Nucleotide-sugars phosphorylation

DNA and RNA are formed of nucleotides. Each nucleotide or nucleoside is composed of a purine or pyrimidine base linked to the 1-position of a ribose sugar in the case of RNA and a 2 -deoxyribose sugar in the case of DNA.155 The 5 position is phosphorylated in the case of a nucleotide, while the nucleoside is not phosphorylated therefore, nucleotides are nucleoside phosphates. Phosphorylation can include one, two, or three phosphate groups. The most familiar example of a phosphorylated nucleotide is phosphorylated adenosine, which occurs as the mono-, di-, and triphosphate (AMP, ADP, and ATP, respectively) and is a principal means of energy storage in biological systems. 

On the other hand, the enzymatic synthesis of glycoconjugates and oligosaccharides leads to high product yields in a short time by stereo- and regioselective one-step reactions. All enzymatic reactions are easy to scale up and are carried out in aqueous media under mild conditions. A whole set of enzymes is now available to build up OAT bonds in monosaccharides, COP bonds in activated monosaccharides e.g. phosphorylated sugars or nucleotide sugars, and C-O-C bonds in di- and oligosaccharides (Fig. 1). However, all these enzymatic reactions are limited by the substrate spectrum of the individual enzyme. 

Salvage pathways of nucleotide sugars represent biochemical short-cuts by which the cellular metabolism circumvents complex pathways for secondary nucleotide sugars. These de novo syntheses involve kinases phosphorylating the... 

The first steps in the pathway accomplish the synthesis of the nucleotide sugar that donates the monosaccharide units to the polymer. Fructose 6-phosphate is isomerized to mannose 6-phosphate the phosphoryl group is then transferred from the 6-position to the 1-position to generate mannose 1-phosphate. Mannose 1-phosphate and GTP are substrates for a pyrophosphorylase that catalyzes formation of GDP-mannose. GDP-mannose is oxidized to GDP-mannuronate in the final step that is catalyzed by a cytoplasmic enzyme. 

Deoxyadenosine is a related nucleoside. It contains deoxyribose instead of ribose as the sugar. Phosphorylation of deoxyadenosine produces a nucleotide found in DNA. 

These sugars are formed hy phosphorylation with ATP, e.g. glucose 6-phosphate. They are extremely important in carbohydrate metaholism. We have already seen that nucleotides contain sugar phosphates. 

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