Structural fluctuations and conformational entropy in proteins: entropy balance in an intramolecular reaction in methemoglobin.
Steinhoff HJ, Schlitter J, Redhardt A, Husmeier D, Zander N
The reversible intramolecular binding of the distal histidine side chain
to the heme iron in methemoglobin is of special interest due to the very
large negative reaction entropy which overcompensates the large reaction
enthalpy. It may be considered as a prominent example of the ability of
proteins (including enzymes) to provide global entropy in a local process.
In this work new experiments and model calculations are reported which
aim at finding the structural elements contributing to the reaction entropy.
Geometrical studies prove the implication of the 20 residue E-helix being
shifted by more than 2 A. Vibrational entropies are calculated by a procedure
derived from the method of Karplus and Kushik. It turns out that neither
the histidine alone nor the complete E-helix contribute more than 15 per
cent of the required entropy.