Thesis (Ph. D. (Biochemistry, Microbiology & Biotechnology))--Rhodes University, 2005.

Given their widespread effects and distribution in both natural and industrial environments, the monitoring of phenolic compounds is of considerable analytical interest. Electrochemical biosensor technologies, in particular those comprising laccase enzymes, afford many potential benefits to address this analytical need. However, several key factors affecting sensor response currently limit their applicability. This Thesis reports on the fabrication and optimisation of an electrochemical laccase-based biosensor towards the application of the monitoring of phenolic compounds. Selected factors considered to affect sensor response were investigated using the optimised biosensor. These included: electrochemical, biochemical and substrate-dependent factors, which were found to intersect in modulating biosensor response signals. Through the application of transducer-dependent and substrate-dependent parameters, the selective and simultaneous detection of a mixture of different phenolic analytes is successfully demonstrated. This Thesis also investigates the use of Quartz-Crystal Microbalance with Dissipation (QCM-D) technology, an analytical technique that measures physical parameters of thin-film structures, towards the successful monitoring of enzyme immobilisation strategies. These strategies are fundamental to the successful fabrication of biosensors, and the real-time monitoring of immobilised film formations is of considerable research interest. In the studies reported on in this Thesis, QCM-D technology was demonstrated to be an effective complementary technology in the prediction of film immobilisation techniques on the resultant biochemical kinetics of immobilised enzymes.

Master of Science / Biochemistry / Gerald Reeck / Laccases belong to the “multicopper oxidase” family of proteins, and can oxidize o-diphenols and p-diphenols in the presence of molecular oxygen. Laccases have been well characterized in wood-rotting fungi where they appear to play a role in lignin degradation, morphogenesis, and stress defense. More recently, laccase-2 has been found to play a role in the insect cuticle sclerotization and tanning. In addition, it has been hypothesized that laccase-1 may be involved in the oxidation of toxic phenolic compounds ingested by insects during feeding. A laccase-type phenoloxidase has been identified in the pea aphid, Acyrthosiphon pisum (Harris) using a combination of substrates that react with laccase. Within the pea aphid, laccase-1 transcript was found to be localized within the gut and the salivary glands. Finally, the specific regions where laccase-1 was present in the salivary gland was visualized using immunohistochemistry.

As global energy demands increase, there is a requirement to decrease our dependency on fossil fuels due to their finite supply and negative environmental impacts. Alternative sources of energy are required that offer sustainability, reduced cost and environmental benefits. Second generation biofuels remove the ‘food vs fuel’ drawback of the first generation. They utilise lignocellulosic biomass, providing cheap and abundant starting materials for energy production. The major biotechnological challenge associated with lignocellulosic processing is the natural recalcitrance of the substrate to sugar conversion (saccharification). This recalcitrance is largely associated with lignin, an aromatic heteropolymer that encases cellulose. To improve bioethanol yields, there is a need for cost effective and environmentally friendly pretreatment methods that can remove lignin. An enzymatic pretreatment strategy was investigated using laccase from the fungus Trametes versicolor (TvL). Expansion of the laccase substrate range towards non-phenolic substrates was explored by screening of a panel of synthetic and naturally derived phenolic compounds as potential redox mediators with laccase. Both groups enabled decolourisation of the recalcitrant dye (RB-5) to varying degrees, which laccase alone was unable to achieve. In the case veratryl alcohol, a lignin model substrate, synthetic compounds 1-HBT, ABTS and violuric acid proved effective laccase mediators. On this basis, TvL with 1-HBT was selected as the most successful laccase mediator system (LMS) and was further explored as a biomass pretreatment method. The effects of LMS treatments towards the saccharification of acid hydrolysed wheat straw were extensively investigated. Optimisation revealed that when both TvL and a TvL LMS of synthetic origin (1-HBT, violuric acid) were applied, saccharification was improved. The observed increase in glucose release was only detected when a second lignin removing technique was applied in succession. Both alkaline-peroxide and organosolv extractions were successfully used to demonstrate the role of laccase/LMS in saccharification improvement, with improvements reaching up to 44.6%. The effect was further demonstrated with additional substrates (corn and sorghum stover) and additional laccases (Pleurotus ostreatus, Agaricus bisporus and Rhus vernicifera). Further studies using β-O-4 structures, py-GC/MS and FTIR analyses provide further information on the structural actions of an LMS towards lignin, including strong evidence for Cα-Cβ cleavage and Cα hydroxyl oxidation mechanisms.

No description available.

