Monosaccharides glyceraldehyde

For example, (fl)-glyceraldehyde, the simplest monosaccharide, can be drawn as in Figure 25.1. 

The presence of an asymmetric carbon atom confers the property of optical activity on the molecule, enabling it to cause the rotation of a beam of plane polarized light in either a clockwise or an anticlockwise direction. Thus all naturally occurring carbohydrates containing asymmetric carbon atoms are optically active and can be designated (+) for clockwise (dextro) rotation or (-) for anticlockwise (laevo) rotation. The designation of d or l to glyceraldehyde, the simplest monosaccharide, which has only one asymmetric centre, refers to... 

Fructose is found in honey and fruit and as part of the disaccharide sucrose (common table sugar). Sucrose is hydrolyzed by intestinal brush border sucrase, and the resulting monosaccharides, glucose and fructose, are absorbed into the portal blood. The liver phosphorylates frurtose and cleaves it into glyceraldehyde and DHAP. Smaller amounts are metabolized in renal proximal tubules. The pathway is shown in Figure 1-12-7 important enzymes to remember are ... 

The most important natural monosaccharide, D-glucose, is an aliphatic aldehyde with six C atoms, five of which carry a hydroxyl group (1). Since C atoms 2 to 5 represent chiral centers (see p. 8), there are 15 further isomeric aldohexoses in addition to D-glucose, although only a few of these are important in nature (see p.38). Most natural monosaccharides have the same configuration at C-5 as D-glyceraldehyde-they belong to the D series. 

The nomenclature and classification of monosaccharides is further complicated by the presence of chiral carbons within monosaccharides. In the late nineteenth century it was ascertained that the configuration of the last chiral carbon in each of the naturally occurring monosaccharides is the same as that for (-i-)-glyceraldehyde. This configuration was designated as D and it was determined that all naturally occurring monosaccharides were in the D configuration. 

All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms (pp. 17-19). The simplest aldose, glyceraldehyde, contains one chiral center (the middle carbon atom) and therefore has two different optical isomers, or enantiomers (Fig. 7-2). 

In general, a molecule with n chiral centers can have 2 stereoisomers. Glyceraldehyde has 21 = 2 the aldohexoses, with four chiral centers, have 24 = 16 stereoisomers. The stereoisomers of monosaccharides... 

The simplest carbohydrates, sometimes referred to as monosaccharides, or sugars, are either polyhydroxyaldehydes (aldoses) or polyhydroxyketones (ketoses). They can be derived from polyalcohols (polyols) by oxidation of one carbinol group to a carbonyl group. For example, the simple three-carbon triol, glycerol, can be converted either to the aldotriose, glyceraldehyde, or to the ketotriose, dihydroxyacetone, by loss of two hydrogens (fig. 12.1). 

Fischer projections are however, unsatisfactory when considering the physical properties and chemical reactivity of monosaccharides for which definitive spatial formulations are necessary. These are given below for D-glyceraldehyde, D-erythrose and D-threose, for which the (/ ,S configuration may be readily assigned at the appropriate chiral carbons. 

Fructose 1,6-biphosphate aldolase from rabbit muscle in nature reversibly catalyzes the addition of dihydroxyacetone phosphate (DHAP) to D-glyceraldehyde 3-phosphate. The tolerance of this DHAP-dependent enzyme towards various aldehyde acceptors made it a versatile tool in the synthesis of monosaccharides and sugar analogs [188], but also of alkaloids [189] and other natural products. For example, the enzyme-mediated aldol reaction of DHAP and an aldehyde is a key step in the total synthesis of the microbial elicitor (—)-syringolide 2 (Fig. 35a) [190]. 

Draw Fischer projections for these monosaccharides a) L-Glyceraldehyde h) L-Mannose... 

We now know the absolute configurations of (+) - and (—) -glyceraldehyde. These structures serve as the configurational standards for all monosaccharides. 

Fig. 11-17 The conversion of the monosaccharide fructose into the glycolytic intermediates dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.

Drawing Monosaccharide Structures The Fischer Projection. From the stereochemical point of view, monosaccharides are considered to be derived from the two trioses, D- and L-glyceraldehyde (see Fig. III-l). These two parent compounds differ only in the steric arrangement of the atoms about the central asymmetric carbon they are mirror images (enantiomorphs, optical antipodes) of one another. 

T12. Thornalley, P., Wolff, S., Crabbe, J., and Stern, A., The autoxidation of glyceraldehyde and other simple monosaccharides under physiological conditions catalysed by buffer ions. Biochim. Biophys. Acta 797, 276-287 (1984). 

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