Structure of Type-I Mycobacterium tuberculosis fatty acid synthase at 3.3 Å resolution.

Tuberculosis (TB) is a devastating and rapidly spreading disease caused by Mycobacterium tuberculosis (Mtb). Therapy requires prolonged treatment with a combination of multiple agents and interruptions in the treatment regimen result in emergence and spread of multi-drug resistant (MDR) Mtb strains. MDR Mtb poses a significant global health problem, calling for urgent development of novel drugs to combat TB. Here, we report the 3.3 Å resolution structure of the ~2 MDa type-I fatty acid synthase (FAS-I) from Mtb, determined by single particle cryo-EM. Mtb FAS-I is an essential enzymatic complex that contributes to the virulence of Mtb, and thus a prime target for anti-TB drugs. The structural information for Mtb FAS-I we have obtained enables computer-based drug discovery approaches, and the resolution achieved by cryo-EM is sufficient for elucidating inhibition mechanisms by putative small molecular weight inhibitors.

Expression of a recombinant, 4'-Phosphopantetheinylated, active M. tuberculosis fatty acid synthase I in E. coli.

BACKGROUND: Fatty acid synthase 1 (FAS I) from Mycobacterium tuberculosis (Mtb) is an essential protein and a promising drug target. FAS I is a multi-functional, multi-domain protein that is organized as a large (1.9 MDa) homohexameric complex. Acyl intermediates produced during fatty acid elongation are attached covalently to an acyl carrier protein (ACP) domain. This domain is activated by the transfer of a 4'-Phosphopantetheine (4'-PP, also termed P-pant) group from CoA to ACP catalyzed by a 4'-PP transferase, termed acyl carrier protein synthase (AcpS). METHODS: In order to obtain an activated FAS I in E. coli, we transformed E. coli with tagged Mtb fas1 and acpS genes encoded by a separate plasmid. We induced the expression of Mtb FAS I following induction of AcpS expression. FAS I was purified by Strep-Tactin affinity chromatography. RESULTS: Activation of Mtb FAS I was confirmed by the identification of a bound P-pant group on serine at position 1808 by mass spectrometry. The purified FAS I displayed biochemical activity shown by spectrophotometric analysis of NADPH oxidation and by CoA production, using the Ellman reaction. The purified Mtb FAS I forms a hexameric complex shown by negative staining and cryo-EM. CONCLUSION: Purified hexameric and active Mtb FAS I is required for binding and drug inhibition studies and for structure-function analysis of this enzyme. This relatively simple and short procedure for Mtb FAS I production should facilitate studies of this enzyme.

A HPLC method for the quantitative determination of N-(2-hydroxy-5-nitrophenylcarbamothioyl)-3,5-dimethylbenzamide in biological samples.

A sensitive and simple HPLC method was developed for the determination of a novel compound, a potential anti-cancer drug, N-(2-hydroxy-5-nitrophenylcarbamothioyl)-3,5-dimethylbenzamide (DM-PIT-1), a member of the new structural class of non-phosphoinositide small molecule antagonist of phosphatidylinositol-3,4,5-trisphosphate-pleckstrin-homology domain interactions, in mouse plasma and tumor tissue homogenates. The chromatographic separation of DM-PIT-1 was achieved on C18 column using isocratic elution with acetonitrile-water (70:30) containing 0.1% formic acid (v/v). DM-PIT-1 was detected by UV absorbance at 320 nm and confirmed by LC-MS. The extraction of the DM-PIT-1 from the plasma and tumor tissue with methylene chloride resulted in its high recovery (70-80%). HPLC calibration curves for DM-PIT-1 based on the extracts from the mouse plasma and tumor tissue samples were linear over a broad concentration range of 0.25-20 µg/ml/g, with intra/inter-day accuracy of 95% and the precision of variation below 10%. The limits of detection and quantification were 0.1 ng and 0.2 ng, respectively. The described method was successfully applied to study the pharmacokinetics of the DM-PIT-1 following the parenteral injections of DM-PIT-1 entrapped in 1,2-disteratoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene-glycol)-2000] (PEG-PE) micelles.

Cell-penetrating TAT peptide in drug delivery systems: proteolytic stability requirements.

The stability and activity of the HIV cell-penetrating TAT peptide (TATp) on the surface of TATp-modified micelles and liposomes in relation to its proteolytic cleavage was investigated. TATp moieties were attached to the surface of these nanocarriers using TATp modified with a conjugate of phosphatidyl ethanolamine with a 'short' PEG (PEG-PE). Following pre-incubation with trypsin, elastase, or collagenase, the proteolytic stability of TATp on the surface of these modified carriers was studied by HPLC with fluorescence detection using fluorenylmethyl chloroformate (FMOC) labeling. All tested enzymes produced a dose-dependent cleavage of TATp as shown by the presence of TATp Arg-Arg fragments. Inhibition of TATp cleavage occurred when these TATp-micelles were modified by the addition of longer PEG-PE blocks, indicating an effective shielding of TATp from proteolysis by these blocks. TATp-modified carriers were also tested for their ability to accumulate in EL-4, HeLa, and B16-F10 cells. Trypsin treatment of TATp-modified liposomes and micelles resulted in decreased uptake and cell interaction, as measured by fluorescence microscopy and fluorescence-activated cell sorting techniques. Furthermore, a decrease in the cytotoxicity of TATp-modified liposomes loaded with doxorubicin (Doxil) was observed following trypsin treatment. In conclusion, steric shielding of TATp is essential to ensure its in vivo therapeutic function.

TAT peptide and its conjugates: proteolytic stability.

The proteolytic cleavage of TATp, TATp-PEG(1000)-PE conjugate (TATp-conjugate), and TATp as TATp-conjugate in mixed micelles made of TATp-conjugate and PEG(5000)-PE (2.5% mol of TATp-conjugate, TATp-Mic) were studied by HPLC with fluorescent detection using fluorenylmethyl chloroformate (FMOC) labeling and by MALDI-TOF MS analysis. The cleavage kinetics were analyzed in human blood plasma and in trypsin-containing phosphate buffered saline (PBS), pH 7.4, to simulate the proteolytic activity of human plasma. The trypsinolysis of free TATp, TATp-conjugate, and TATp-Mic revealed that the main initial fragmentation is an endocleavage at the carboxyl terminus resulting in an Arg-Arg (RR) dimer. The trypsinolysis followed pseudo-first-order kinetics. The cleavage of the free TATp was relatively fast with a half-life of a few minutes (t(1/2) ∼ 3.5 min). The TATp-conjugate showed more stability with about a 3-fold increase in half-life (t(1/2) ∼ 10 min). TATp in TATp-Mic was highly protected against proteolysis with an over 100-fold increase in half-life (t(1/2) ∼ 430 min). The shielding of TATp by PEG moieties in the proposed TATp-Mic is of great importance for its potential use as a cell-penetrating moiety for multifunctional "smart" drug delivery systems with detachable PEG.

Distribution of amphotericin B-arabinogalactan conjugate in mouse tissue and its therapeutic efficacy against murine aspergillosis.

The pharmacokinetic characteristics and tissue distribution of a novel water-soluble, nontoxic amphotericin B-arabinogalactan (AMB-AG) conjugate exhibited distinct differences from those of micellar and liposomal amphotericin B formulations. These differences included extended persistence of drug in the body and its accumulation in the lungs; thus, the AMB-AG conjugate was highly effective in treating systemic murine aspergillosis.