Ces travaux de thèse visent à valoriser la lignine, biopolymère aromatique le plus abondant sur terre. Pour cela, la dépolymérisation oxydante de la lignine par voie enzymatique a été explorée afin d’obtenir des synthons aromatiques fonctionnalisés. La laccase et le système laccase médiateur (LMS) ont été sélectionnés comme système enzymatique. Dans une première partie, les paramètres réactionnels (choix du médiateurs, température, co-solvant…) de fonctionnement optimal du LMS ont été déterminés notamment via l’utilisation de molécules modèles de lignine. Ces conditions optimales ont été directement appliquées pour l’oxydation d’une lignine organosolv issue d’une plante herbacée. Une étape de fractionnement organique a été conduite sur la lignine avant oxydation afin d’éliminer les populations à l’origine de réactions de couplage. La lignine a ensuite été oxydée par le LMS en milieu biphasique, puis traitée au peroxyde d’hydrogène. Ce procédé en trois étapes a permis de générer des composés aromatiques monomères à trimères (mis en évidence par chromatographies SEC, HPLC, GC et LC-MS) et a été appliqué avec succès à une seconde lignine issue de conifère. Dans les deux cas, des rendements élevés ont été constatés comparés à ceux obtenus dans la littérature. Après isolation, les composés aromatiques produits pourraient trouver des applications comme précurseurs dans les industries de la chimie fine et des polymères. / This work aims at valorizing lignin, the most abundant aromatic biopolymer on earth. For that purpose, an enzymatic approach for the oxidative depolymerization of lignin was investigated in order to obtain fine chemicals. Laccase and the laccase-mediator system (LMS) were selected for the enzymatic oxidation. In the first part, optimal conditions (type of mediator, temperature, co-solvent…) were determined especially by studying reactions on lignin model molecules. These conditions were applied for the oxidation of an organosolv grass lignin. Prior to the oxidation, an organic fractionation was conducted on the lignin in order to remove the population responsible for radical coupling. Then, the lignin was oxidized by the LMS in a biphasic medium followed by a mild hydrogen peroxide treatment. This three-step process allowed the production of monomeric to trimeric aromatic compounds (as shown by SEC, HPLC, GC and LC-MS) and was successfully applied to a different organosolv lignin coming from hardwood. High yield were obtained in both cases as compared with literature results. After isolation, the obtained aromatic molecules could be of interest as precursors for the fine chemistry and polymer industries.

La laccase est une enzyme ayant une faible spécificité qui peut dégrader de nombreux contaminants présents dans l'environnement. Son utilisation dans des procédés de bioremédiation est donc appelée à augmenter dans les années à venir. Il est donc important d'améliorer le potentiel d'utilisation de cette enzyme en la rendant réutilisable et plus stable qu'à l'état naturel. Plusieurs techniques d'immobilisation/insolubilisation sont déjà disponibles mais la plupart utilisent un agent toxique pour plusieurs espèces. Cette étude cherche donc à remplacer ce produit toxique par un agent biodégradable, le chitosane, ayant peu d'effets sur l'environnement dans la formation d'agrégats d'enzymes réticulés [ cross-linked enzyme aggregates (CLEAs)]. Les conditions de formation de ces biocatalyseurs ont été optimisées dans le but d'obtenir un produit actif et stable. Ces CLEAs ont également été caractérisées en termes de capacité catalytiques [i.e. catalytique], de stabilité thermique et face aux dénaturants chimiques ainsi qu'en terme de dimensions.

Thesis (doctoral)--University of Helsinki, 2007. / Includes bibliographical references. Also available on the World Wide Web.

L'utilisation d'enzymes dans différentes applications biotechnologiques et industrielles s'est avérée prometteuse durant les dernières décennies. La laccase est une enzyme qui suscite l'attention, notamment, grâce à sa capacité à dégrader une vaste gamme de polluants présents dans l'environnement, à son apparente stabilité, et au fait qu'elle peut d' [sic] être utilisée dans une variété d'applications comme : les cosmétiques, l'industrie du papier, ou encore le traitement des eaux et la bioremédiation de sols. Il apparaît donc pertinent d'améliorer le potentiel d'utilisation de cette enzyme en augmentant sa stabilité et sa capacité à être réutilisée. À ces fins, plusieurs méthodes l'immobilisation/insolubilisation sous forme de cross-linked enzyme agregates (CLEAs) ou encore la formation d'une structure polymérique autours [sic] des biocatalyseurs se sont avérés efficaces. Cette étude vise à stabiliser la laccase libre et sous forme de CLEAs via la formation de réseau polymérique organique/inorganique formé de chitosane et de silane. Cette technique a permis de 1) stabiliser significativement l'activité enzymatique face aux dénaturants chimiques et thermique [i.e. thermiques]; 2) augmenter la résistance thermique de la laccase et des CLEAs modifiées; 3) augmenter l'efficacité catalytique de la laccase et CLEAs modifiées ainsi que leur affinité pour le substrat utilisé. De plus, les biocatalyseurs formés s'avèrent insolubles ce qui permettrait leur utilisation/réutilisation dans des bioréacteurs opérés en continu.

by Shicheng Chen. / "July, 2003." / Thesis (Ph.D.)--Chinese University of Hong Kong, 2003. / Includes bibliographical references (p. 206-232). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.