Fab portion

In addition to the covalent binding, some methods derived from bioaffinity chromatography can be used for non covalent attachment of antibodies to a surface by the inactive Fc portion. The advantage is that antigen binding sites stay undamaged and accessible for the analytes due to the orientation of antibody with the active Fab portions towards the tested medium. 

It is very important in PAP techniques for the secondary antibody to be applied in excess. This way, one arm of the divalent Fab portion of the immunoglobulin molecule can bind the primary antibody, and the other arm is free to bind the PAP complex. If not in excess, both arms of the molecule may bind the primary antibody, creating a prozone-type of effect. Without a free secondary antibody Fab site to capture the PAP complex, the assay will not work. 

Three monoclonal antibodies to human TNF are approved for the treatment of inflammatory bowel disease infliximab, adalimumab, and certolizumab (Table 62-3). Infliximab and adalimumab are antibodies of the IgGi subclass. Certolizumab is a recombinant antibody that contains an Fab fragment that is conjugated to polyethylene glycol (PEG) but lacks an Fc portion. The Fab portions of infliximab and certolizumab are chimeric mouse-human antibodies but adalimumab is fully humanized. Infliximab is administered as an intravenous infusion. At therapeutic doses of 5-10 mg/kg, the half-life of infliximab is approximately 8-10 days, resulting in plasma disappearance of antibodies over 8-12 weeks. Adalimumab and certolizumab are administered by subcutaneous injection. The half-life for both is approximately 2 weeks. 

In this exercise you will examine the interactions between the enzyme lysozyme (Chapter 4) and the Fab portion of the anti-lysozyme antibody. Use the PDB identifier 1FDL to explore the structure of the IgGl Fab fragment-lysozyme complex (antibody-antigen complex). View the structure using Protein Explorer, and also use the information in the PDBsum summary of the structure to answer the following questions. 

There are both functional and structural domains. A functional domain may consist of one or more structural domains. A functional domain is a functionally autonomous region of the molecule. In the case of the IgG, the Fab portion is the functional antigen recognition domain while the Fc region is an effector domain. Structural domains are geometrically separate. 

An additional site for binding of SpA on the immunoglobulin molecules has been identified in the Fab portion. This property can easily be exploited for the separation of Fc fragments after proteolytic breakdown with papain (see Fig. 16 on page 581). This site has been demonstrated to be present in all... 

Fab portion (Fab fragment) That part of the antibody molecule containing the antigen binding site, facilitated diffusion Carrier-mediated transport of molecules along a concentration gradient across the cell membrane with no expenditure of energy, facilitation Increase in responsiveness of a post-synaptic membrane to successive stimuli. 

The effector sites that interact with cells (e.g., IgE with mast cells) and with complement are on the constant (Fc) region of the heavy chains. Variations in the Fc region result in the classes and subclasses into which immunoglobulins are grouped IgM, IgG (four subclasses), IgA (two subclasses), IgD, and IgE. Their respective heavy chains are called p, y, a, 6, and e. The hinge region between the Fc and Fab portions, which is susceptible to proteolytic cleavage, controls the interaction between the Fab and Fc parts. The... 

Most clinical isolates of S. aureus synthesize protein A, a cell wall protein endowed with unique immunoglobulin-binding properties. Protein A has a classical site that binds the Fey of IgG [60], and an alternative site that binds the Fab portion of 15-50% of human polyclonal IgG, IgM, IgA, and IgE [61]. 

Fig. 2.1 The antibody. An IgG antibody has a single constant region (white) with the Fc portion and the species-specific antigens. The variable region (gray) contains the Fab portion that binds the epitope portion of the antigen. The small protein, only in the variable region, is known as the light chain the large protein that is part of the constant and variable region is the heavy chain. The IgG can be digested by the protease enzyme, papain, into an Fc end (constant end) and a Fab end (variable end)...

The entire structure is Y-shaped and has a molecular weight of about 160,000. The common end of IgG, sometimes refered to as the F(. portion of the molecule, has a fixed structure (i.e., does not vary between different IgG molecules). Conversely, the two separate heavy-light chain combinations in IgG, sometimes refered to as the Fab portions of the molecule, contain variable amino acid sequences that have a tertiary structure which permits the molecule to bind strongly to a specific region, "epitope", of the foreign substance, "antigen", for which it was generated. 

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