Absorption spectra hemes

This report was complemented by a mediated electrochemistry study using horse cytochrome c as the electron acceptor. This was followed up by an unmediated thin layer optical spectroelectrochemistry study of chicken liver SO using a Au electrode modified by 4-pyridinethiol (Aldrithiol). Reversible changes in the heme absorption spectrum were seen and the caleulated heme redox potential (-115 mV vs. Ag/AgCl) was consistent with previous studies using optical potentiometry and cyclic voltammetry. ... 

The biochemical activity and accessibility of biomolecule-intercalated AMP clays to small molecules was retained in the hybrid nanocomposites. For example, the absorption spectrum of the intercalated Mb-AMP nanocomposite showed a characteristic soret band at 408 nm associated with the intact prosthetic heme group of the oxidised protein (Fe(III), met-myoglobin) (Figure 8.9). Treatment of Mb with sodium dithionite solution resulted in a red shift of the soret band from 408 to 427 nm, consistent with the formation of intercalated deoxy-Mb. Reversible binding of CO under argon to the deoxy-Mb-AMP lamellar nanocomposite was demonstrated by a shift in the soret band from 427 to 422 nm. Subsequent dissociation of CO from the heme centre due to competitive 02 binding shifted the soret band to 416nm on formation of intercalated oxy-Mb. 

A quite different approach came from Chance and others using heme enzymes (1947). Purified horseradish peroxidase has a characteristic absorption spectrum which was visibly altered in the presence of hydrogen peroxide. When an appropriate substrate was added it was oxidized by the hydrogen peroxide and the spectrum reverted to that of the original state of the enzyme. Similar studies were performed with catalase, showing that prosthetic groups in enzymes underwent reversible changes in the course of their reactions. 

However, in contrast to the human His25Ala HO-l heme complex, which has no detectable activity in the absence of imidazole (78), the His20Ala Hmu O rheme complex in the presence of NAD PH and NADPH-cytochrome P450 reductase was foimd to catalyze the initial meso-hydroxylation of the heme (151). The product of the reaction was Fe verdoheme, as judged by the electronic absorption spectrum and the detection of carbon monoxide as a product of the reaction. Hydrolytic conversion of the verdoheme product to biliverdin and subsequent HPLC analysis confirmed that the oxidative cleavage of the porphyrin macrocycle was specific for the a-meso-carbon. 

There is some experimental evidence showing the effects of heme-linked ionization on lignin peroxidase. The redox potential of the Fe +/Fe2+ couple of lignin and Mn-dependent peroxidases is pH-dependent, as is the absorption spectrum of the ferrous form of the lignin and Mn-dependent peroxidases (6). The pKa values determined from both experiments were 6.6. Detailed studies were performed studying O2 binding to the ferrous lignin peroxidase (79). The pKa for the ionization was measured at different... 

Knowing that carbon monoxide complexes of hemes are dissociated by light, Warburg and Negelein, in 1928, determined the photochemical action spectrum (see Chapter 23) for reversal of the carbon monoxide inhibition of respiration of the yeast Torula utilis. The spectrum closely resembled the absorption spectrum of known heme derivatives (Fig. 16-7). Thus, it was proposed that 02, as well as CO, combines with the iron of the heme group in the Atmungsferment. 

In IQQS.Bartsch 15) isolated from the photosynthetic bacterium, Chro-matium, a brown protein which contained non-heme iron, but was not a ferredoxin. It differed from ferredoxin by absorption spectrum, redox potential, and EPR signal at g = 1.94 Bearden et al. 17)). 

The CD spectra of peroxidases in the heme absorption region are highly sensitive to the conformation of the heme group inside the protein cavity and to the dipole-dipole coupling interactions between the porphyrin and the surrounding aromatic amino acid side chains [122, 123]. In this region, GS horseradish A2 peroxidase presents a band at 311 and the intense CD Soret band at 412 nm with a shoulder at 354 nm (Fig. 11.3). The main differences between the CD spectrum of CIII compared to that of GS are a reduction of the CD band at 311 nm and a red-shift of the Soret CD band to 430 nm with the disappearance of the 354 nm shoulder. Additionally a broad band, possibly composed by a series of shoulders, appears from 350 to 400 nm. During spontaneous decay, the CD spectrum of CIII slowly returns to that of GS, with the characteristic reduction of the Soret band due to porphyrin destruction (see above). 

Little is known about the redox partners of rusticyanin, although a diheme cytochrome, cyt c4, has been imphcated because of its abihty to form a complex with rusticyanin. The complex formation between these two proteins at pH 4.8 induces a dramatic decrease, by almost 100 mV, in the redox potential of rusticyanin, while the potentials of both hemes in the cytochrome remain unchanged. Interestingly, complex formation is also accompanied by changes in the electronic absorption spectrum of rusticyanin that are reminiscent of those observed for the uncomplexed protein at high pH (above pH 7.0) (Giudici-Orticoni et al., 1999). 

The absorption spectrum of Cua in the visible-Soret region is much weaker than the spectra of the two hemes. However, it is well established that Cua in the oxidized state contributes to the broad absorption band near 830 nm significantly (Wharton and Tzagoloff, 1964 Beinert et al., 1980). However, the contribution of heme a to the 830-nm band should not be ignored (Caughey et al., 1976). 

The sixth coordination position of the heme iron in cytochrome c peroxidase, which is normally occupied by H2O, is available for reactions with extraneous ligands such as fluoride, cyanide, and azide, as well as substrates and substrate analogs. The acidic-alkaline transition of a hemo-protein, which is caused by the ionization of the bound water ligand, is usually accompanied by significant spectral changes. However, the visible absorption spectrum of cytochrome c peroxidase is not appreciably... 

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