Glucose phosphorylation and

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. 

Phosphate esters of glucose, fructose, and other monosaccharides are important metabolic intermediates, and the ribose moiety of nucleotides such as ATP and GTP is phosphorylated at the 5 -position (Figure 7.13). 

Glycolysis and the citric acid cycle (to be discussed in Chapter 20) are coupled via phosphofructokinase, because citrate, an intermediate in the citric acid cycle, is an allosteric inhibitor of phosphofructokinase. When the citric acid cycle reaches saturation, glycolysis (which feeds the citric acid cycle under aerobic conditions) slows down. The citric acid cycle directs electrons into the electron transport chain (for the purpose of ATP synthesis in oxidative phosphorylation) and also provides precursor molecules for biosynthetic pathways. Inhibition of glycolysis by citrate ensures that glucose will not be committed to these activities if the citric acid cycle is already saturated. 

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.

There are no new messengers which act as cell-membrane transmitters in plants except cytokinins calcium is more widely used than in unicellular organisms but much less so than in animals. Plants respond to light via phosphorylation and changes from dormancy requires die change of cell calcium. Response times >1 s. There are several other sensors which are sometimes described as hormones, e.g. glucose and NO. 

Protein phosphorylation in response to rising cAMP levels activates enzymes involved in maintaining blood glucose levels and in making ATP. 

PROTEIN PHOSPHORYLATION IN RESPONSE TO RISING cAMP LEVELS ACTIVATES ENZYMES THAT MAINTAIN GLUCOSE LEVELS AND RETRIEVE ENERGY. PHOSPHORYLATION INACTIVATES ENZYMES THAT STORE GLUCOSE, FAT, AND PROTEIN. 

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