Thesis (Ph.D.)(Biochemistry))--University of Pretoria, 2008. / Includes bibliographical references.

Thesis (M. Sc.)(Biochemistry)--University of Pretoria, 2006. / Includes bibliographical references. Available on the Internet via the World Wide Web.

Thesis (Ph. D.)--Washington State University, December 2008. / Title from PDF title page (viewed on July 6, 2009). "School of Molecular Biosciences." Includes bibliographical references.

Dissertation (PhD)--University of Stellenbosch, 2007. / Bibliography. Also available via the Internet.

Dissertation (PhD)--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

Tuberculosis (TB) is a disease that the World Health Organisation (WHO) regards as a global pandemic. There is a great need for new drugs to combat this threat. Drug resistant strains of the causative agent, Mycobacterium tuberculosis (Mtb), have increased the urgency of this quest for novel anti-mycobacterial medicines. Publication of the Mtb genome sequence revealed a large number of cytochrome P450 (CYP) enzymes [Cole, S. T. et al. 1998]. These mono-oxygenase enzymes have been studied for many years and are responsible for metabolic functions in every kingdom of life. Research on the Mtb P450s to date has highlighted several of them as having critcal roles within the organism. CYP121 and CYP128 have been implicated as essential through gene knockout studies. It has been demonstrated that CYP125 is not essential for viability. However, it is part of a gene cluster highly important for Mtb infectivity and virulence. Due to the prospective importance of P450s to Mtb, this group of enzymes is under investigation as a source of novel drug targets. CYP142 was discovered as a potential drug target after it was located to a gene cluster involved in cholesterol catabolism during Mtb dormancy. As part of this PhD project, it was demonstrated that CYP142 performs an almost identical role to that reported for CYP125. These enzymes both perform C27 hydroxylation and carboxylation of the cholesterol side chain. However, variations in the level of oxidation have been identified, dependent upon the redox system with which these P450s are associated. A crystal structure of CYP142 showing high similarity in active site architecture to CYP125 supports the physiological role of CYP142 in cholesterol catabolism. Combining this with in vitro data which demonstrates that CYP142 possesses high affinity for a range of azole anti-fungal agents [Ahmad, Z. et al. 2005, 2006] supports the suggestion that it is a candidate target for the next generation of anti-mycobacterial drugs. CYP144 was highlighted as being important during the latent phase of Mtb growth, a phase that is not targeted by any of the current antimycobacterials. Work performed as part of this PhD has shown that many characteristics of CYP144 are highly comparable to those reported for other MtbP450s. CYP144 shows high affinity and specificity towards many azole molecules. Econazole, clotrimazole and miconazole have repeatedly been shown to bind to MtbP450s, including CYP144 and CYP142, with high affinity and are excellent potential candidates as novel anti-mycobacterial agents. An N-terminally truncated form of CYP144, CYP144-T, has been investigated in the pursuit of a CYP144 crystal structure. It is hoped that this will enable the elucidation of a physiological role for CYP144. Both CYP142 and CYP144 have demonstrated biochemical and biophysical characteristics that contribute to our knowledge of P450 enzymes. This PhD has established that CYP142 exhibits an equilibrium between P450 and P420 species in its CO-bound, ferrous form. A conversion from P420, and stabilisation of P450, upon substrate binding was also demonstrated. CYP144 displays unusual azole coordination characteristics when examined by EPR and removal of the CYP144 gene from Mtb increased sensitivity of the strain to clotrimazole. Studies of these enzymes has advanced knowledge of P450 and Mtb redox chemistry, established roles for the MtbP450 cohort and identified the potential of anti-mycobacterial drugs and associated targets.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (leaves [135]-168).

Tuberculosis (TB) affects nearly a third of the world’s population and has been termed a ‘Global Emergency’ by the WHO. The emergence of multi/extensively drug resistant (M/XDR) strains of Mycobacterium tuberculosis (Mtb), the causative agent of TB, and the increasing incidences of azole drug resistant sterol demethylases (CYP51) from pathogenic fungi has propelled studies to understand mechanisms of azole drug resistance on the drug target CYP51. Since Mtb is devoid of a sterol biosynthetic pathway, the presence and study of CYP51B1 and 19 other Cytochrome P450s in its genome is important to clarify host-pathogen mechanism of infection and the potential of using azole drugs to treat TB. In this study, CYP51B1 from Mtb was used as the model enzyme to study CYP51 mutants from Candida albicans fluconazole-resistant clinical strains. By protein engineering methods, F89H, L100F, S348F, G388S and R391K CYP51B1 mutants were made and azole drug binding properties were investigated using stopped-flow kinetics and static equilibrium methods. Dissociation constant (Kd) values were derived for a range of commercially available azole drugs by fitting the equilibrium binding data to a hyperbolic equation. Kd values for stopped-flow kinetics were derived by plotting observed binding rates (kobs) across different azole drug concentrations against time, followed by fitting multiple kobs data to a linear equation to derive azole drug de-binding (koff) and binding (kon) rate constants – the Kd was obtained by koff/kon. Extinction coefficient for heme b content in mutants and Wild Type (WT) CYP51B1 were an average of ɛ419 = 96.1 mM-1 cm-1. Biochemical characterisation of the mutants were carried out using established experiments on CYP51 – reduction of Fe(III)-heme to Fe(II)-heme, NO binding to Fe(III)-heme, rates of CO-Fe(II) adduct formation and rates of collapse of the P450 to P420 species in the presence of CO and estriol with redox partners from Mtb. In order to elucidate the effects of the above mutations on the iron-heme catalytic region, electron paramagnetic resonance (EPR) experiments were carried out with and without azole drugs. Circular dichroism (CD), differential scanning calorimetry (DSC) and multi-angled laser light scattering (MALLS) analysis confirmed that F89H, R391K and L100F mutants were stable and homogeneous. Crystallogenesis was successful for the above mentioned mutants and atomic structures were obtained for all mutants and WT CYP51B1 (in ligand-bound and substrate-free forms), except for S348F and G388S mutants which were expressed as inclusion bodies and 60% holoenzyme, respectively. Reconstituted catalytic assays to determine the sterol demethylating propensity of the mutants were carried out using redox partners from Mtb or E. coli, and with lanosterol and dihydrolanosterol as the surrogate substrates. Redox potentiometry showed similar potentials to WT for all mutants except for the G388S mutant which was relatively positive (–102 mV). Redox cycling experiments followed by EPR analysis for mutants and WT resulted in a novel P450 high-spin species at g value 5.84 (80 %) which gradually collapsed to the initial low spin state over 48 h. Expression trials were concurrently carried out on two other Mtb P450 genes – CYP123 (Rv0744c) and CYP136 (Rv3059) products of which may have similar functions to CYP51B1 or may share similar redox partners. CYP123 is located on the same operon as CYP51B1 while CYP136 has a 29% sequence identity to another CYP51 from a marine slime bacterium. Although further work is necessary, in this study CYP123 was expressed totally as inclusion bodies while CYP136 was expressed as soluble apoprotein fused with trigger factor chaperone.

Thesis (Ph. D.)--University of Alabama at Birmingham, 2007. / Title from first page of PDF file (viewed Feb. 19, 2008). Includes bibliographical references.

Diss. (sammanfattning) Stockholm : Karol. inst., 2006. / Härtill 5 uppsatser.