The presence of thiols in living systems is critical for the maintenance of cellular redox homeostasis, the maintenance of protein thiol-disulfide ratios and the protection of cells from reactive oxygen species. In addition to the well studied tripeptide glutathione (γ-Glu-Cys-Gly), a number of compounds have been identified that contribute to these essential cellular roles. Many of these molecules are of great clinical interest due to their essential role in the biochemistry of a number of deadly pathogens, as well as their possible role as therapeutic agents in the treatment of a number of diseases. A series of studies were undertaken using theoretical, chemical and biochemical approaches on a selection of thiols, ergothioneine, the ovothiols and mycothiol, to further our understanding of these necessary biological components. Ergothioneine is present at significant physiological levels in humans and other mammals; however, a definitive role for this thiol has yet to be determined. It has been implicated in radical scavenging in vivo and shows promise as a therapeutic agent against disease states caused by oxidative damage. Given the clinical importance of this intracellular thiol, further investigation into the behaviour of ergothioneine appeared warranted. A high level theoretical study was performed to determine the thermodynamic driving force behind the instability of the ergothioneine disulfide, as well as the thermodynamics of the reactions of ergothioneine with a selection of biologically relevant reactive oxygen species. These results were compared to those determined for a glutathione model compound, as well as the related ovothiols. The latter are believed to act as hydrogen peroxide scavengers in vivo and are currently under review as possible therapeutics against oxidative damage. The structural differences between the ovothiols and ergothioneine dramatically affect their reactivity and this study investigates the thermodynamic driving forces behind these differences. Mycothiol is the major thiol found in the Actinomycetales bacteria, which include the causative agent of tuberculosis, and the enzymes which use mycothiol have been identified as important targets for the development of novel antimicrobials. To better understand the in vivo behaviour of mycothiol, a thorough conformational search was performed to determine what, if any, trends exist among the low energy conformers expected to be present in solution. Knowledge of the conformations preferred by mycothiol may aid in the design of substrate-based inhibitors targeted at mycothiol-dependent enzymes. In addition, the efforts towards the identification of a mycothiol-dependent glyoxalase system are described. The glyoxalase system is essential for the detoxification of methylglyoxal, a toxic by-product of glycolysis, and this system would serve as a target for the design of new therapeutics against tuberculosis and other pathogenic Actinomycetales bacteria. In addition to the study of intracellular thiols, this work details a preliminary theoretical study of the thermodynamics of the phosphorylation of proteinaceous serine residues by inositol pyrophosphates in eukaryotic cell-free extracts. It has been postulated that this observed activity may represent a novel signalling pathway in eukaryotes. This study focused on the effect of inositol pyrophosphate structure and overall charge on the thermodynamics of these reactions. This information should contribute to our understanding of this novel cellular phosphorylation process.

Thesis (Ph. D.)--University of Hong Kong, 2001. / Includes bibliographical references (leaves 203-238).

Thesis (M. Phil.)--University of Hong Kong, 1989.

published_or_final_version / Physiology / Master / Master of Philosophy

Thesis (MSc)--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet.

LOW TEMPERATURE SYNTHESIS AND CHARACTERIZATION OF SOME LOW POSITIVE AND NEGATIVE THERMAL EXPANSION MATERIALS Kathleen Madara White 151 pages Directed by Dr. Angus P. Wilkinson Low temperature non-hydrolytic sol-gel synthesis was used to explore the possibility of lowering the crystallization temperatures of some known AIVMV2O7 compounds. Crystallization temperatures for ZrP2O7 and ZrP2O7 were unaffected by the use of non-hydrolytic sol-gel methods; however, successful synthesis of these compounds broadens the range of materials that can be produced using this method and suggests the possibility of synthesizing solid solutions (or composites) including ZrP2O7 or ZrV2O7. This research presents for the first time the direct synthesis of ZrP2O7 from separate zirconium and phosphorus starting materials using mild autoclave methods. Characterization of some AIVMV2O7 compounds, using lab and high resolution synchrotron powder XRD, led to the assignment of a new symmetry for CeP2O7 and to the suggestion that the reported structure for PbP2O7 was inadequate. Studies using in situ high temperature lab and synchrotron powder XRD for PbP2O7 and CeP2O7 provided the opportunity to report their thermal properties for the first time, and to compare their behavior to that of some other AIVMV2O7. High pressure diffraction measurements on CeP2O7 provided data for the estimation of bulk moduli and suggested two possible pressure-induced phase transitions. A broad range of MIIIMVP4O14 compounds were prepared using low temperature hydrolytic sol-gel synthesis. Thermal studies revealed nearly linear trends in CTEs and lattice constants with respect to the sizes of MIIIMV cations. Some lower ionic radii compounds had CTEs comparable to that of ZrP2O7 at low temperature, suggesting a similar superstructure. Three compounds were found to exhibit temperature-induced phase transitions.

