Download Serum proteins are against a number of important bacterial infections.

Immunoglobulins
- Serum proteins are against a number of important bacterial
infections.
Introduction
• All vertebrates possess immunoglobulin-like
molecules.
• Immunoglobulins are synthesized and secreted by
end cells of the B cell lineage, i.e. plasma cell.
• Immunoglobulins are largely confined to the broad
and heterogeneous band of -globulins and show
considerable diversity of structure and function.
The first evidence to show antibody is one
of serum protein fractions
1939, A. Tiselius and E. A. Kabat
To immunize rabbits with ovalbumin
Belong to -globulin fractionimmunoglobulin
Untreated
antiserum
Treated
antiserum
Structure of Immunoglobulins
• The typical immunoglobulin molecule is
– Asymmetrically composed of four polypeptide chains
– Linked by disulphide bridges
– The larger chains are designated heavy(MW about 50~77 kDa)
and the smaller light (MW about 25 kDa).
– The two most important of their features in the immune
response are specificity and biologic activity.
• Several immunoglobulin fragments can be prepared
using proteolytic enzymes and these have been of value
in unravelling the functional activities of different parts
of these molecules
Schematic diagram of structure of
immunoglobulins
– Papain digestion：
• To cleave on the amino-terminal side of the inter-heavy chain
disulphide bonds.
• To yield two Fab（Fragment antigen-binding）fragments and one Fc
（Fragment crystallizable）fragment.
– Pepsin digestion ：
• To cleave on the carboxy-terminal side of the inter-heavy chain
disulphide bonds.
• To yield a Fab dimer（F(ab' )2）and a rather smaller Fc fragment
（pFc'）
A MW of 150,000 Da
A MW of 100,000 Da
A MW of 25,000 Da
A MW of 45,000 Da
A MW of 50,000 Da
A MW of 50,000 Da
• The immunoglobulin molecules
are in Y configuration and have
the flexible hinge region which
permits considerable movement
of the Fab arms.
• Both light and heavy
polypeptide chains consist of a
series of similar subunits,
designated domains.
– 每一個domain約由110 amino acids及a single intrachain disulphide
bridge堆疊而成之polypeptide chain。
– Light and heavy polypeptide chains之amino-terminal domain的氨基酸
組成有較高之變化性，稱為variable region(可變區)；和可變區相較
之下氨基酸組成變化低的domain，稱為constant region(不可變區)。
– light chain具有單一可變區及不可變區domains(VL及CL)；heavy chain
具有單一可變區及三到四個不可變區domains (VH及CH1、CH2、CH3、
CH4，其不可變區domain數目依immunoglobulin種類而定)。
Ribbon representation of an intact monoclonal
antibody depicting the heavy and light chain
Qutternary structure immunoglobulin and
interactions between domains
1.
2.
3.
Extended peptide
sequence between
CH1 and CH2 of , ,
and  heavy chain
Poline rich sequence
Giving IgG, IgD and
IgA segmental
flexibility
• Variable region為immunoglobulin與抗原接觸的部位
(antigen-binding site)，其中具有三區hot spots
(hypervariable regions，HVR1、HVR2、HVR3 or
complementarity-determining regions, CDR1、CDR2、
CDR3)。
• Affinity: the strength of binging or association of a single
epitope for a single combining site
• Avidity: indicate the average strength of binding
Diagram of an immunoglobulin light chain
depicting the fold structure
Held teogether
by hydrophobic
interaction and
disulfide bond
Antigenic Deteminants on
Immunoglobulin
• Antibodies also are potent immunogens to induce an
antibody response
• Anti-Ig antibodies are powerful tools
• Antigenic determinants (epitopes) on Ig:
– Isotypic determinant: constant-region determinants, to distinguish
each Ig class and subclass within a species
– Allotypic determinant: be able to distinguish the subtle amino acid
difference between the same set of isotype genes
– Idiotypic determinant: be generated by conformation of amino acid
sequences of heavy- and light-chain variable regions specific for
each antigen
1.
2.
Individual
determinant is
called an idiotope.
