Haemoglobins of Mycobacteria: structural features and biological functions.

The genus Mycobacterium is comprised of Gram-positive bacteria occupying a wide range of natural habitats and includes species that range from severe intracellular pathogens to economically useful and harmless microbes. The recent upsurge in the availability of microbial genome data has shown that genes encoding haemoglobin-like proteins are ubiquitous among Mycobacteria and that multiple haemoglobins (Hbs) of different classes may be present in pathogenic and non-pathogenic species. The occurrence of truncated haemoglobins (trHbs) and flavohaemoglobins (flavoHbs) showing distinct haem active site structures and ligand-binding properties suggests that these Hbs may be playing diverse functions in the cellular metabolism of Mycobacteria. TrHbs and flavoHbs from some of the severe human pathogens such as Mycobacterium tuberculosis and Mycobacterium leprae have been studied recently and their roles in effective detoxification of reactive nitrogen and oxygen species, electron cycling, modulation of redox state of the cell and facilitation of aerobic respiration have been proposed. This multiplicity in the function of Hbs may aid these pathogens to cope with various environmental stresses and survive during their intracellular regime. This chapter provides recent updates on genomic, structural and functional aspects of Mycobacterial Hbs to address their role in Mycobacteria.

Peroxynitrite detoxification by ferryl Mycobacterium leprae truncated hemoglobin O.

During infection, Mycobacterium leprae is faced with the host macrophagic environment limiting the growth of the bacilli. However, (pseudo-)enzymatic detoxification systems, including truncated hemoglobin O (Ml-trHbO), could allow this mycobacterium to persist in vivo. Here, kinetics of peroxynitrite (ONOOH/ONOO(-)) detoxification by ferryl Ml-trHbO (Ml-trHbO-Fe(IV)=O), obtained by treatment with H(2)O(2), is reported. Values of the second-order rate constant for peroxynitrite detoxification by Ml-trHbO-Fe(IV)=O (i.e., of Ml-trHbO-Fe(III) formation; k(on)), at pH 7.2 and 22.0 degrees C, are 1.5x10(4) M(-1) s(-1), and 2.2x10(4) M(-1) s(-1), in the absence of and presence of physiological levels of CO(2) (approximately 1.2x10(-3) M), respectively. Values of k(on) increase on decreasing pH with a pK(a) value of 6.7, this suggests that ONOOH reacts preferentially with Ml-trHbO-Fe(IV)=O. In turn, peroxynitrite acts as an antioxidant of Ml-trHbO-Fe(IV)=O, which could be responsible for the oxidative damage of the mycobacterium. As a whole, Ml-trHbO can undertake within the same cycle H(2)O(2) and peroxynitrite detoxification.

H2O2 and (.)NO scavenging by Mycobacterium leprae truncated hemoglobin O.

Kinetics of ferric Mycobacterium leprae truncated hemoglobin O (trHbOFe(III)) oxidation by H2O2 and of trHbOFe(IV)O reduction by (.)NO and NO2- are reported. The value of the second-order rate constant for H2O2-mediated oxidation of trHbOFe(III) is 2.4 x 10(3) M(-1) s(-1). The value of the second-order rate constant for (.)NO-mediated reduction of trHbOFe(IV)O is 7.8 x 10(6) M(-1) s(-1). The value of the first-order rate constant for trHbOFe(III)ONO decay to the resting form trHbOFe(III) is 2.1 x 10(1) s(-1). The value of the second-order rate constant for NO2--mediated reduction of trHbOFe(IV)=O is 3.1 x 10(3) M(-1) s(-1). As a whole, trHbOFe(IV)O, generated upon reaction with H2O2, catalyzes (.)NO reduction to NO2-. In turn, (.)NO and NO2- act as antioxidants of trHbOFe(IV)O, which could be responsible for the oxidative damage of the mycobacterium. Therefore, Mycobacterium leprae trHbO could be involved in both H2O2 and (.)NO scavenging, protecting from nitrosative and oxidative stress, and sustaining mycobacterial respiration.