The Interconversion of Sugars in Nature

Leloir, L.F. The interconversion of sugar in nature. In Currents in biochemical research, p. 585-608. New York Interscience Publishers 1960... 

This discussion will deal with some aspects of the interconversion of the cyclic forms of a carbohydrate to their acyclic forms and to each other. Since a topic of this nature cannot be well covered in a single Chapter, the discussions will be limited to the consideration of a cross-section of closely related reactions which include the formation and hydrolysis of aldono-lactones and aldosides and the deamination of amino sugars. 

Scheme I illustrates the interesting position of n-fructose in the sugar series, and the widely differing products obtained in the past few years by treatment with acid or alkali. Only intermediates and end products are given the reaction conditions are described in the text. Emphasis is placed on D-fructose because of its abundant occurrence in Nature and its rapid conversion into other isomers. The 1,2-enediol (c) and its ionized form (f) have central positions, indispensable to explaining hexose interconversions in alkaline media. The enediolate ion was mentioned for the first time during work on the saccharinic acids, and, in 1944, IsbeU used this same mechanism during development of his electron-displacement theory. ...

Isopentenyl pyrophosphate is potentially a bifunctional molecule. Its terminal vinyl group gives a nucleophilic character whereas when it isomerizes to 3,3-dimethylallyl diphosphate, the latter is electrophilic. Thus, longer-chain polyprenyls are formed by a favourable condensation of isopentenyl pyrophosphate first with dimethylallyl pyrophosphate and later with other allylic diphosphates. The initial interconversion of isopentenyl diphosphate and dimethylallyl diphosphate is promoted by an isomerase. The successive condensations yield the Cio compound geranyl diphosphate and then the C15 farnesyl diphosphate. The two molecules of famesyl diphosphate condense to form presqualene pyrophosphate which is reduced by NADPH to give the C30 open chain terpenoid squalene. The condensation reactions with IPP are a rather novel method of C-C bond formation since in the formation of other types of natural products (peptides, sugars, fatty acids, etc.) the reactions involve Claisen- or aldol-type condensations. 

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