Glucose, phosphorylation

GK is present in the cells in both an active (GKa) and an inactive (GKb) form. The inactive form is bound to intracellular membrane or to special regulating proteins, and the balance between the two forms is glucose sensitive [21-24]. The result is a varying activity of the amount of GK present in the cell, and a general finding is a relatively fast variation with time constants in the order of 20-60 min [21, 23, 24]. Also the total amount of GK may vary, but most investigators find that the total amount of glucokinase in the beta cell is relatively constant over the day [25, 26]. 

The glucose dependence of the GK activity is likely to be an essential part of glucose sensing, but as free glucose appears not to play a role inside the cell except 

The phosphorylation rate (v) of the isolated glucokinase enzyme is given by the Michaelis-Menten expression [27]  

The removal of the inflowing glucose may be rather complex, as described later, but as a first approximation, it is assumed to be proportional to S, so  

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Figure 19-5. Variation in glucose phosphorylating activity of hexokinase and glucokinase with increase of blood glucose concentration. The for glucose of hexokinase is 0.05 mmol/L and of glucokinase is 10 mmol/L.

Benveniste H, Drejer J, Schousboe A, Diemer NH. 1987. Regional cerebral glucose phosphorylation and blood flow after insertion of a microdialysis fiber through the dorsal hippocampus in the rat. J Neurochem 49(3) 729-734. 

There are two different glucose-phosphorylating enzymes, hexokinase and glucokinase, which catalyse the reaction ... 

There are several different glucose transporters. There are two glucose phosphorylating enzymes glucokinase and hexokinase (Chapter 6). 

Figure 6-1. The steps of glycolysis. Feedback inhibition of glucose phosphorylation by hexokinase, inhibition of pyruvate kinase, and the main regulatory, rate-limiting step catalyzed by phosphofructoki-nase (PFK-I) are indicated, pyruvate formation and substrate-level phosphorylation are the main outcomes of these reactions. Regeneration of NAD occurs by reduction of pyruvate to lactate during anaerobic glycolysis.

Fig. 1. Some interrelationships between glucose-6-phosphatase and other enzymes of carbohydrate metabolism [from Nordlie (10) copyright (1968), Academic Press, Inc. Reproduced by permission]. Numbers in parentheses indicate the relative disposition per 100 molecules of glucose phosphorylated of glucose-6-P via four alternate metabolic pathways, according to Ashmore et al. (69). Further details are given in the text.

It previously has been pointed out that, potentially, PP glucose phosphotransferase and ATP-glucose phosphotransferase activities of glucose-6-phosphatase are the most potent glucose phosphorylating systems which have been characterized for liver (9, 10, 41, 118). Such a conclusion appears to have possible validity principally at and below pH 7 however (see Fig. 9) because of the nature of the pH-activity profiles of the phosphatase-associated phosphotransferase activities. 

The glucose phosphorylation reaction (16.5) has a large positive ArG0 of 13.8 kJ-mol-1 of glucose. This ArG° is large enough that the relative... 

Notice that below the threshold, the glucose phosphorylation is not sufficient to sustain the beta cells energy expenditures. This is particularly seen in the vertical jumps for small values of Rox. However, in practice the threshold is smoother, because other nutrients like fatty acids and amino acids can take over [32]. 

To summarise, it should be said that the PolyP content and chain length in Propionibac-teria are strongly dependent on the carbon source. These bacteria possess polyphosphate glucokinase and are able to directly utilize PolyP for glucose phosphorylation. 

Fig. 11-4 The dependence of the rate of glucose phosphorylation on glucose concentration for hexokinase and glucokinase. The Km for hexokinase is much lower than that for glucokinase.

Phosphorylation of glucose Phosphorylation of fructose 6-phosphate Dephosphorylation of 2 molecules of 1,3-BPG Dephosphorylation of 2 molecules of phosphoenolpyruvate... 

The rate of D-glucosamine phosphorylation is about 70% of that for D-glucose phosphorylation. Free D-glucosamine and adenosine-5-tri-phosphoric acid disappear at similar rates in the presence of yeast hexokinase. ... 

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