Thesis (PhD (Genetics. Plant Biotechnology))--University of Stellenbosch, 2006. / The main aim of the work presented in this thesis was to elucidate the apparent role of pyrophosphate fructose 6-phosphate 1-phosphotransferase (PFP) in sucrose accumulation in sugarcane. PFP activity in sugarcane internodal tissue is inversely correlated to the sucrose content and positively to the water-insoluble component across varieties which differ in their capacities to accumulate sucrose. This apparent well defined and important role of PFP seems to stand in contrast to the ambiguity regarding PFP’s role in the general literature as well as the results of various transgenic studies where neither the downregulation nor the over-expression of PFP activity had a major influence on the phenotype of transgenic potato and tobacco plants. Based on this it was therefore thought that either the kinetic properties of sugarcane PFP is significantly different than that of other plant PFPs or that PFP’s role in sucrose accumulating tissues is different from that in starch accumulating tissues. In the first part of the study sugarcane PFP was therefore purified and its molecular and kinetic properties were determined. It consisted of two subunits which aggregated in dimeric, tetrameric and octameric forms depending on the presence of Fru 2,6-P2. Both the glycolytic and gluconeogenic reactions had broad pH optima and the kinetic parameters for all the substrates were comparable to that of other plant PFPs. The conclusion was therefore that sugarcane PFP’s molecular and kinetic characteristics do not differ significantly from that of other plant PFPs. The only direct way to confirm if PFP is involved in sucrose accumulation in sugarcane is to alter its levels in the same genetic background through genetic engineering. This was therefore the second focus of this study. PFP activity was successfully down-regulated in sugarcane. The transgenic plants showed no visible phenotype under greenhouse and field conditions and sucrose concentrations in their immature internodes were significantly increased. PFP activity was inversely correlated with sucrose content in the immature internodes of the transgenic lines. Both the immature and mature internodes of the transgenic plants had significantly higher fibre contents. This study suggests that PFP plays a significant role in glycolytic carbon flux in immature, metabolically active sugarcane internodal tissues. The data presented here confirm that PFP can indeed have an influence on the rate of glycolysis and carbon partitioning in these tissues. It also implies that there are no differences between the functions of PFP in starch and sucrose storing tissues and it supports the hypothesis that PFP provides additional glycolytic capacity to PFK at times of high metabolic flux in biosynthetically active tissue. This work will serve as a basis to refine future genetic manipulation strategies and could make a valuable contribution to the productivity of South African sugarcane varieties.

published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

No description available.

La déshydrogénation oxydante du n-butane a été étudiée sur des catalyseurs à base de pyrophosphates de Ti, Zr, Ce et Sn. Leurs propriétés physico-chimiques (morphologie, structure, composition) ont été suivies par une série de techniques physiques et leurs propriétés catalytiques évaluées entre 410 et 570°C. TiP2O7 présente, à 530°C, le meilleur rendement : les sélectivités en butènes et butadiène atteignent respectivement 42% et 14% pour une conversion de 25%. L'ajout d'eau a un effet négatif sur les performances catalytiques. Mis à part CeP2O7, qui se transforme en deux phases phosphate de Ce(III) durant la réaction, les catalyseurs restent inchangés et peuvent être classifiés en deux groupes en fonction de mécanisme réactionnel impliqué : le premier avec ZrP2O7, SnP2O7 et TiP2O7 à basse température et le deuxième avec TiP2O7 à haute température. Des mesures de conductivité électrique et des caractérisations par RPE, spectroscopie Raman et TAP ont été mises en œuvre pour étudier le mécanisme réactionnel. Sur les deux types de catalyseur l'étape initiale d'activation de l'alcane correspond à l'attaque de l'alcane par une espèce O– menant à un radical, mais tandis que le radical formé est rapidement transformé en butène suite à une deuxième abstraction d'hydrogène par un mécanisme similaire sur le premier type de catalyseur, il est piégé dans une lacune anionique ou sur un site acide de Lewis Ti4+, étant transformé en une espèce alcoxyde ou directement en butènes par l'abstraction d'un proton, sur le deuxième type. Dans le premier mécanisme l'étape limitante est l'attaque de l'alcane par une espèce O– alors que dans le second c'est la réoxydation du site par diffusion de l'oxygène réticulaire ou directement par l'oxygène de la phase gaz. On observe pour TiP2O7 entre 400 et 450°C une situation particulière dont l'étape limitante est toujours l'attaque de l'alcane par une espèce O– mais l'intermédiaire réactionnel peut également être piégé sur le catalyseur.