The Mycobacterium tuberculosis cytochrome P450 system

Abstract

Tuberculosis remains a leading cause of human mortality. The emergence of strains of Mycobacterium tuberculosis, the causative agent, that are resistant to the major frontline antitubercular drugs increases the urgency for the development of new therapeutic agents. Sequencing of the M. tuberculosis genome revealed the existence of 20 cytochrome P450 enzymes, some of which are potential candidates for drug targeting. The recent burst of studies reporting microarray-based gene essentiality and transcriptome analyses under in vitro, ex vivo and in vivo conditions highlight the importance of selected P450 isoforms for M. tuberculosis viability and pathogenicity. Current knowledge of the structural and biochemical properties of the M. tuberculosis P450 enzymes and their putative redox partners is reviewed, with an emphasis on findings related to their physiological function(s) as well as their potential as drug targets.

Figure 1. General P450 catalytic cycle depicting oxygen activation and subsequent hydroxylation of substrate (RH) into product (ROH)

For simplicity the cysteine thiolate proximal ligand is omitted and thick horizontal lines denote the heme protoporphyrin framework. The shunt pathway is depicted by the dotted line and Compound I is highlighted in blue.

Figure 2. Schematic representation of the cell envelope of Mtb

Figure 3. Phylogenetic analysis of the amino acid sequences of the twenty Mtb P450 enzymes

Figure 4. Crystal structure of ligand-free and econazole-bound CYP130A1

(A) The dimerization interface of the econazole-bound structure (2000 ų) is formed largely via interactions between the G helices in anti-parallel orientations, overlapping N termini of the I helices, and multiple contacts in the BC-loop region. The monomers are colored in green and blue, heme is in blue and red, and econazole in yellow. (B) Superimposition of the active site of ligand-free (green cartoon) and econazole-bound (cyan cartoon) structures. The residues P87, M89 and Y392 are in contact with the ligand. The heme for the ligand-free form is in red while that of the econazole-bound form is in blue. The econazole ligand is colored in yellow. The pdb files for the ligand-free and econazole-bound structures are 2UUQ and 2UVN, respectively.

Figure 5. Structures of the active sites of the (A) fluconazole- and (B) cYY-bound CYP121A1 complexes and (C) the chemical structures of the proposed substrate and reaction product in the CYP121A1-catalyzed reaction

For the fluconazole-bound structure, a conformer (conformer B) in which the ligand is not coordinated was chosen. The respective ligands are colored in yellow. The water molecules are labeled w1 to w5. In the fluconazole-bound structure, the azole moiety of the ligand occupies the position of w2. The pdb files for the fluconazole- and the cYY-bound structure are 2IJ7 and 3G5H, respectively.

Figure 6. Biosynthesis of S881

As proposed by Holsclaw et al. the first synthetic step would be the hydroxylation of the terminal ω-position of MK-9 (DH2) by CYP128, followed by PAPS-dependent sulfation by Sft3 to form the S881 sulfolipid.