HEAT-INDUCED STRUCTURAL-CHANGES IN THE FAB FRAGMENT OF IGG RECOGNIZEDBY MOLECULAR-DYNAMICS SIMULATION - IMPLICATIONS FOR SIGNAL-TRANSDUCTION IN ANTIBODIES

Molecular dynamics simulation was used to identify the conformational alterations in the Fab fragment (Kol), driven by heating at 300, 320, and 340 K. Comparison of heat-modified V-H, C(H)1 and V-L, C-L domain structures with the corresponding crystal conformations revealed speci fic differences, most definitely expressed in the C(H)1 domain. These are dislocations of predominantly peripheral peptide loops exposed to the V - C interdomain interface, comprising in particular the 175-185 amino acids of the heavy chain, as well as the 112-123 amino acids of the interdomain hinge fragment. The deviations, limited to peripheral domain regions at 300 and 320 K, spread at 340 K. The resulting relaxa tion of the tertiary packing in the protein (including the hydrophobic core) initiates global melting of the domain. These theoretical resul ts were supported by experimental findings concerning penetration and binding of dyes (Congo Red, Trypan Blue, ANS) to the protein. Packing relaxation of the C(H)1 domain is turned on after disruption of the sp ecific peptide arrangement formed at the V-C interdomain interface bas ically at the hinge portion(117-122) and at fragments of adjacent pept ide loops (149-154, 171-179) of C(H)1 origin, most probably playing th e role: of a switching mechanism. The dislocations also comprise the 1 31-141 amino acids of the loops accommodated at the C(H)1-C(H)2 interf ace. However, the lack of crystallographic data concerning the Fab-Fc interface limits discussion of this effect to speculation It was concl uded that the unconcerted movements of the V and C parts of the Fab fr agment are an intrinsic driving mechanism, introducing structural alte rations into the C domains. It is suggested that the domain relaxation , induced by healing or mechanical constraints, allows for intermolecu lar interactions, affecting in this way the stability of the immune co mplex.