Fluctuation of the Myoglobin Structure

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The kinetics of hydrogen-deuterium exchange in sperm whale metmyoglobin has been followed in aqueous solutions of various pD values (5.6–7.7) and at various temperatures (0–55 °C) by an infrared absorption measurement. At pD=5.6 and at 0 °C, 117 out of the total 148 peptide hydrogen atoms exchange relatively slowly with a rate of an apparent first-order reaction for one hour of the reaction time. This number (117) is almost equal to the sum of the number of peptide NH-groups (108∼111) which are found to be involved in hydrogen bonds with the carbonyls of other peptide groups and those (8) which are in hydrogen bonds with oxygen atoms of some sidechains in the myoglobin molecule in the crystalline state. At higher temperatures and/or at higher pD's, however, only 109 peptide NH groups (assignable to those involved in the peptidepeptide hydrogen bonds) are found to exchange relatively slowly. The kinetics of the exchange reactions of these 109 peptide NH groups are explained by assuming that the fluctuation amplitude (<I>i.e.</I>, the probability of finding the peptidepeptide hydrogen bonds broken) becomes lower on going from the terminus towards the inner portion in each of the eight α-helices involved in the myoglobin structure.
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