Resonance Raman investigation of cyanide ligated beef liver and Aspergillus niger catalases

Abstract

Resonance Raman spectroscopy has been used to investigate the properties of cyanide-bound beef liver catalase (BLC) and Aspergillus niger catalase (ANC) in the pH range 4.9-11.5. Evidence has been obtained for the binding of cyanide to both BLC and ANC in two binding geometries. The first conformer, exhibiting the v[Fe-CN] stretching mode at a higher frequency than the δ[Fe- C-N] bending mode, exists as an essentially linear Fe-C-N linkage. For both BLC-CN and ANC-CN, the v[Fe-CN] and δ[Fe-C-N] frequencies of this conformer were practically identical and observed at ~434 and ~413 cm-1, respectively. The second conformer exhibits a v[Fe-CN] mode at lower frequency than the δ[Fe-C-N] mode, and is thus characteristic of a bent Fe- C-N linkage. The υ[Fe-CN] and δ[Fe-C-N] modes were identified at 349 and 445 cm-1, respectively, for BLC-CN, and at 350 and 456 cm-1, respectively, for ANC-CN. The two conformers persist in the pH range 4.9- 11.5. Furthermore, upon raising the pH to 11.5, the v[Fe-CN] mode of the linear conformer of BLC-CN downshifts to 429 cm-1 while that of the bent conformer remains unchanged. The observed pH-dependent shift is attributed to the deprotonation of a distal-side amino acid residue, probably a distal histidine. The Fe-C-N axial vibrations of the two conformers identified for ANC-CN did not show any significant pH-dependent shifts, indicating a more stable hydrogen bonding interaction relative to BLC-CN.