Amino acid hemoproteins

NADPH)-cytochrome P450 reductase. The other is P450arom. a hemoprotein of 530 amino acids with a molecular weight of ca. 55 kDa. 

The development of synthetic enzymes and proteins has also been achieved through the preparation of structurally defined peptide nanostructures. A nice example, reported by DeGrado and co workers [67], is the construction (Fig. 27) of a four a-helix bundle system (72) that was shown to complex four metalloporphyrins by their axial coordination with the imidazole of the properly oriented histidines. This type of structure could be used as an artificial photosynthetic center. Along the same lines, Benson and co-workers [68] recently prepared a miniature hemoprotein, 73, by linking two units of a 13-amino acid peptide to a porphyrin. UV-visible and CD studies confirmed that the metalloporphyrin is indeed sandwiched between the a-helical peptides, as depicted in 73. 

Amino acid side-chains may have a role in electron transfer in proteins through the well-known hopping pathway . In this process electrons could move between certain residues such as tyrosine and tryptophan, with the generation of free radical intermediates. Such free radical residues are known in certain high oxidation state species of hemoproteins. 

A more detailed treatment, including the effects of spin delocalization on the pseudocontact shifts, might be warranted once single crystal EPR data will become available for several of the low spin ferric heme compounds. In the hemoproteins it would then be of special interest to investigate pseudocontact shifts for amino acid residues near the heme groups which could yield structural information in a similar way as the ring current shifts. 

All the proteins described in this chapter and that catalyze very different reactions involving 02 exhibit the following common characteristics (1) the presence of an iron porphyrin cofactor, (2) an axial iron ligand coming from the protein that is in most cases a histidine imidazole (except for cytochrome P-450), and (3) the intermediate formation of high-valent iron-oxo species, formally equivalent to Fe(IV)=0 or Fe(V)=0, which are key intermediates in the enzymatic reactions. The different reactions performed by these iron-oxo species and the different uses of 02 by the hemoproteins are due to the very different environments of the heme in these hemoproteins and to a more or less rapid electron transfer. As a function of its environment, the iron-oxo species may either be reduced into H20 if electrons are easily transferred to the heme or oxidize a substrate or a protein amino acid that could be present in close proximity in the active site (Figure 14). 

Figure 14 Main possible fates of the high-valent iron-oxo species formed as intermediates in 02-aetivating hemoproteins (RH is a substrate AmAH is an amino acid residue close to the heme in the active site).

By treatment with 5 M urea and chromatography on DEAE-cellulose, it has been possible to dissociate the E. coli NADPH-sulfite reductase into a flavoprotein and a hemoprotein fraction. The flavoprotein fraction has been shown to be an octamer of a single polypeptide of molecular weight 58,000-60,000 and to contain FMN and FAD in equimolar amounts, but no heme, nonheme iron, or labile sulfide. The hemoprotein fraction is a tetramer of a polypeptide of molecular weight 54,000-67,000, and contains heme, nonheme iron, and labile sulfide, but no flavin. Thus NADPH-sulfite reductase is considered to be an enzyme of agfit subunit composition. The amino acid composition of the whole enzyme and the flavoprotein and hemoprotein fractions have been determined (414). 

Hemoglobin, absorption spectra, 317-319 Hemoprotein(s), amino acid sequences, 371... 

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