MOTIONS IN HEMOGLOBIN STUDIED BY NORMAL-MODE ANALYSIS AND ENERGY MINIMIZATION - EVIDENCE FOR THE EXISTENCE OF TERTIARY T-LIKE, QUATERNARY R-LIKE INTERMEDIATE STRUCTURES

The normal mode analysis of human hemoglobin showed the presence in th e deoxy T-state of one main preferential direction that brings the str ucture close to the R-state, with a low-energy variation, while in the oxy R-state there are several modes that point towards the T-state, b ut with higher energy variations and less contribution to the transiti on. The displacement along a combination of normal modes, followed by energy minimization, starting from the R-state, did not allow one to o btain a structure significantly different from that of R, showing that the fully oxygenated hemoglobin is trapped in a deep and narrow poten tial energy minimum. On the contrary, starting from the deoxy T-state, the displacement along a combination of normal modes, followed by ene rgy minimization, yielded an intermediate structure, that we designate T-d1(min), indicated that the potential energy minimum in the vicinit y of this structure is as narrow as that of R but less deep. The proce dure of displacement along the modes, followed by energy minimization, was applied to T-d1(min), yielding T-d2(min); then the procedure was repeated, yielding the intermediate structures T-d3(min) and T-d4(min) . The structures T-d2(min), T-d3(min) and T-d4(min) are not significan tly different from each other, indicating that they are trapped in a n arrow, deep energy minimum. This procedure revealed the existence of a t least two intermediate sets of structures between T and R: the first one, T-d1(min), is different from the T and R structures, while the s econd set, T-d2(min), T-d3(min) and T-d4(min), is quaternary R-like an d tertiary T-like, where the contacts at the interfaces alpha(1) beta( 1) and alpha(1) beta(2) are R-like, and the alpha and beta heme enviro nments are still T-like.