Thèse de doctorat : Génétique Moléculaire : Nancy 1 : 2008. / Titre provenant de l'écran-titre.

Characterization of allosteric properties of phosphofructokinase from the extreme thermophile Thermus thermophilus (TtPFK) using thermodynamic linkage analysis revealed several peculiarities. Inhibition and activation of Fru-6-P binding by the allosteric effectors phosphoenolpyruvate (PEP) and MgADP are entropically-driven in TtPFK. It is also curious that PEP binding affinity is unusually strong in TtPFK when compared to PFKs from Escherichia coli, Bacillus stearothermophilus, and Lactobacillus delbrueckii, while the magnitude of the allosteric inhibition by PEP is much smaller in TtPFK. In an effort to understand the source of weak inhibition, a putative network of residues between the allosteric site and the nearest active site was identified from the three-dimensional structures of BsPFK. Three of the residues in this network, D59, T158, and H215, are not conserved in TtPFK, and, due to their nature (N59, A158, S215), are unlikely to be involved in the same non-covalent interactions seen in BsPFK. The triple chimeric substitution N59D/A158T/S215H, results in a 2.5 kcal mol-1 increase in the coupling free energy, suggesting that the region containing these residues may be important for propagation of inhibitory response. The individual substitutions at each position resulted in an increase in the coupling free energy, and the double substitutions displayed additivity of these changes. The chimeric substitution made at N59 suggests that the polar nature of the asparagine at position 59 is key for the enhanced binding of PEP. The non-conserved R55 was found to be particularly important for the enhanced binding of PEP in TtPFK, as chimeric substitutions R55G and R55E resulted in a 3.5 kcal mol-1 and 4.5 kcal mol-1 decrease in the binding affinity for PEP, respectively. Our results also confirm the observations previously made in PFKs from E. coli and B. stearothermophilus, that the ability of the effector to bind is independent of its ability to produce allosteric response. We show that several substitutions result in a decrease in binding affinity of PEP to TtPFK, while dramatically enhancing its ability to inhibit (N59D, R55G, R55E). Similarly, some substitutions, like S215H and A158T show an enhanced inhibition by PEP, while having no effect on its binding affinity.

Frankfurt (Main), Universiẗat, Diss., 2008. / Erscheinungsjahr an der Haupttitelstelle: 2007.

Frankfurt (Main), Univ., Diss., 2004. / Erscheinungsjahr an der Haupttitelstelle: 2003. Computerdatei im Fernzugriff.

Frankfurt (Main), Universiẗat, Diss., 2004. / Erscheinungsjahr an der Haupttitelstelle: 2003.

The biochemical activity of Streptococcus thermophilus and Lactobacillus bulgaricus in a 1:1 ratio (Chr. Hansen's Lab - CH-III) were studied at 43 C in goat and cow milk during the manufacture and storage of yogurt. Determination of pH, titratable acidity, lactose content, proteolytic activity, and enumeration of the starters were performed on samples taken at hourly intervals until the yogurt became set, and after 8, 16, 24 hours, 11 and 22 days of storage at 4 C. In goat milk the number of starter culture organisms showed higher values than those in cow milk. Fermentation was considered to be finished when the pH decreased to 4.7 and 0.7% of lactic acid was produced. Those values were reached after 3 and 4 hours for goat and cow milk respectively. The values for pH in both milks were stable at 4.7 during storage at 4 C. Goat milk and goat milk yogurt had higher levels of free amino acids (12.65 mg/dl and 24.39 mg/dl respectively) than cow milk (10.31 mg/dl) or cow milk yogurt (20.20 mg/dl). During storage at 4 C free amino acids in goat milk decreased by 20%, in cow milk they increased by 70% from values found in fresh yogurt. The proteinase activity in goat milk during elaboration of yogurt was from 2 to 3 hours after incubation, was then stable until 24 hours of storage at 4 C and then decreased. No change in the proteinase activity in cow milk yogurt was seen until 4 hours of incubation for experiments 1 and 2 and 3 hours in experiment 3. During the storage at 4 C, proteinase activity in cow milk yogurt was stable. Polyacrylamide gel electrophoresis of caseins failed to reveal marked changes during the manufacture of yogurt and storage after 22 days at 4 C. However, a scanning gel densitometer showed that the area corresponding to alpha-casein decreased by 22% and for beta-casein 13.32% during the elaboration of goat milk yogurt. After 22 days of storage at 4 C, the alpha-casein fraction decreased an additional 14%, but no change was observed in beta-casein. The area corresponding to alpha- and beta-caseins decreased by 7.16 and 14.48% respectively during yogurt manufacture and an additional 6.74 and 8% during storage of cow milk yogurt. The overall results found in this study suggest that the metabolic activity of S. thermophilus and L. bulgaricus is greater in goat milk than in cow milk during elaboration and storage of yogurt.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1944. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Stuttgart, Univ., Diss., 2002.

Konstanz, Univ., Bachelorarb., 2006.

University, Diss., 2002--Stuttgart. / Erscheinungsjahr an der Haupttitelstelle: 2002.