Antibody Structure and Forms

• General structure of an antibody is described

Antibody Forms

• Antibodies exist in two forms: membrane-bound (BCR) and secreted.
• The membrane-bound form is a B-cell receptor (BCR).
• The secreted form is the antibody.
• The membrane-bound form has a spacer, hydrophobic segment, and cytosolic segment.
• The secreted form has a hydrophilic segment.

Antibody Molecule Digestion

• Intact monoclonal antibodies (mAbs) of ~150 kDa can be cleaved enzymatically.
• Papain digestion results in Fab (~50 kDa) and Fc (~50 kDa) fragments.
• Pepsin digestion results in F(ab')2 (~100 kDa) fragments.
• Reduction of disulfide bonds via DTT results in light chains (LC, ~25 kDa) and heavy chains (HC, ~50 kDa).

Polypeptide Chains

• Antibodies consist of four polypeptide chains.
• These include two identical light chains and two identical heavy chains.
• The chains are connected with disulfide bonds.

Antibody Regions

• Antibodies possess three regions: variable, constant, and an exceptional hinge region.
• They also contain a carbohydrate chain.

Ig Domain

• Light and heavy chains contain repeating homologous structural units folded into a globular motif called an Ig domain.
• The Ig domain is a "sandwich" of two pleated sheets with antiparallel strands of amino acids connected by loops.
• The constant domain of the light chain has seven β strands (four form one sheet, three form another).
• The variable domains contain nine β strands (four form one sheet, five form another).

Variable Regions

• Three highly variable regions exist in the V region of the heavy and light chains.
• These regions, complementary to the shape of the bound antigen, are called complementarity-determining regions (CDRs).
• CDR3s are the most variable.
• 3 hypervariable regions of VL and VH domains form an antigen-binding surface.
• Wide-ranging antibody specificities arise from variations in the length and amino acid sequence of the six CDRs in each Fab fragment.
• Heavy-chain CDRs have more residues that contact the antigen than light-chain CDRs do.
• There are two isotopes of the light chain: kappa or lambda.

Constant Regions

• CH1 and CL domains extend the Fab arms, enabling interaction with the antigen and increasing rotation of the Fab arms.
• The "Hinge region" is located between the CH1 and CH2 domains and is rich in proline.
• The hinge region provides flexibility to IgG, IgD, and IgA and is vulnerable to cleavage by proteolytic enzymes.
• Flexibility at the hinge and V-C junction allows the antibody's arms to bind to sites some distance apart.

Chain Characteristics

• μ and ϵ chains lack a hinge region.
• They possess an additional domain of 110 amino acids CH2/CH2 with hingelike characteristics.
• Isotypes differentiate antibody molecules into distinct classes and subclasses based on the heavy chain C regions.
• IgA, IgD, IgE, IgG, and IgM are designated by letters of the Greek alphabet.
• Additional Fc-linked polypeptide called the J (joining) chain is disulfide-bonded to the carboxyl-terminal cysteine residue of two of the ten chains.