Eskygpp Cpscp

APEFLGGP Gm4

domains of the light and y chains to engage bivalently with randomly spaced epitopes. Bivalent binding greatly enhances the functional affinity (avidity) of the antibody. The function of the hinge region, which is the site of most of the sequence differences between the y chains of different subclasses is yet to be fully understood. Interest is focused on the variation in length of the hinge and its possible importance in spacing the efferent and afferent functional parts of the molecule.

The heavy chains are linked together at their midsection, i.e. in the hinge region, by disulfide bonds and also through the pairing of the C\3 domains by strong noncovalent interactions. Each light chain is linked through its C-terminal cysteine to a heavy chain by a single disulfide bond and adheres also as a result of extensive noncovalent interaction through the pairing of the VM and Vj domains, and of the Cv, and C[ domains. The strength of these noncovalent interactions is such that the polypeptide chains do not normally dissociate when the interchain bonds are broken unless dénaturants are present.

The domains may be regarded as compact cylinders and the interdomain regions as short sections of extended chain permitting a degree of flexibilitv in the interdomain regions, especially in the hinge region. The cylinders are resistant to proteases but proteolytic enzymes such as papain, trypsin and fibrinolysin can cleave IgG molecules at the hinge region, dividing the molecule into two basic functional units: a pair of fragments (Fab), which continue to express the antigen-binding function, and a single fragment (Fc) which readily crystallizes at low ionic strength and which interacts with complement (Clq) and continues to engage most of the effector functions of IgG. Pepsin cleaves C-terminal to the hinge disulfide leaving the two Fab fragments still disulfide linked as the F(ab')2 bivalent antigen-binding fragment, but degrades the N-terminal domain of the Fc fragment leaving only the intact C-terminal domain, C73, as the pFc' fragment. Fragments similar to the pFc' fragment (Fc1) are also obtained from Fc by prolonged exposure to trypsin, papain and fibrinolysin. Specific proteolytic cleavage in the interdomain regions can be achieved by carefully controlled proteolysis of Fab or Fc following brief exposure to denaturing conditions such as low pH (-4.0). Fragments corresponding to individual intact domains appear to retain their native conformation and continue to exhibit interdomain association as well as some of the biological properties associated with that domain.

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