Evidence for a Proximal Histidine Interaction in the Structure of Cytochromes c‘ in Solution:  A Resonance Raman Study

Abstract

Soret-excited resonance Raman (RR) spectra of oxidized and reduced cytochromes ć from Rhodospirillum molischianum and Rhodobacter sphaeroides, in solution, are reported. The spectra of the type I ferricytochromes ć in both species contain different extents of two forms. One of these is readily assignable to a "normal" five-coordinated high-spin heme. The second species with v3 and v10 modes at 1502 and 1635 cm-1, respectively, is attributed to a five-coordinated intermediate-spin heme. The RR data show that the equilibrium between these two forms is species-dependent at neutral pH and 20 degrees C. The v(Fe-His) mode of the a form of reduced cytochromes ć is assigned to a band at 228-231 cm-1, indicating that the proximal His has a strong electronegative character. X-ray crystallographic data on R. molischianum ferricyt ć show that the proximal His has no interaction with either the protein or water molecules [Finzel, B.C., Weber, P.C., Hardman, K.D., & Salemme, F.R.(1985) J. Mol. Biol. 186, 627-643]. Considering that the absence of H bonding at the coordinated histidine corresponds to a low frequency for the v(Fe-His) mode (195-205 cm-1), the structure and/or environment of the proximal histidine appears different for cyt ć (III) in the crystal and cyt ć (II) in aqueous solution. To account for the elevated frequency of the v(Fe-His) mode of cyt ć (II), several possibilities have been examined. Among these, we propose that a conserved Lys residue, located in the protein sequence three residues before the His ligand, can form an electrostatic interaction with the (His)N1 atom, directly or through a water molecule. It is further suggested that this electrostatic interaction could also play a role in the high-spin <--> intermediate-spin equilibrium of oxidized cytochromes ć.

The pH Dependent Protein Structure Transitions and Related Spin-State Transition of Cytochrome c′ from Alcaligenes xylosoxidans NCIMB 11015

“…The imidazole group of the heme ligand (His120) is exposed to solvent, and the N δ H makes a hydrogen bond with water in the structure of AxCyt c¤ and other Cyt c¤ species. 12,31,58 The correlation between the imidazoleH 2 O interaction and Fe(III) heme oxidation states has been reported. 57 The H 2 O molecule forming the hydrogen bond with His120 would become OH ¹ at alkaline pH in AxCyt c¤.…”

“…58 The imidazole orientation strongly reduces the effect of the His electronegativity on the ν(FeHis) frequency. The lower frequency shift of the ν(Fe His) also reflects the weaker FeHis bond.…”

Activation of Heme-regulated Eukaryotic Initiation Factor 2α Kinase by Nitric Oxide Is Induced by the Formation of a Five-coordinate NO-Heme Complex

“…In general, the Fe-His frequency is sensitive to a number of factors, including the hydrogen-bonding status of N ␦ nitrogen, strain imposed on the axial His by the protein moiety, and the geometry of bound imidazole (47)(48)(49)(50). For example, the high Fe-His frequency at 245 cm Ϫ1 for cytochrome c peroxidase (a result of the strong hydrogen bond between the axial His and Asp 235 ) shifts to 205 cm Ϫ1 following disruption of the hydrogen bond (51,52).…”

“…The Fe-imidazole frequency of the cavity mutant shifted to 225 cm Ϫ1 , and a geometrical change of imidazole was observed concomitantly in that dihedral angles () between the projection of imidazole plane and the nearest N(pyrrole)-Fe-N(pyrrole) axis altered from 0°(wild type) to 45°(H93G) (54 -56). Therefore, the geometry of His bound to the heme iron is also an important factor in the origin of the Fe-His frequency associated with hydrogen-bonded imidazole coordination (49 the imidazole ring of His 120 is oriented such that is ϳ33°in the Fe(II) state (57). In the crystal structure of ovine prostaglandin endoperoxide H synthase, the imidazole ring of His 388 is oriented such that is ϳ26°in the Fe(III) state (50).…”

The Critical Role of the Proximal Calcium Ion in the Structural Properties of Horseradish Peroxidase

“…This heme species is uncommon in biological systems. Known examples include the neutral pH form of cytochrome cЈ (51,52) and class III peroxidases (18, 23). Unfortunately, the biological significance and structural properties of the heme that give rise to the QS state remain obscure (24).…”