Of a hexose

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... 

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. 

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... 

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. 

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. 

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

FIGURE 20-11 Transketolase-catalyzed reactions of the Calvin cycle, (a) General reaction catalyzed by transketolase the transfer of a two-carbon group, carried temporarily on enzyme-bound TPP, from a ketose donor to an aldose acceptor, (b) Conversion of a hexose and a triose to a four-carbon and a five-carbon sugar (step of Fig. 20-10). (c) Conversion of seven-carbon and three-carbon sugars to two pentoses (step of Fig. 20-10). 

Removal of the hydroxyl group from C-2 of a hexose also increases the furanose content for example, allose, 8% and 2-deoxy-rtho-hexose, 27%. The hydroxyl groups on C-2 and C-3 are cis in allose there is therefore a substantial interaction between vicinal, cis-hydroxyl groups in furanoses. This interaction has been estimated70 to be 3.6 kj/mol. Even when the hydroxyl groups on C-2 and C-3 are trans, there is some increase in the furanose content on removal of OH-2 for example, galactose, 6% 2-deoxy-Zj/xo-hexose, 16% (Table II). 

It has been said that inositol is a sugar with carbon in the ring. Indeed it is interesting to reflect on the effect of such a simple atom-for-atom replacement on the chemical anc enzymatic reactivities of a hexose, when compared with a rather inert cyclitol. 

The propensity for -elimination of an anionic sites at C-6 of a hexose derivativ requires the judicious choice of reaction conditions, protective groups, and aldehyde activt tion modes for intramolecular carbocyclization. In addition, one must appreciate that... 

Compound 83 is a diene that comprises four carbon atoms of a hexose chain, and it takes part in the Diels-Alder reaction with maleic anhydride to give 86% of the product 84, the stereochemistry of the addition being directed by the acetal ring [42]. 

Figure 17-8 The pentose phosphate pathways. (A) Oxidation of a hexose (C6) to three molecules of C02 and a three-carbon fragment with the option of removing C3, C4, C5, and C7 units for biosynthesis (dashed arrows). (B) Non-oxidative pentose pathways 2 1/2 C6 —> 3 C5 or 2 C6 —> 3 C4 or 3 V2C6 —> 3 C7.

Glycosaminoglycans (GAGs) are unbranched chains having repeating disaccharide units, which, with the exception of keratan sulfate, contain an acid and a base. The KS disaccharide unit consists of a hexose and a base. The structures of the different classes of GAG disaccharide units are given in Fig. 1, and summarized in Table I. 

The nature and position of the carbon-nitrogen linkage in the 2-amino sugars endows them with unique properties and biological function that differ widely for example, from those of the nucleotide and nucleoside type of molecule. These substances are theoretically derived by condensation of an amine with carbon atom 1 of a hexose. Although no biogenetic relationship between these two groups has yet been discovered such a possibility cannot be discounted. 

Figure III-3 Physical meaning of the Fischer projection of a hexose.

There is good evidence that genes coding for NDP-hexose synthetases (hexose-1-phosphate nucleotidyltransferases, NDP-hexose pyrophosphorylases) are involved in the formation of deoxysugars in various organisms. As was mentioned before (see Sect. 2.1 Scheme 6b), these enzymes catalyze the conversion of a hexose 1-phosphate to a NDP-hexose. 

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