Hexoses carbon 6, oxidation

Iodic acid in strong sulfuric acid at 100°C. is reported to show a rather remarkable specificity ketoses, sucrose, and pentoses are oxidized, but aldohexoses and lactose are not attacked 206). At still higher temperatures, hexoses are oxidized quantitatively to carbon dioxide and water 207). 

Both the fermentation of hexose sugars to ethanol and carbon dioxide and the oxidation of ethanol to acetic acid are exothermic (heat yielding) processes (see Sugar). The first reaction is expressed as foUows ... 

There are three possible classes of sugar acids which may be produced by the oxidation of monosaccharides (Figure 9.11). The aldonic acids are produced from aldoses when the aldehyde group at carbon 1 is oxidised to a carboxylic acid. If, however, the aldehyde group remains intact and only a primary alcohol group (usually at carbon 6 in the case of hexoses) is oxidised then a uronic acid is formed. Both aldonic and uronic acids occur in nature as intermediates in... 

This pathway is variously known as the pentose phosphate, hexose monophosphate or phosphogluconate pathway, cycle or shunt. Although the pentose phosphate pathway achieves oxidation of glucose, this is not its function, as indicated by the distribution of the pathway in different tissues. Only one of the carbons is released as CO2, the key products are NADPH and ribose 5-phosphate, both of which are important for nucleotide phosphate formation and hence for synthesis of nucleic acids (Chapter 20). The... 

Carbohydrate metabolism in a typical plant cell is more complex in several ways than that in a typical animal cell. The plant cell carries out the same processes that generate energy in animal cells (glycolysis, citric acid cycle, and oxidative phosphorylation) it can generate hexoses from three- or four-carbon compounds by glu-coneogenesis it can oxidize hexose phosphates to pentose phosphates with the generation of NADPH (the ox-... 

An oxidative pentose phosphate cycle. Putting the three enzyme systems together, we can form a cycle that oxidizes hexose phosphates. Three carbon... 

Figure 17-8 The pentose phosphate pathways. (A) Oxidation of a hexose (C6) to three molecules of C02 and a three-carbon fragment with the option of removing C3, C4, C5, and C7 units for biosynthesis (dashed arrows). (B) Non-oxidative pentose pathways 2 1/2 C6 —> 3 C5 or 2 C6 —> 3 C4 or 3 V2C6 —> 3 C7.

Isbell and co-workers (51) have tried to minimize the oxygen reaction and to maximize the peroxide reaction. When a large excess of peroxide and a low temperature were maintained, they found that the monosaccharides are converted almost quantitatively to formic and two-carbon acids. Table II presents results for the peroxide oxidation of 14 sugars. The total acid produced from aldo-hexoses under favorable conditions is about six moles, consisting almost entirely of formic acid. Aldopentoses react more rapidly than aldo-hexoses and yield about five moles of formic acid per mole of pentose. Fructose and sorbose yield approximately five moles of total acid of which four moles are formic acid. Glycolic acid was identified qualitatively but not determined quantitatively. L-Rham-nose and L-fucose yield about five moles of acid, including four moles of formic acid. Acetic acid was identified only qualitatively. 

Many kinds of organisms and some mammalian organs, notably liver, possess an alternative pathway for the oxidation of hexoses which results in a pentose phosphate and carbon dioxide. This pentose can be used as a precursor of the ribose found in nucleic acids or other sugars containing from three to seven carbon atoms which are needed in smaller amounts. The first and third reactions in the pentose phosphate pathway generate NADPH which is a major source of reducing power in many cells. 

---