3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item An investigation into the use of smart grid technologies for the Northern Cape Province of South Africa, where extensive solar generation is planned in a constrained network(2015-05-04) Parbhoo, NishalLarge scale distributed generation is expected to be implemented in South Africa for the first time due to government initiated programmes aimed at increasing the utilization of renewable energy resources for power generation. Solar photovoltaic distributed generation is proposed in the vast majority of renewable energy power generation independent power producer applications received in the Northern Cape of South Africa; therefore the integration of these units into the distribution networks presents a great challenge. The current distribution network was not originally designed for integration of distributed generation at such high penetration levels. Expensive and lengthy network reinforcement and strengthening projects will need to be implemented in the distribution network to facilitate safe and reliable interconnection of distributed generation at higher penetration levels. This report gathers and documents the major challenges such as: voltage regulation, equipment thermal ratings, power quality, and protection coordination related to solar photovoltaic distributed generation interconnection to constrained distribution networks. Smart grid technologies which enable higher penetration of distributed generation and provide a cost-effective alternative to network strengthening are investigated. Based on the South African context, a Smart Grid technology approach is proposed and the Smart Grid technologies which offer tangible and direct benefits in the short term are prioritised for implementation.Item Topologically conserved hydrophobic residues of the thioredoxin C-subdomain stabilise GSTs(2014-06-12) Parbhoo, NishalThe thioredoxin-like fold is a well conserved fold that is present in many families of proteins. One such superfamily of proteins include the GSTs which are involved in phase II detoxification. GSTs primarily catalyse the metabolism of xenobiotics but are also involved in transporting non–substrate ligands and reactive compounds. The GST fold comprises an N-terminal thioredoxin domain and an all alpha helical C-terminal domain and is present in at least 18 classes of proteins. The N-terminal thioredoxin domain is characterised by the βαβαββα topology and can be further divided into two structural motifs, an N-terminal (βαβ) and a C-terminal (αββα) motif. A well conserved hydrophobic network exists between these two motifs and the role of the C-terminal motif is elucidated in this study using class Alpha GST as a model protein. A topologically conserved valine (Val58) and an isoleucine (Ile75) located on β3 and α3, respectively, were mutated to alanine. Secondary and tertiary structural characterisation as well as ligandin function of the mutant enzymes displayed no major structural alteration with respect to the wild-type enzyme. This was confirmed with high resolution crystal structures obtained. Enzymatic activity was maintained indicating that no structural alterations have occured that affects the active site dynamics and the domain interface as a result of the induced mutations. Thermal denaturation studies, however, indicated a slight destabilisation in the enzyme in the case of the valine mutation, but a large destabilisation was witnessed as a result of the isoleucine mutation. This is further observed in denaturant-induced equilibrium studies where the thermodynamic stability of proteins can be determined. Furthermore, as a result of the isoleucine mutation, the enzyme unfolds via a populated intermediate in contrast to the wild-type which globally unfolds via a two-state mechanism with no stable intermediates being populated. Pulse-proteolysis was employed as an additional probe for thermodynamic stability where the enzyme was digested by thermolysin at varying denaturant concentrations. Pulse-proteolysis results were in agreement with the thermal and denaturant-induced stability studies further confirming that the isoleucince substitution causes a large destabilisation. Thus these conserved hydrophobic residues of the thioredoxin C-subdomain play a crucial stabilising role in the GST fold.Item The role of a conserved interdomain interaction in Escherichia coli glutaredoxin-2(2010-08-26) Parbhoo, NishalDomain interfaces play an important role in protein stability and folding. A major structural feature at the interdomain interface of the GST class of proteins is the conserved hydrophobic ‘lock-and-key’ motif. In a monomeric homologue of the GSTs, Grx2, the hydrophobic interdomain ‘lock-and-key’ motif is formed by insertion of the side-chain of methionine 17 (Met17) from domain 1 into a hydrophobic pocket in domain 2. This study evaluates the contribution of the Met17 residue to the stability of Glutaredoxin-2 (Grx2). Protein engineering techniques were employed to generate a Met17 to Alanine (M17A) mutant protein and comparative studies with wild-type and M17A Grx2 were performed. The spectral properties of M17A Grx2 monitored using far and near-ultraviolet circular dichroism and tryptophan fluorescence indicated no significant changes in secondary or tertiary structure in the native state. Conformational stability studies were performed to determine the contribution of the ‘lock-and-key’ motif to protein stability. Equilibrium unfolding studies, displayed significant impact on the conformational stability of the protein with a DDG(H2O) of 4 kcal.mol-1 as a result of the replacement of the Met17 residue with alanine. The co-operativity of unfolding is slightly decreased, with the mvalue being reduced by 0.3 kcal.mol-1.M-1 suggesting an intermediate formation. This intermediate becomes more prominent during equilibrium unfolding in the presence of ANS which showed an increase in intensity in the unfolding transition for M17A Grx2 but was absent for wild-type Grx2. The kinetics of unfolding of both Grx2 proteins are complex, both displaying two observable phases (fast and slow) which occur in parallel as confirmed by performing initial conditions test. The slow phase involves structural rearrangements that expose small amounts of surface area while the fast phase represents gross structural unfolding exposing large amounts of surface area. The rate of the fast unfolding phase is increased for M17A Grx2, as the time constant decreased from 2.4s (wild-type) to 830ms, however there is negligible change in the rate of the slow phase. The increase in the unfolding rate of the fast phase is in agreement with the equilibrium studies which highlights the destabilisation as a result of the mutation.