Wavelength-dependent spectral changes accompany CN-hemin binding to human apohemoglobin

The interaction of apohemoglobin with two heme derivatives, CN-protohemin and CN-deutero-hemin, was monitored at multiple Soret wavelengths (417-423 and 406-412 nm, respectively) in 0.05 M potassium phosphate buffer, pH 7.0, at 10 degrees C and revealed, as previously reported, a multiphasic kinetic reaction. Wavelength-dependent reactions were observed for both CN-protohemin and CN-deuterohemin derivatives with the alpha chain (bathochromic entity) displaying faster (4- to 7-fold) rates throughout the courses of both heme-binding reactions. The basis of this spectrally heterogeneous kinetic phenomenon could be deduced from molecular modeling studies of alpha- and beta-chain structures. Key differences in the number of stabilizing contacts of the two chains with the peripheral alpha propionyl 45(CE3); 58(E7); 61(E10) as well as the beta vinyl 38(C4); 71(E15); 106(G8) groups were found. Furthermore, RMS plots comparing apo- and heme-containing subunits reveal substantial structural disparities in the C-CD-F-FG helical regions of the alphabeta dimer interface.