Substituted hemes

Fig. 3. Absorbance spectra of ferri- and ferro-heme-hemopexin in the Soret and visible regions. The spectra of rabbit ferri- and ferro-mesoheme-hemopexin (solid hne and dashed line, respectively) are shown and are typical of low-spin bis-histidyl heme proteins. Other species display similar spectra, and a variety of other 2,4-substituted heme analogs are also bound by hemopexin.

Fig. 10. a-meso-Methyl-substituted heme groups and their oxidation to biliverdin products. The substituent R in the heme structure is a methyl in the symmetric porphyrin and an ethyl in mesoheme. 

If the a-meso regioselectivity of hydroxylation is not inherent to the heme group, as indicated by the fact that all four isomers are obtained by coupled heme oxidation (40), how is the regiochemistry of heme oxidation influenced by electronic factors One possibility is that the puckering or ruffling of the porphyrin caused by steric clashes between the meso and flanking substituents in the meso-substituted heme probes... 

Figure 4 Introduction of non-native metal Schiff base and substituted heme cofactors by noncovalent attachment in myoglobin. (Adapted from...

A commonexperimental strategy for studying electron transfers between proteins uses a metal-substituted heme protein as one of the reactants. In particular, the substitution of zinc for iron in one of the porphyrin redox centers allows facile initiation of electron transfer through photoexcitation of the zinc porphyrin (ZnP). The excited zinc porphyrin, ZnP in Equation (6.32),... 

Other Studies of Heme Proteins. Monzani et al. have used H relaxometry to study Mn(III)-substituted heme complexes with cyanogen bromide fragments of human serum albumin. 

The long-range electron transfer reactions in ruthenium-modified myoglobin, in which the labile heme unit has been replaced by various metalloporphyrins, have been reviewed. The reductions of the [Ru(NH3)5] moiety, attached at His-48, by Pd- and Pt-substituted hemes in myoglobin proceed at rates of 9 1 x 1(P and 1.2x lO s", respectively. The difference in rates for electron transfer between Fe (heme) and Mg or Zn(porphyrin) centers in [a(Fe(II)P),j3(M T)] hemoglobin hybrids indicates a direct process as opposed to the involvement of a conformational gate. Using [Co(NH3)5Cl] to quench the Zn state, a rate constant of 2.4 x 10 s has been measured for back electron transfer within [a(Zn- -P)i8(Fe(III)CN)]. ... 

Bruha A, Kincaid JR (1988) Resonance Raman studies of dioxygen adducts of cobalt-substituted heme proteins and model compounds. Vibrationally coupled dioxygen and the issues of multiple structures and distal side hydrogen bonding. J Am Chem Soc 110 6006-6014... 

Independent proofs of a varying electron density at iron, e.g. as a function of 2,4 substituents, may be given by chemical shifts of core level excitation energies. ESC A studies on ferrous carbonyl compounds cannot be performed due to the high rate of CO dissociation. More stable ferric compounds, e.g. //-oxo-bishemins, are on the other hand less sensitive to the relatively small perturbations by peripheral substituents. EXAFS edge studies may thus prove to be a more efficient tool. ESCA studies of BF4 doped polypyrrole have shown that the removal of one electron from each unit of four pyrrol groups is completely smeared out over the entire unit much in the same manner as is expected in the present electron-gas model of substituted hemes [30]. 

---