Abstract
Choline oxidase catalyzes the oxidation of choline to glycine betaine. This reaction is of considerable medical and biotechnological applications, because the accumulation of glycine betaine in the cytoplasm of many plants and human pathogens enables them to counteract hyperosmotic environments. In this respect, the study of choline oxidase has potential for the development of a therapeutic agent that can specifically inhibit the formation of glycine betaine, and therefore render pathogens more susceptible to conventional treatment. The study of choline oxidase has also potential for the improvement of the stress resistance of plant by introducing an efficient biosynthetic pathway for glycine betaine in genetically engineered economically relevant crop plant. In this study, codA gene encoding for choline oxidase was cloned. The cloned gene was then used to express and purify the wild-type enzyme as well as to prepare selected mutant forms of choline oxidase. In all cases, the resulting enzymes were purified to high levels, allowing for detailed characterizations. The biophysical and biochemical analyses of choline oxidase variants in which the positively charged residue close to the flavin N(1) locus (His466) was removed (H466A) or reversed (H466D) suggest that in choline oxidase, His466 modulates the electrophilicity of the bound flavin and the polarity of the active site, and contributes to the flavinylation process of the covalently bound FAD as well as to the stabilization of the negative charges in the active site.
Biochemical, structural, and mechanistic relevant properties of selected flavoproteins with special attention to flavoprotein oxidases, as well as the biotechnological and medical relevance of choline oxidase, are presented in Chapter I. Chapter II summarizes all the experimental techniques used in this study. Chapter III-VII illustrate my studies on choline oxidase, including cloning, expression, purification and preliminary characterizations (Chapter III), spectroscopic and steady state kinetics (Chapter IV), the catalytic roles of His466 and the effects of reversing the protein positive charge close to the flavin N(1) locus (Chapter V and VI), and the roles of His310 with a special attention to its involvement in a proton-transfer network (Chapter VII). Chapter VIII presents a general discussion of the data presented.
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