Hexose ring

General acid catalysis by glutamic acid-35 represents at present the mechanism for lysozyme best able to explain the kinetic and structural data. For it to occur, however, distortion of the hexose ring in subsite D to a half-chair must take place so that relief of strain in the transition state will make bond breaking sufficiently easy. A question that must be answered for this picture to be tenable is whether relief of strain can so greatly facilitate bond breaking when the carbonium ion is a glycosyl ion [see also the discussion in Fife (1972) and Atkinson and Bruice (1974)]. 

Aminoglycosides have a hexose ring, either streptidine (in streptomycin) or 2-deoxystreptamine (in other aminoglycosides), to which various amino sugars are attached by glycosidic linkages (Figures 45-1 and 45-2). They are water-soluble, stable in solution, and more active at alkaline than at acid pH. 

Deoxy-D-arabtno-hexose ( 2-deoxy-D-glucose ) is not a substrate for the growth, respiration, or fermentation of yeasts. However, it is metabolized, although the extent to which the hexose ring is cleaved is small. The compound is phosphorylated by hexokinase, with ATP as the donor.3660 2-Deoxy-D-arabtno-hexose is employed as an ana-... 

In subsequent steps this intermediate is likely converted to an activated nucleoside-diphosphate, (NDP)-mannose, followed by dehydration to generate an NDP-4-keto-6-deoxy-D-maimose, a reaction catalyzed by GDP-D-mannose-4,6-dehydratase (MDH). This intermediate would then be converted to GDP-L-hicose by an NDP-L-fiicose synthetase, which catalyzes epimerization at the C-3 and C-S positions of the hexose ring and an NADPH-dependent reduction at the C-4 position (22, 23). In the proposed pathway (Figure 2), conversion of the NDP-L-fiicose (a pyranose) to the fiiranose, and oxidation of the S-keto group would provide the activated S-dehydro-a-L-fiicofuranose for attachment to the C4 hydroxyl of ( )-3-(3,4-dihydroxyphenyl)-2-methylacrylic acid-derived moiety of hygromycin A. 

Pentose Rings (Figure 9.10, Figure 9.11, Table 9.2, Structure) Hexose Rings (Structure, Figure 9.12, Figure 9,13)... 

The hexose rings in these analogues have a different regiochemistry of attachment to the backbone - C-6, C-4 in h-DNA and C-4, C-2 in p-RNA - and the base stacking is different. For example, studies of self-complementary sequences, such as 5 -TTTTAAAA-3 and 5 -AAAATTTT-3, which show equivalent energies with native duplexes, are quite different with the modified backbones. In p-RNA, the for the duplex of the first sequence is much higher than that of the second. In h-DNA, the situation is reversed and the duplex of the second sequence is more stable than the first. The different backbones have been proposed to alter the inclination between the backbone... 

Another variation of the classical primary polynucleotide structure (10.90a) was established by the synthesis of derivatives based on hexose rings (10.92b). The use of the latter in place of ribose rings produces a more linear chain which forms a non-helical double-stranded arrangement. However, some of these polyhexose chains appear to form double-stranded arrangements more stable than duplex DNA built from (10.92a). Similarly for RNA analogues [62]. 

Cellulose acetate is a cellulose derivative in which each hexose ring of the polysaccharide chain has two hydroxyl groups esterified to acetate (in general, in the C-3 and C-6 positions). This medium was developed for electrophoresis largely through the work of Kohn [II], who also designed the original tank, produced by Shandon Southern... 

A pair of ions arising the further loss of a hexose ring (i.e., 162 amu) from the [M-I-NH4-17] and [M-I-NH4-35] ions, respectively. 

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