3- Keto pentose

An exception to the normal behavior of ketoses with alkaline sodium hypoiodite has been noted in the case of D-xyloketose. This keto-pentose reduces very strongly and could not be distinguished in a mixture with D-xylose by the Willstatter-Schudel method. 

Base Total sugar (%) Reducing sugar (%) Aldo- pentose (%) Keto- pentose (%) Aldo- hexose (%) Keto- hexose (%)... 

Sugars with four carbon atoms are called tetroses, those with five are pentoses, those with six are hexoses, and so on. According to the above rule, there have to be two keto pentoses, each with one d and one l form, or four altogether (cf. Table 31-1). In addition, there are four keto hexoses, four aldopentoses, and eight aldohexoses, each occurring once in the d form and once in the l form. These sugars are given trivial names (Table 31-1). 

In this organism, the product of oxidation of glucose-6-phosphate and of the phosphorylation of gluconate has been demonstrated to be 6-phos-phogluconate. This substance, as will be discussed below, is oxidized and is decarboxylated with the loss of the first carbon, Ci, to form the keto-pentose phosphate, ribulose-5-phosphate (Fig. 11 A). The loss of this car-... 

Chapter 13, Carbohydrates, describes the carbohydrate molecules monosaccharides, disaccharides, and polysaccharides and their formation by photosynthesis. Monosaccharides are classified as aldo or keto pentoses or hexoses. Chiral molecules, moved from Chapter 12 to Chapter 13, are discussed along with Eischer projections and d and l notations. An Explore Your World feature models chiral objects using gumdrops and toothpicks. Carbohydrates used as sweeteners are described and carbohydrates used in blood typing are discussed. The formation of glycosidic bonds in disaccharides and polysaccharides is described. 

TPP-dependent enzymes are involved in oxidative decarboxylation of a-keto acids, making them available for energy metabolism. Transketolase is involved in the formation of NADPH and pentose in the pentose phosphate pathway. This reaction is important for several other synthetic pathways. It is furthermore assumed that the above-mentioned enzymes are involved in the function of neurotransmitters and nerve conduction, though the exact mechanisms remain unclear. 

Pyruvate-dependent lyases serve catabolic functions in vivo in the degradation of sialic acids and KDO (2-keto-3-deoxy-manno-octosonate), and in that of 2-keto-3-deoxy aldonic acid intermediates from hexose or pentose catabolism. 

Figure 2. Hypothetical scheme for the selectivity-conferring interactions (indicated as wavy lines) of hexokinase with keto- and ald-hexoses. This model accounts for all known specificity and selectivity of yeast hexokinase toward hexose substrates and pentose ligands.

Gelidium pacificum Okam. Chrondus ocellatus Holmes D-galactose D-galactose, methyl pentose, 2-keto-hexonic acid (7), sulfate, D- and irerythrose 111... 

In vivo, pyruvate lyases perform a catabolic function. The synthetically most interesting types are those involved in the degradation of sialic acids or the structurally related octulosonic acid KDO, which are higher sugars typically found in mammalian or bacterial glycoconjugates [62-64], respectively. Also, hexose or pentose catabolism may proceed via pyruvate cleavage from intermediate 2-keto-3-deoxy derivatives which result from dehydration of the corresponding aldonic acids. Since these aldol additions are freely reversible, the often unfavourable equilibrium constants require that reactions in the direction of synthesis have to be driven by an excess of one of the components, preferably pyruvate for economic reasons, in order to achieve a satisfactory conversion. 

In higher mammalian organisms, thiamine is transformed to the coenzyme thiamine pyrophosphate by direct pyrophosphate transfer from ATP. This coenzyme performs important metabolic functions, for example, as cocarboxylase in the decarboxylation of rr-keto acids (such as pyruvate to form acctyl-CoA) and in tran.sketolases (such as use of pentoses in the hexose monophosphate shunt). 

Thiamine pyrophosphate (Figure 4-8A) is involved in decarboxylation of a-keto acids and is the cofactor for the transketolase of the pentose phosphate pathway. 

The growing fatty acid chain on the fatty acid synthase complex is elongated, two carbons at a time, by the addition of the three-carbon compound, malonyl CoA, which is subsequently decarboxylated. With each two-carbon addition, the growing chain, which initially contains a P-keto group, is reduced in a series of steps that require NADPH. NADPH is produced by the pentose phosphate pathway and by the reaction catalyzed by the malic enzyme. 

Lower monosaccharides, i.e., aldo- and keto-bioses, -trioses, and -tetroses, do not exist naturally in a free state. Glyceroaldehyde and hydroxyacetone in phospho-rylated forms are the products of alcoholic fermentation and glycolytic sequence. Erythrose and erythrulose also appear in phosphorylated forms in the pentose cycle of glucose, while ketopentose-ribulose can be found as its phosphate ester (Table 5.1). 

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