Sugars diphosphates

Each of these compounds, 53-56, was shown to be a very effective competitive inhibitor of the enzyme with respect to the fructose 1,6-diphosphate, whereas several other analogs, including acyclic structures, had no effect. These and other results suggest that the furanose form of the sugar diphosphate is the active form in the enzymatic reaction (105). More recent studies using rapid quenching techniques and C-nmr measurements have confirmed this hypothesis and indicate that the enzyme uses the a anomer 52 much more rapidly than the 3 anomer 50 and probably uses the a anomer exclusively (106). 

Crystalline phosphoric acid has also been used to prepare sugar diphosphates, e.g. a-D-ribose 1,5-diphosphate (62) and hexose 1,6-diphosphates. 

Fig. 10 Two schematic representations of a polaron-like species in DNA. In the top drawing, the base pairs of DNA are represented by the horizontal lines the sugar diphosphate backbone is represented by the vertical lines. The polaronic distortion is enclosed in the box and extends over some number of base pairs. This is shown schematically by drawing the base-pair lines closer together. In the lower figure, a specific potential po-laron is identified, AAGGAA, and the radical cation is presented as being delocalized over this sequence. Movement of the polaron from one AAGGAA sequence to the next requires thermal activation...

The stabilization of the radical cation by forming a polaron is a trade-off between its delocalization and the energy required to distort the DNA structure. The former lowers the kinetic energy of the intrinsically quantum mechanical migrating radical cation, and the latter will be determined by factors that are independent of specific base sequence, such as the force constants of bonds in the sugar diphosphate backbone. 

Fig. 4. Radioautogram of products of 60 seconds photosynthesis with C 0-. Radioautograph of two-dimensional paper chromatogram of products formed by Chlorella pyrenoidosa during 60 seconds of photosynthesis with C 0 . Abbreviations P, POaH" UDPG, uridine diphosphoglucose PGA, 3-phosphoglyceric acid PEPA, phosphoenolpyruvic acid. Sugar diphosphate includes ribulose-l,5-diphosphate, sedoheptulose-1,7-diphosphate, and fructose-1,6-diphosphate.

The solubility of the barium salts depends on the number of acid groups and therefore on the pH of the solution. At neutral pH where the primary and secondary acid groups of phosphate are dissociated, inorganic phosphate and the sugar diphosphates form water-insoluble barium salts. At a pH of about 4 the secondary acid groups are not appreciably dissociated so that the salts become water-soluble. 

There is a second large gene duster, about 35kb, also involved in the synthesis of xanthan. The predse function is unknown but it is thought to encode for proteins involved in the synthesis of sugar nucleotide diphosphates. 

If a sugar is esterified with two or more phosphate groups, the compound is termed bisphosphate, trisphosphate etc. (e.g. fructofuranose 1,6-bisphosphate). The term diphosphate denotes an ester with diphosphoric acid, e.g. adenosine S -diphosphate. 

For most of the sugar components, little or nothing is known about their biosynthesis. Nucleoside hexosyl-4-ulose diphosphates are, however, almost certainly key intermediates in the biosynthesis of several of these sugars, as discussed in Ref 7. The biosynthesis of the 6-deoxyheptoses is probably analogous to that of the 6-deoxyhexoses, and proceeds by way of nucleoside heptosyl-4-ulose diphosphates. 

Other nucleoside diphosphate sugar compounds are known, eg, UDPGal. In addition, the same sugar may be linked to different nucleotides. For example, glucose may be linked to uridine (as shown above) as well as to guanosine, thymidine, adenosine, or cy-tidine nucleotides. 

Figure 20-7. Summary of the interrelationships in metabolism of amino sugars. (At asterisk Analogous to UDPGIc.) Other purine or pyrimidine nucleotides may be similarly linked to sugars or amino sugars. Examples are thymidine diphosphate (TDP)-glucosamine and TDP-N-acetylglucosamine.

A primed numeral locates the position of the phosphate on the sugars of mononucleotides (eg, 3 -GMP, 5 -dCMP). Additional phosphoryi groups linked to the first by acid anhydride bonds form nucleoside diphosphates and triphosphates. 

Most nucleotide sugars are formed in the cytosol, generally from reactions involving the corresponding nucleoside triphosphate. GMP-sialic acids are formed in the nucleus. Formation of uridine diphosphate galactose (UDP-Gal) requires the following two reactions in mammahan tissues ... 

C. Nucleoside Diphosphate Sugars.—A polyprenol phosphate containing eleven isoprene units is involved in the biosynthesis of various bacterial cell-wall components.As mentioned in last year s Report, another isoprenoid phosphate, dolichol monophosphate (40), is an intermediate in sugar... 

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