Investigations of Ferric Heme Cyanide Photodissociation in Myoglobin and Horseradish Peroxidase
Zeng | Weiqiao | Sun | Yuhan | Benabbas | Abdelkrim | Champion | Paul M.
The photodissociation of cyanide from ferric myoglobin (MbCN) and horseradish peroxidase (HRPCN) has definitively been observed. This has implications for the interpretation of ultrafast IR (Helbing et al. Biophys. J. 2004, 87, 1881â1891) and optical (Gruia et al. Biophys. J. 2008, 94, 2252â2268) studies that had previously suggested the FeâCN bond was photostable in MbCN. The photolysis of ferric MbCN takes place with a quantum yield of â¼75%, and the resonance Raman spectrum of the photoproduct observed in steady-state experiments as a function of laser power and sample spinning rate is identical to that of ferric Mb (metMb). The data are quantitatively analyzed using a simple model where cyanide is photodissociated and, although geminate rebinding with a rate of kBA â (3.6 ps)â»Â¹ is the dominant process, some CNâ exits from the distal heme pocket and is replaced by water. Using independently determined values for the CNâ association rate, we find that the CNâ escape rate from the ferric myoglobin pocket to the solution at 293 K is kâáµ¤â â (1â2) Ã 10â· sâÂ¹. This value is very similar to, but slightly larger than, the histidine gated escape rate of CO from Mb (1.1 Ã 10â· sâÂ¹) under the same conditions. The analysis leads to an escape probability kâáµ¤â/(kâáµ¤â + kBA) â¼ 10ââ´, which is unobservable in most time domain kinetic measurements. However, the photolysis is surprisingly easy to detect in Mb using cw resonance Raman measurements. This is due to the anomalously slow CNâ bimolecular association rate (170 MâÂ¹ sâÂ¹), which arises from the need for water to exchange at the ferric heme binding site of Mb. In contrast, ferric HRP does not have a heme bound water molecule and its CNâ bimolecular association rate is larger by â¼10Â³, making the CNâ photolysis more difficult to observe.Show more [+] Less [-]