Question

Why would predicted molecular weights of heavy and light chains not add up evenly to match the molecular weight of undenatured IgG antibody

Answer 1

The strucuture of IgG antibody shows to have two major parts, a pair of heavy and light chains. The heavy chains extend upto the length of the antibody whereas the light chains are found exclusively on the antign-binding site of the antibody. Thus, it is expected that the total summation of molecular weights of the pairs of heavy and light chains must add up to give the total molecular weight of the antibody. However, structural composition of the antibody revelas that summation of the individual chains of the antibody is different from the actual weight of the antibody. This is because the heavy and light chains are bound to each other by sulphide linkages and appear to have excess weight. On the contrary, degradation/denaturation of an antibody releases the heavy and light chains also causes degradation of some of the structural proteins as well. Thus, the final weight of the net antibody counts less than the expected summation weight.