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Heme iron centers are found in a wide range of proteins where they play different roles for many crucial biological processes, including catalysis. Among heme-containing enzymes, the cytochromes P450 superfamily comprises members distributed in all domains of life where they participate in the metabolism of endogenous and exogenous compounds. These enzymes can perform a series of oxidative reactions on a broad range of chemically different substrates and for this reason are optimal candidates for biocatalytic purposes and, in general, technological applications. In this review, the general features of these enzymes will be discussed with particular emphasis on the structural insights obtained through X-ray crystallography to understand the key steps of their catalytic mechanism where oxygen is activated. Moreover, one of the finest multi-step reactions catalyzed by the cytochrome P450 aromatase, dealing with the conversion of androgens into estrogens, will be discussed in details. The X-ray structure of this enzyme, together with site-directed mutagenesis experiments has elucidated the role of key residues involved in substrate binding and catalysis. This last example shows how the function and structure in cytochromes P450 are closely inter-correlated to achieve a complex finely tuned catalytic mechanism. It is possible to exploit them for biotechnological applications, but careful attention must be paid in not altering their delicate structure on which their correct function strictly depends. [ABSTRACT FROM AUTHOR]

This review covers our current knowledge on catalytically self-sufficient cytochromes P450. These enzymes present all the advantages of the regio- and stereo-selectivity of typical P450s combined with the fact that, due to the natural fusion with their reductases, their application in biocatalysis does not need the addition of other proteins: only NAD(P)H and substrate are needed to trigger catalysis. To this date only few catalytically self-sufficient P450s have been isolated from bacteria, fungi and plants. Their low number does not detract from their importance, as the reactions they catalyse and the turnover numbers of which they are capable (in some cases higher than 15,000 s) makes them highly relevant to the chemical and pharmaceutical industry. These enzymes will be covered in this review, together with their specific reactions relevant to the field of biocatalysis. [ABSTRACT FROM AUTHOR]

Abstract: Cytochrome bd is a prokaryotic respiratory quinol:O2 oxidoreductase, phylogenetically unrelated to the extensively studied heme–copper oxidases (HCOs). The enzyme contributes to energy conservation by generating a proton motive force, though working with a lower energetic efficiency as compared to HCOs. Relevant to patho-physiology, members of the bd-family were shown to promote virulence in some pathogenic bacteria, which makes these enzymes of interest also as potential drug targets. Beyond its role in cell bioenergetics, cytochrome bd accomplishes several additional physiological functions, being apparently implicated in the response of the bacterial cell to a number of stress conditions. Compelling experimental evidence suggests that the enzyme enhances bacterial tolerance to oxidative and nitrosative stress conditions, owing to its unusually high nitric oxide (NO) dissociation rate and a notable catalase activity; the latter has been recently documented in one of the two bd-type oxidases of Escherichia coli. Current knowledge on cytochrome bd and its reactivity with O2, NO and H2O2 is summarized in this review in the light of the hypothesis that the preferential (over HCOs) expression of cytochrome bd in pathogenic bacteria may represent a strategy to evade the host immune attack based on production of NO and reactive oxygen species (ROS). This article is part of a Special Issue entitled: 18th European Bioenergetic Conference. [Copyright &y& Elsevier]

The nerve tissue mini-hemoglobin from Cerebratulus lacteus (CerHb) displays an essential globin fold hosting a protein matrix tunnel held to allow traffic of small ligands to and from the heme. CerHb heme pocket hosts the distal TyrB10/GlnE7 pair, normally linked to low rates of O2 dissociation and ultra-high O2 affinity. However, CerHb affinity for O2 is similar to that of mammalian myoglobins, due to a dynamic equilibrium between high and low affinity states driven by the ability of ThrE11 to orient the TyrB10 OH group relative to the heme ligand. We present here the high resolution crystal structures of CerHb in the unligated and carbomonoxy states. Although CO binds to the heme with an orientation different from the O2 ligand, the overall binding schemes for CO and O2 are essentially the same, both ligands being stabilized through a network of hydrogen bonds based on TyrB10, GlnE7, and ThrE11. No dramatic protein structural changes are needed to support binding of the ligands, which can freely reach the heme distal site through the apolar tunnel. A lack of main conformational changes between the heme-unligated and -ligated states grants stability to the folded mini-Hb and is a prerequisite for fast ligand diffusion to/from the heme. [ABSTRACT FROM AUTHOR]

Protoglobin from Methanosarcina acetivorans C2A ( MaPgb), a strictly anaerobic methanogenic Archaea, displays peculiar structural and functional properties within members of the hemoglobin superfamily. In fact, MaPgb-specific loops and a N-terminal extension (20 amino acid residues) completely bury the heme within the protein matrix. Therefore, the access of diatomic gaseous molecules to the heme is granted by two apolar tunnels reaching the heme distal site from locations at the B/G and B/E helix interfaces. The presence of two tunnels within the protein matrix could be partly responsible for the slightly biphasic ligand binding behavior. Unusually, MaPgb oxygenation is favored with respect to carbonylation. Here, the crucial role of Tyr(B10)61 and Ile(G11)149 residues, located in the heme distal site and lining the protein matrix tunnels 1 and 2, respectively, on ligand binding to the heme-Fe-atom and on distal site structural organization is reported. In particular, tunnel 1 accessibility is modulated by a complex reorganization of the Trp(B9)60 and Phe(E11)93 side-chains, triggered by mutations of the Tyr(B10)61 and Ile(G11)149 residues, and affected by the presence and type of the distal heme-bound ligand. © 2011 IUBMB IUBMB Life, 63(5): 287-294, 2011 [ABSTRACT FROM AUTHOR]