Electron paramagnetic resonance and optical absorption spectrum of the pentacoordinated ferrihemoproteins.

The photolysis of NO-fedhemoprotein complexes was performed at liquid helium (4.2 K) and liquid nitrogen (?? K) temperatures.On illumination of light at 4.2 K, the optical absorption spectrum of NO-ferrimyoglobin turns into the spectrum characteristic of the ferric high spin state, which exhibits a broad maximum at 380 nm, a peak at 500 nm, and a broad absorption band from 600 to 680 nm.This spectrum differs from that of native ferrimyoglobin in both acid and alkaline forms.The EPR spectrum of the photolyzed product of NOferrimyoglobin has a broad absorption at g = 5.8 (halfwidth, 110 G ) characteristic of the ferric high spin state together with a very broad absorption in the range from g = 4 to 2. This spectrum differs from that of native acid-ferrimyoglobin (g = 6.0 and half-width = 21 G).Taking account of the fact that the water molecule coordinates at the 6th position in the native ferrimyoglobin at acidic and neutral pHs, we conclude that the photodissociated product of NO-ferrimyoglobin at 4.2 K is present in the pentacoordinated form.In contrast to the results on NO-ferrimyoglobin, we cannot detect any significant difference in optical and EPR spectra between the photodissociated product of NO-ferrihorseradish peroxidase and the native ferrienzyme.This indicates that native ferrihorseradish peroxidase is in the pentacoordinated form.Except for certain "type abnormal hemoglobins (l), the pentacoordinated form of the ferric state of hemoproteins has not yet been realized.This comes from the fact that the water molecule occupies the 6th position in the ferric unligated state in contrast to the vacancy at this position in the ferrous unligated state.The recent discovery of the photodissociability of NO-fenihemoprotein complexes (2, 3) has opened up the possibility of observing such a pentacoordinated form of the ferric state.Thus, we performed the photolysis of NOfenihemoprotein complexes at liquid helium temperature (4.