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papain, a selective proteolytic enzyme produced by the papaya
fruit, breaks one peptic bond midway in the heavy chains
disturbing neither the inter and intra-chain noncovalent bonds
(S-S) nor the others; as a result, the molecule is divided into
three fragments, two identical Fabs which retain one paratope
with antigen binding function each, and one Fc, which is labeled
‘c’ for its tendency to crystallise spontaneously in the tube. The
paratopes are formed by the folding together of the C-terminals of
the heavy and light chains of each. The terminals are the ‘variable
parts’ of the two chains, and consist of about one hundred
amino acids whose sequence refl ects the rearrangement—in the
maturing B-cell—of three fragments of the germ line ...