Sugars hexose

A similar family tree could be constructed for the less common ketose sugars, but only fructose, 16.43 vide supra), and sorbose are commonly encountered. Fructose is the sweetest of all the naturally occurring monosaccharides, and hence, high-fructose corn syrup, a mixture of fructose and glucose, is widely used as a sweetening agent for food. Concerns have been raised as it is linked to obesity and diabetes. The natural isomer of sorbose, unusually, is the L-isomer, 16.55 it can be isolated from mountain ash berries or prepared by oxidation of sorbitol, 16.56, and it is used in the commercial synthesis of vitamin C. 

---

D-glucose, dextrose, C Hi20 . The most common hexose sugar. It is present in many plants, and is the sugar of the blood. It is a constituent of starch, cellulose, glycogen, sucrose and many glycosides, from all of which it can be obtained by hydrolysis with acids or enzymes. 

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 ... 

In contrast with the well-known Embden-Meyerhof-Pamass glycolysis pathway for the conversion of hexose sugars to alcohol, the steps in conversion of ethanol to acetic acid remain in some doubt. Likely, ethanol is first oxidized to acetaldehyde and water (39). For further oxidation, two alternative routes are proposed more likely, hydration of the acetaldehyde gives CH2CH(OH)2, which is oxidized to acetic acid. An alternative is the Cannizzaro-type disproportionation of two molecules of acetaldehyde to one molecule of ethanol and one molecule of acetic acid. Jicetobacter... 

A somewhat more complicated route into glycolysis is followed by galactose, another simple hexose sugar. The process, called the Leloir pathway after Luis Leloir, its discoverer, begins with phosphorylation from ATP at the C-1 position by galactokinase ... 

Note that in this scheme, the six hexose sugars have been converted to six pentose sugars with release of six molecules of COg, and the six pentoses are reconverted to five glucose molecules. 

In extrahepatic tissues, hexokinase catalyzes the phosphorylation of most hexose sugars, including fruc-... 

Attack of the OH radical on carbohydrates of low molecular mass gives rise to a variety of products. Indeed, the reaction of radiolytically-generated OH radical with lower hexose sugars produces lower saccharides (for di- and higher saccharide species), uronic and aldonic acids, and 3-, 2- and 1-carbon aldehydic fragments, e.g. 

The compactness and complexity of (ligno)cellulose makes it much more difficult to attack by enzymes with respect to starch. Therefore, the cost of bioethanol production is higher [23], To be cost competitive with grain-derived ethanol, the enzymes used for biomass hydrolysis must become more efficient and far less expensive. In addition, the presence of non-glucose sugars in the feedstock complicates the fermentation process, because conversion of pentose sugars into ethanol is less efficient than conversion of the hexose sugars. 

The structure of the complex formed between the enzyme lysozyme and its substrate. The crevice that forms the site for substrate binding (the active site) runs horizontally across the enzyme molecule. The individual hexose sugars of the hexasaccharide substrate are shown in a darker color and labeled A-F. (Coordinates courtesy of D. C. Philips, Oxford, England.) (Illustration copyright by Irving Geis. Reprinted by permission.)... 

Table II shows similar relative rates and AFImax measured over saturated aqueous NaCl (water activity 0.76). Limited studies were also made over saturated KI solutions (water activity 0.60). Rate was somewhat greater at the higher water activity, but there was not sufficient difference to warrant further comparative study. The same relative rate order for sugar type occurs over NaCl as over water. Table III reports similar studies over activated silica. The experimental scheme does not permit rigid water activity control at near zero activity, but it is clear that rates are much lower and that the same relative rate order for pentose and hexose sugars is preserved, although the differences are much less pronounced. The triose is relatively less reactive here.

Commercially, lactic acid is manufactured by controlled fermentation of the hexose sugars from molasses, corn, or milk. Lactates are made by synthetic methods from acetaldehyde and lactonitrile, a by-product acrylonitrile production. 

Although the tegument contains specific systems for molecular and ion transport - especially amino acids, hexose sugars, vitamins, purines, pyrimidines, nucleotides, and lipids - it probably also serves a number of other vital functions (647) (a) it is a major site of catalytic activity and... 

---