H-1-NMR STUDY OF DYNAMICS AND THERMODYNAMICS OF ACID-ALKALINE TRANSITION IN FERRIC HEMOGLOBIN OF A MIDGE LARVA (TOKUNAGAYUSURIKA-AKAMUSI)

One of the components of hemoglobin from the larval hemolyph of Tokuna gayusurika akamusi possesses naturally occurring substitution at the E 7 helical position (Leu E7) [M. Fukuda, T. Takagi, K. Shikama, Biochim . Biophys. Acta 1157 (1993) 185-191]. Its oxygen affinity is almost co mparable to those of mammalian myoglobins and it exhibits Bohr effect. Both acidic and alkaline forms of the ferric hemoglobin have been inv estigated using H-1 NMR in order to gain insight into molecular mechan isms for relatively high oxygen affinity and Bohr effect of this prote in. The NMR data indicated that the acidic form of the protein possess es pentacoordinated heme, and that the alkaline form possessing OH- ap pears with increasing the pH value. pH titration yielded a pK value of 7.2 for the acid-alkaline transition, and this value is the lowest am ong the values reported so far for various myoglobins and hemoglobins. The kinetic measurements of the transition revealed that the activati on energy for the dissociation of the Fe-bound OH-, as well as the dis sociation and association rates, decrease with increasing the pH value . These pH dependence properties are Likely to be related to the Bohr effect of this protein. (C) 1998 Elsevier Science B.V. All rights rese rved.