Hemoglobin conformational changes

Movement of the Heme Iron hy Less Than 0.04 nm Induces the Conformational Change in Hemoglobin... 

FIGURE 15.32 Changes in the position of the heme iron atom upon oxygenation lead to conformational changes in the hemoglobin molecnle. 

Oxygenation of Hemoglobin Triggers Conformational Changes in the Apoprotein... 

Oxygenation of Hemoglobin Is Accompanied by Large Conformational Changes... 

Using the 241-cm value for the Ni-histidine frequency in Mb, the T- R shift from the Hb- Mb comparison is 5 cm. This value is consistent with the increase observed in a comparison of the corresponding Fe proteins and with the T R shifts based on other Fe hemoglobins (3-8 cm ) (31). The similarity of the increases observed in the metal-histi3 e frequencies for the nickel and iron hemoglobins indicates that the effect of quaternary structure on the Ni-histidine bond is similar to the Fe case. Also, the effect of the protein conformation change is virturally independent of the particular metal in the porphyrin core. 

Protein-Conformation Effects on Electronic Structure of the Porphyrin Ring. In spite of the similarity in the R/T changes at the metal-histidine bond for Ni and Fe hemoglobins, the effect of the R/T conformational change on the porphyrin s electronic structure in the two cases is quite different. In a wide variety of hemoglobins the T- R increase in i/( e-his) is associated with a... 

In the crystalline state these two forms are not isomorphous, and adding substrate to ConA crystals causes them to shatter.48 (This is a common observation and occurs even in hemoglobin crystals.) The differences between the conformers in the ConA part of the complex are not fully known, but there is considerable rearrangement of the protein. The rates of the reaction are fast, and so in solution the protein must be able to fluctuate readily between its different forms. Comparison with lysozyme (earlier in this article) shows that when lysozyme binds its inhibitors, a conformational change does occur, but it is not so gross that in the solid state shattering of crystals occurs. 

The situation for Fe11 porphyrin is rather similar, with the existence of 5 = 0, 1 and 2 states. The iron in Fe(TPP)(THF)2 lies in the plane of the porphyrin.647 This result is of considerable relevance to model systems for hemoglobin and the question of the trigger for the conformational change associated with the cooperative uptake of dioxygen. In this example the presence of a symmetrical weak axial field is the crucial factor. While this particular situation probably does not occur in any natural hemoprotein, these results demonstrate conclusively that high-spin Fe11 will fit into the porphyrin plane. 

Uptake of 02 is associated with substantial conformational changes, which are central to an understanding of the cooperative efEect. These conformational changes are associated with the reversible interconversion of a tensed (T), low affinity form of hemoglobin into a relaxed (R) high affinity form. 

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