Fundamental Ketoses

The most important ketoses are the hexos-2-uloses H0CH2(CH2) 3C0CH20H, such as fructose. They have one less chiral centre than the aldoses of the same chain length i.e., there are only four diastereomerically different hexos-2-uloses. 

---

As the name implies, carbohydrates are characterized by a nominal (CH20) composition. Next to proteins, carbohydrates combined into polysaccharides are generally the most abundant (Table 8.6) and widely distributed biopol5miers in living organisms. (However, there are some plankton in which the lipid concentration exceeds that of carbohydrates (Wakeham et al, 1997).) The fundamental monomeric rmits of most carbohydrates are five- (pentose) and six-(hexose) carbon sugars, of which there are roughly ten common forms (Fig. 8.9). These structural units usually occur as either aldoses or ketoses, in which either the first (aldose) or second (ketose) carbon... 

Fructose-1,6-diphosphate aldolase of rabbit muscle has been studied very extensively-and it is now commercially available. That of spinach leaves, obviously very accessible, was recently examined (Valentin and Bolte 1993). In the fundamental reaction of glycolysis (reaction 6.8), the donor is dihydroxyacetone phosphate. It can scarcely be varied, but there is more flexibility with the acceptor (David et al. 1991 Bednarski et al. 1989), and sometimes we can wander considerably from the subject of sugar chemistry. In any case, the vicinaZ-diol created at positions 3 and 4 (uloses numbering) has the D-threo configuration. Hence the condensation of the keto aldehyde 6.28 gives ketose 6.29 which, after isolation, is dephosphorylated enzymically in the presence of acid phosphatase. 

The name carbohydrates comes from their composition which is represented by the general formula C (H20) which resembles the combination of carbon and water. This composition implies that a molecule of monosaccharide consists of a hydrocarbon chain with attached hydroxyl groups. More detailed analysis shows that most of the monosaccharide molecules have hydrocarbon chains which are from five to seven carbon atoms long. From the study of the chemical behavior of different monosaccharides it follows that some of them exhibit reactions typical for aldehydes and others show reactions typical for ketones. The monosaccharides can therefore be classified into two groups aldoses and ketoses. In their nomenclature, the names of carbohydrates are formed from the root based on the number of C-atoms, the fundamental functional group (ketone or aldehyde) and the suffix -ose. While the aldehyde group always contains the first carbon atom of the chain, keto-group in all known carbohydrates appears at the second carbon atom of the chain. Some monosaccharides named in accordance with these rules are represented in the scheme below. 

In the case of ketoses too trivial names are still frequently preferred for the fundamental types although systematic... ulose names can easily be generated in a manner similar to that used for the systematic names of aldoses (see Table 16). 

---