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

The interaction of apohemoglobin with two heme derivatives, CN-protohemin a nd CN-deuterohemin, was monitored at multiple Soret wavelengths (417-423 an d 406-412 nm, respectively) in 0.05 M potassium phosphate buffer, pH 7.0, a t 10 degreesC and revealed, as previously reported, a multiphasic kinetic r eaction. Wavelength-dependent reactions were observed for both CN-protohemi n and CN-deuterohemin derivatives with the alpha chain (bathochromic entity ) displaying faster (4- to 7-fold) rates throughout the courses of both hem e-binding reactions. The basis of this spectrally heterogeneous kinetic phe nomenon could be deduced from molecular modeling studies of alpha- and beta -chain structures. Key differences in the number of stabilizing contacts o f the two chains with the peripheral alpha propionyl 45(CE3); 58(E7); 61(E1 0) as well as the beta vinyl 38(C4); 71(E15); 106(G8) groups were found. Fu rthermore, RMS plots comparing apo- and heme-containing subunits reveal sub stantial structural disparities in the C-CD-F-FG helical regions of the alp ha beta dimer interface.