The deoxy sugars which have been found ubiquitously in plants, fungi and bacteria, are an important class of carbohydrate. The widespread occurrence of deoxy sugars in different organisms suggest diverse biological functions. Numerous reports have shown that they indeed exhibit various vital and potent biological activities. Interest in the chemistry and biochemistry of deoxy sugars has expanded considerably due to the studies of bacterial surface glycans in which a number of dideoxy hexoses have been identified as important antigenic determinants in bacteria. Although the importance of this type of sugar in affecting the immune response is well recognized, little is known about their biosynthetic formation. Furthermore, in the studies of 3,6-dideoxyhexose, many unique features of the enzymes involved in this biological transformation have been found. These unusual properties are not readily explicable, and thus offer a challenge to our understanding of fundamental enzymatic mechanisms. Inspired by the intriguing biological activities of this type sugar and the unique catalytic properties of the enzymes involved in its biosynthesis, we will examine this biosynthetic process at the molecular level. Initial efforts will be directed to the biosynthesis of 3,6-dideoxyhexose, and emphasis will be placed on the mechanistic and stereochemical aspects. Specifically, we will (1) purify the enzymes involved in the biosynthetic formation of ascarylose (3,6-dideoxy-L-arabino-hexose); (2) synthesize isotopically labeled substrates and cofactors, and potential alternate substrates containing mechanistically informative reactive functionality at key sites of the molecules; (3) incubate these compounds with the enzymes, determine their competence as substrates and/or inhibitors, and determine the structures of the incubation products in order to address the following mechanistic and stereochemical points of general importance to the understanding of all deoxy sugar biosynthesis: (a) the overall stereochemical outcome of this dehydration-reduction process; (b) mechanism of sugar reduction; and (c) mechanism of NAD(P)H oxidation. The possible presence and role of metal ion in this catalysis will be carefully examined. With everything well under way, we will proceed with the study of other deoxy sugar biosynthesis, such as the formation of 2,6-dideoxyhexoses. Evaluation of the results from these experiments will aid in forming a conclusion about the nature and sequence of events at the active-site of these important enzymes during these biosynthetic transformations.
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