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

Biosynthesis; how new molecules are formed
Selected pathways of carbohydrate and lipid metabolism
Interconversions of monosaccharides

The previous chapter discussed the General Principles of biosynthetic processes, specific pathways for The formation of the main precursors of Introduction/36.html">CARBOHYDRATES and Lipids, as well as the processes of Gluconeogenesis, the Synthesis of glucose-6-phosphate from free glucose, and the formation of Fatty acids. In addition, typical polymerization reaction sequences and the main mechanisms regulating Glycogen Synthesis and degradation were examined. This chapter is devoted to specific aspects of the METABOLISM of Monosaccharides, Oligosaccharides, lipid Polysaccharides, and related compounds.

As previously noted (Chapter 11, Section D,1), chemical interconversions of substances proceed most readily at the level of carbohydrate oxidation processes. Numerous reactions are known in which one sugar is converted into another. For the most part, such transformations involve a sugar linked to a nucleotide. Fig. 12-1 summarizes the pathways of sugar interconversions and illustrates how glucose-6-phosphate or fructose-6-phosphate gives rise to many other sugars found in living organisms. Galactose and mannose undergo analogous transformations. In the presence of the appropriate kinase, mannose is converted into mannose-6-phosphate, which isomerizes to an equilibrium state with fructose-6-phosphate. From free galactose, in the presence of galactokinase, galactose-1-phosphate is formed, which is converted into UDP-galactose and subsequently into UDP-glucose. On the other hand, the metabolism of free fructose—an essential component of the human diet (usually present in food as sucrose)—differs from the scheme described above. Under the action of a specialized enzyme, fructokinase, fructose is phosphorylated in the Liver to yield fructose-1-phosphate; however, the latter cannot be converted into fructose-6-phosphate due to the absence of the corresponding mutase. Instead, a specific enzyme, aldolase, cleaves fructose-1-phosphate into two components: dihydroxyacetone phosphate and free glyceraldehyde. [In rare cases of deficiency of this aldolase, a condition of fructose intolerance arises.] The glyceraldehyde formed from fructose is reduced to glycerol, which is then phosphorylated (in the presence of glycerokinase) and reoxidized to dihydroxyacetone phosphate.