Univalent Fab

Fig. 2. Papain digestion of an antibody molecule yields two univalent Fab fragments and an Fc fragment whereas pepsin digestion yields a bivalent F(ab )2 fragment.

The equilibrium constant for the univalent Fab-adsorbed Mb reaction is 5 x 10° M ". This compares very favorably with the value for the clone 4 equilibrium constant in solution, 7.1 X 10 M (27). That the IgG isotherm is significantly steeper than the Fab isotherm indicates that bivalent binding of the IgG molecules occurs. 

It was first noted by Greenbury et al. that bivalent antibodies exhibit a higher effective energy of interaction ( avidity ) in their reaction with red blood cells than the corresponding univalent (Fab ) fragments the ratio of effective combining affinities was 150 1 to 450 1... 

Fig. 5.16. The three particles at the left are polyoma virus coated with univalent. Fab fragments of rabbit antibody. Numerous V s can be seen, suggesting attachment of two Fab fragments to individual capsomeres on the virus. At the right are two particles of polyoma virus treated first with univalent rabbit antibody, then with bivalent goat antibodies directed to the rabbit antibody. The width of the combined antibody layer is about 300 A. Reprinted by permission, from reference 67.

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