Biochemistry - Chemical Reactions in Living Cells, Volume 2 - D. Metzler 1980

Biosynthesis: How New Molecules Are Formed
Selected pathways of carbohydrate and lipid metabolism
Synthesis of deoxysugars

Sugar METABOLISM frequently involves the dehydration of α,β-unsaturated carbonyl compounds. A classic example is the synthesis of 2-keto-3-deoxy sugar acid derivatives [Eq. (7-59)]. In certain instances, a carbonyl group is introduced via The oxidation of an OH group—a reaction whose apparent sole purpose is to create conditions favorable for subsequent dehydration. A case in point is The Biosynthesis of L-rhamnose from D-glucose, a multistep pathway [Eq. (12-7)] utilizing sugars linked to deoxythymidine diphosphate [5, 6]. The initial step entails introducing a carbonyl group into the carbohydrate moiety through oxidation. This is followed by dehydration [Eq. (12-7), reaction b], after which the newly formed double bond is reduced. The complex multienzyme system catalyzing steps a through c in Eq. (12-7) incorporates its own oxidizing agent (NAD⁺), which stores the reducing equivalents generated in step a and subsequently utilizes them to reduce the double bond in step c. Specific stages of this reaction sequence feature inversion at the C-3 position. A separate enzyme is required for the second reduction [Eq. (12-7), reaction d].

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The biosynthesis of dideoxysugars, essential constituents of bacterial Cell-surface Polysaccharides, is discussed in Chapter 8, Section D.4.