**5. References**


**1. Introduction**

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*Chile*

**On the Specialization History of the**

Felipe Merino and Victoria Guixé *Laboratorio de Bioqímica y Biología Molecular*

> *Facultad de Ciencias Universidad de Chile*

**ADP-Dependent Sugar Kinase Family**

Sugars are one of the most common carbon sources used by heterotrophic organisms. Indeed, sugar phosphorylation is thought to be a key step in the cellular metabolism since, just after transport into the cell, these molecules are phosphorylated to trap them for further metabolic processing. There are several known pathways used to produce pyruvate from the incoming sugar (like glucose or galactose) which is accompanied by the synthesis of ATP and the production of reductive power. Amongst them, the Embden-Meyerhof pathway, or glycolysis, seems to be the most commonly used. Some microorganisms can also use the Entner-Doudoroff pathway. Also, although the pentose phosphate pathway is generally associated with nucleotide synthesis and reductive power in the form of NADPH it also can be linked to the flux from glucose to pyruvate as this pathway has fructose-6-phosphate and glyceraldehyde-3-phosphate as intermediates. Some microorganisms, such as *Lactococcus lactis*, use a pathway very similar to glycolysis, but instead of start from glucose they use galactose as main carbon source. In this fashion glucokinases are replaced by galactokinases and phosphosfructokinases by tagatose-6-phosphate kinases (van Rooijen et al., 1991). Interestingly, all the above mentioned pathways ultimately converge through glyceraldehyde-3-phosphate. In this way, the main difference between them is what happens with the hexoses. Here, one of the most important reactions are the initial phosphorylations, e.g. phosphorylation of glucose, fructose-6-phosphate, galactose, tagatose-6-phosphate. Early on the 90s it was already recognized that the transfer of the *γ*-phosphate of ATP to several sugars was catalyzed by at least three different non-homologous protein families: the hexokinase family, the ribokinase family, and the galactokinase family (Bork et al., 1993). The hexokinase family contains enzymes with wide specificities including glucokinases, ribulokinases, gluconokinases, xylulokinases, glycerokinases, fructokinases, rhamnokinases, and fucokinases (Bork et al., 1993). The galactokinase family contains enzymes that catalyze the phosphorylation of galactose, mevalonate, P-mevalonate, and homoserine (Bork et al., 1993). The ribokinase family on the other hand is very interesting since its members catalyze the transfer of the terminal phosphate of ATP to sugars like ribose, fructose, sugar containing molecules such as nucleosides, and sugar phosphate molecules like fructose-6-phosphate, fructose-1-phosphate, and tagatose-6-phosphate (Bork et al., 1993). This makes the ribokinase family the group with the broadest specificity amongst the above mentioned. It is clear that while the three groups share some similar substrates and hence are a great example of

