Ligand specificity of group I biotin protein ligase of Mycobacterium tuberculosis

Abstract

Background: Fatty acids are indispendable constituents of mycolic acids that impart toughness & permeability barrier to the cell envelope of M. tuberculosis is an essential co-factor for acetyl-CoA carboxylase (ACC) the enzyme involved in the synthesis of malonyl-CoA, a commited precursor, needed for fatty acid synthesis. Biotin carboxyl carrier protein (BCCP) provides the co-factor for catalytic activity of ACC. Methodology/Principal Findings: BPL/BirA (Biotin Protein Ligase), and its substrate, biotin carboxyl carrier protein (BCCP) of Mycobacterium tuberculosis (Mt), were cloned and expressed in E. coli BL21. In contrast to EcBirA and PhBPL, the ∼29.5 kDa MtBPL exists as a monomer in native, biotin and bio-5′AMP liganded forms. This was confirmed by molecular weight profiling by gel filtration of Superdex S-200 and Dynamic Light Scattering (DLS). Computation docking of biotin and bio-5′AMP to MtBPL show that adenylation alters the contact residues for biotin. MtBPL forms 11 H-bonds with biotin, relative to 35 with bio-5′AMP. Docking simulation also suggest that bio-5′AMP hydrogen bonds to the conserved 'GRGRRG' sequence but no biotin. The enzyme catalyzed transfer of bioton to BCCP was confirmed by incorporation of radioactive biotin and by Avidin blot. The Km for BCCP was ∼5.2 μM and ≄20 nM for biotin. MtBPL has low affinity (Kb = 1.06 × 10-6 M) for biotin relative for EcBirA but their Km are almost comparable suggesting that while the major function of MtBPL is biotinylation of BCCP, tight binding of biotin/bio-5′AMP by EcBirA is channeled for its repressor activity. Conclusions/Significance: Three studies thus open up avenues for understanding the unique features of MtBPL and the role it plays in biotin utilization in M. tuberculosis. © 2008 Purushothaman et al.