SIMULATION OF A COMPLEX PROTEIN STRUCTURAL-CHANGE - THE T[--]R TRANSITION IN THE INSULIN HEXAMER

The T <-> R transition in the insulin hexamer is an outstanding model for protein structural changes in terms of its extent and complexity: the limiting structures T-6, T(3)R(3) and R(6) have been defined by X- ray crystallography. The transition occurs cooperatively within trimer s. It involves displacements of >30 Angstrom and a secondary structura l rearrangement of 15% of the peptide chain between extended and helic al conformations. Experimental data for the transition are plentiful, Theoretical methods to simulate pathways without constraints would nev er succeed with such substantial transitions. We have developed two ap proaches, targeted energy minimization (TEM) and targeted molecular dy namics (TMD). Previously successful in simulating the T <-> R transiti on of the insulin monomer, these procedures are also shown here to be effective in the hexamer, With TMD, more conformational space is explo red and pathways are found at 500 kJ/mol lower energy than with TEM. B ecause the atoms have to meet distance constraints in sum rather than individually, a high degree of conformational freedom and independence is implied. T-6 --> T(3)R(3) and T(3)R(3) --> T-6 pathways do not coi ncide because the transformation is directed. One subunit enters a dea d end pathway in one direction of the TMD simulation, which shows that constraint and freedom are critically balanced. The ensemble of produ ctive pathways represents a plausible corridor for the transition, A v ideo display of the transformations is available.