A-hexoses

Other aldohexoses behave similarly m adopting chair conformations that permit the CH2OH substituent to occupy an equatorial orientation Normally the CH2OH group is the bulkiest most conformationally demanding substituent m the pyranose form of a hexose... 

Because a hexose contains four chiral carbon atoms, there are 2 = 16 different possible arrangements of the hydroxyl groups in space, ie, there are 16 different stereoisomers. The stmctures of half of these, the eight D isomers, are shown in Figure 1. Only three of these 16 stereoisomers are commonly found in nature D-glucose [50-99-7] D-galactose [59-23-4] and D-mannose [3458-28-4]. 

The parent with the greatest number of carbon atoms in the chain, e.g. a heptose rather than a hexose. 

Monosaccharides have the formula (CH2 0) , where n is between 3 and 6. Of the 70 or so monosaccharides that are known, 20 occur in nature. The most important naturally occurring monosaccharides contain five carbons (pentoses) or six carbons (hexoses). Structures of ribose, an important pentose, and a-glucose, a hexose that is the most common monosaccharide, are shown in Figure 13-14. As shown in the figure, it is customary to number the carbon atoms in a monosaccharide, beginning with the HCOH group adjacent to the ether linkage. 

The six-membered ring of a hexose such as a-glucose takes on a puckered structure. A molecular plane can be defined that passes through the midpoints of all the bonds of the ring. Bonds to non-ring atoms are either perpendicular to this plane (shown in green) or roughly parallel to the plane (shown in brown). 

Hexose monosaccharides can form both five- and six-membered rings. In most cases, the six-membered ring structure is more stable, but fructose is an important example of a hexose that is more stable as a five-membered ring. The structure ofy6-fhictose is shown in Figure 13-17. Notice that there are — CH2 OH fragments bonded to two positions of this five-membered ring. Examples and explore the structures of monosaccharides in more detail. 

Fractose is a hexose monosaccharide that forms a five-membered ring stmcture. In the ball-and-stick model, the ring is highlighted in blue. 

Although the chemical formulas indicate that ribose is a pentose and fructose is a hexose, the ring portions of the structures are the same size. Proceeding clockwise around the rings from the oxygen atom, we see that the structures differ at the first two positions. In the first position, ribose has a carbon atom bonded to —H and —OH, while)S-fructose has a carbon bonded to —OH and — CH2 OH. In the second position the molecules have the same two bonds but in different orientations. The OH group points up in y6-fructose and down in ribose. The two molecules have the same structures at the other positions. 

This procedure provided the most important method for the synthesis of anhydro sugars of the hydrofuranol type. 3,6-Anhydro sugars have resulted from the saponification of hexoses esterified at C6 by the following cationogens Br- 3 36 14- 36 CH3 C H4 S020 37, 3S- 39> 40 HO-SCVO-41 42 02N-0-.43 It is very probable that the phosphate ion is also cationogenic and that saponification of a hexose 6-phosphate... 

In the well known "transketolase reaction" [9] for instance, the transfer of the fragment H0CH2-C=0 from a hexose to a triose takes place via the "active glycoaldehyde" (Scheme 5.8) ... 

Both D-[l- C]xylose and D-[5- C]arabinose were exposed to a concentrated phosphate buffer solution (pH 6.7). 1-Hydroxy-2-propanone (ace-tol) was distilled from the heated solution. Radioassay indicated that similar labeling [3- C] occurred in the acetol from both pentoses, with loss of the configurational difference thus, a 3-ketopentose or its enediol was suggested as an intermediate. Further work with 3-0- and 6-0-methyl-D-glucose and with 1-0-methyl-D-fructose indicated that /3-elimination from a 3-ketose or, in the case of a hexose, from a 3-ketose or a 4-ketose, or both, tautomerization of the resulting a-diketone to a /3-diketone, and hydrolytic cleavage are essential steps in the formation of acetol. 

This enzyme [EC 2.7.1.61] catalyzes the reaction of an acyl phosphate with a hexose to produce an acid and hexose phosphate. If the sugar is D-glucose or D-man-nose, phosphorylation is on 06. If the sugar is D-fructose, phosphorylation is on 01 or 06. 

While this model explained the action of the brain enzyme on a number of hexose substrates and nonsubstrate inhibitory analogs, the mode had its weaknesses. It assumed that the other conformations of a hexose that are in equilibrium with the active conformer act as competitive inhibitors relative to this conformer. One cannot evaluate the effect of a competitive inhibitor which is present in a constant proportion relative to the active substrate by initial velocity measurements. Moreover, the use of apparent Michaelis constants may not provide accurate estimates of affinity, which is more directly related to a dissociation constant. The chief limitation of the model, however, is that an equally great number of experimental facts can be satisfactorily explained in terms of a simpler scheme involving the binding and phosphorylation of the Cl conformer. Furthermore, one can understand more directly how the enzyme can phosphorylate glucopyranose and fructofuranose equally well. 

This enzyme [EC 2.7.7.28], also known as NDP-hexose pyrophosphorylase, catalyzes the reaction of a nucleoside triphosphate with a hexose 1-phosphate to produce a NDP-hexose and pyrophosphate (or, diphosphate). In the reverse reaction the NDP-hexose can be, in decreasing order of activity, guanosine, inosine, and adenosine diphosphate hexoses in which the sugar is either glucose or mannose. 

Common table sugar, sucrose, is a disaccharide composed of a hexose form of glucose bonded to a pentose form of fructose. 

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