garland Science.
• Cellular and molecular Immunology – Abul .K. Abbas, Andrew H. Lichtman, 2003, Saunders
Company.
• Structure and Function of Immunoglobulins - HarryW Schroeder Jr and Lisa Cavacini, J Allergy
Clin Immunol, 2010 February.
37

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