Modeling low-pH hemoproteins

• Published in: The Journal of biological chemistry 1989-11, Vol.264 (32), p.19099-19102
• Main Authors: Andersson, L A, Mylrajan, M, Sullivan, E P, Strauss, S H
• Format: Article
• Language: English
• Subjects: Analytical, structural and metabolic biochemistry; Biological and medical sciences; Cytochromes b5; Fundamental and applied biological sciences. Psychology; Hemeproteins; Hemoproteins; Hydrogen-Ion Concentration; Metalloproteins; Models, Chemical; Porphyrins; Proteins; Raman spectroscopy; Spectrum Analysis, Raman - methods
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/S0021-9258(19)47272-4

A tetracoordinate ferrous heme (iron-porphyrin) has been proposed as an intermediate at low pH (< 3.0) for respiratory hemoproteins, peroxidases, and model heme complexes. This intermediate is believed to arise via protonation of the N(≥) atom of the proximal histidine and consequent cleavage of the Fe-N(ϵ) bond. To establish a spectral signature for the proposed low-pH tetracoordinate species, we have obtained Soret excitation resonance Raman spectra on samples of crystallographically defined, tetracoordinate iron(II)-octaethylporphyrin (Fe.OEP; S = 1). The high-frequency (greater than or equal to 900 cm−1) resonance Raman spectral features of Fe.OEP are clearly distinct from those of high-spin pentacoordinate or low-spin hexacoordinate ferrous hemes. Rather, they are at frequencies more typically observed for low-spin hexacoordinate ferric porphyrins. Comparative spectral analysis of tetracoordinate Fe.OEP and other proposed tetracoordinate ferrous hemes (e.g. iron(II)-protoporphyrin IX) demonstrates little or no macrocycle effect on the resonance Raman frequencies above 900 cm−1. This work thus serves to provide a testable spectral signature by which the existence of the proposed tetracoordinate biological intermediate may be verified and by which its functional significance may be tested.