The sum of the
individual idiotope
is the idiotype
The Classes and Subclasses of
Immunoglobulins
• 利用heavy chain constant
region的結構和功能的不
同可將Immunoglobulins
分為IgG、IgA、IgM、
IgD、IgE等五類；其中
IgG及IgA具有subclasses。
• IgG、IgD、IgE為monomer；IgA為dimer；IgM為
pentamer。
• J chain：
– A small acidic protein，15 kDa，其結構與light and heavy chain
無關。
– 由plasma cell分泌而來。
– J chain is disulfide-bonded to the penultimate cysteine residue in
the tail piece of theαorμchain。主要用來聚合immunoglobulin
monomer。
• Secretory component：
– A single polypeptide，about 70 kDa，僅可與IgA聚合在一起；
由黏膜皮層細胞產生，這些細胞會將血液中的IgA抓住，使其
與secretory component結合後轉移進入分泌系統。
– 其結構中具有高量的carbohydrate；氨基酸組成和 J chain及
immunoglobulins沒有相似處。
– 利用非共價鍵的接出方式與游離或被鍵結的IgA相接。
IgG
• 具有γ1、γ2、γ3、γ4 四種heavy chains，κ或λlight chain；因此有
IgG1(60-70%)、IgG2(14-20%)、IgG3(4-8%)、IgG4(2-6%)四種次種。
• IgG accounts for about 75-80% of the total serum immunoglobulin in
normal adult and is the abundant antibody produced during secondary
humoral immune responses in the blood。
• IgG1、IgG2、IgG3可活化classical complement pathway，IgG2可額
外活化alternative complement pathway。
IgM
• 具有μheavy chain，κ或λlight
chain。
• 為pentamer，具有10 antigenbinding sites，五個單元由一個J
chain連結在一起。
• 重鏈具有四個constant regions。
• IgM accounts for only about 10% of serum immunoglobulin，is the
key immunoglobulin of the primary response and an efficient activator
of the classical complement pathway，and is the most common
immunoglobulin expressed on the surfaces of B cells。
• 因為分子很大，無法由循環系統進入組織液中，但因其對poly-Ig
recoptor有高親和性，因此在IgA deficiency時會出現在分泌系統中
與secretory piece連結扮演IgA的角色。
IgA
• 具有α1、α2兩種heavy chains，
κ或λlight chain；因此有
IgA1、IgA2(兩者存在的比
例為5：1)兩種次種。
• 釋出時為dimer (secretory
IgA, 最多可為pentamer)加上
secretory component游離於
組織及血液中，其間具有J
chain連接；如果為
membrane-bound form則為
monomer。
• IgA accounts for only about 10-15%
of serum immunoglobulin
• The major immunoglobulin of
external secretions
• The most abundant antibody class
was found in saliva, tear, intestinal
mucus, bronchial secretions, milk,
prostatic fluid, and other secretions。
• 不會活化classical complement
pathway，但可以協助alternative
complement pathway中C3
convertase的stabilization。
• Specific Fc receptors for IgA have
been observed but are not well
charaterized。
Formation of secretory IgA:
Dimer IgA binds to a poly-Ig receptor
Internalized by receptor-mediated endocytosis
Ploy-Ig receptor is enzymatically cleaved
Secretory component bound to the dimer IgA
IgE
• 具有εheavy chain，κ或λlight
chain，about 180 kDa，為所有Ig
量最少的一種(約佔0.004%)，為
偵測allergic disorder(過敏性失調)
之臨床標準指標值。
• 重鏈具有四個constant regions。
• 會與mast cells及basophils表面higtaffinity Fc receptor (FcεRI)接合，
當有特殊抗原出現時，會刺激該
細胞釋出inflammatory mediators。
• 某些細胞(such as eosinophils)表面
受器FcεRII可與IgE接合(親和性
低)，對於parasitic worms感染之
免疫性有重要關係。
Allergen cross-linkage of receptor-bound IgE on mast
cell induces degranulation, causingrelease of substances
that mediate allergic manifestations
IgD
• 具有δheavy chain，κ或
λlight chain，在血液及體
液中出現濃度相當低。
• 目前研究發現在B細胞表
面具有IgD，與IgM相連
在一起；扮演B細胞表面
的antigen receptor。
• 其與其他免疫球蛋白間之
功能尚未明瞭。