Hexose, phosphorylation

Hanson and Anderson170 resolved the phosphorylation system of Acetobacter aerogenes into four components enzyme I, HPr, and two components required for the activity of enzyme II. The components of enzyme II are a protein of high molecular weight and a smaller, inducible protein that increases the affinity of the system for D-fructose. The D-fructose-PEP-transferase system is similar to those involved with D-fructose phosphorylation in Arthrobacter pyridinolis, and with hexose phosphorylation in Staphylococcus aureus.171... 

In other glycolytic reactions, the radioactively labeled ATP can phosphorylate at C-1 of fructose 6-phosphate and C-6 of glucose, both of which are equivalent to C-3 in 1,3-BPG. Thus, after prolonged incubation, both labeled inorganic phosphate and labeled ATP will be present in the mixture, and 1,3-BPG with a radioactive label at both C-1 and C-3 will be present in the extract. One must assume that a small amount of unlabeled ATP is available at the start to initiate hexose phosphorylation. 

Meyerhof has shown that hexose phosphorylation requires Mg ions and co-zymase, which reacts with the sugar to yield hexose diphosphate and adenylic acid in a manner comparable to the action of adenine triphosphate as a phosphate carrier in muscle glycolysis. 

T"he extraordinary ability of an enzyme to catalyze only one particular reaction is a quality known as specificity (Chapter 14). Specificity means an enzyme acts only on a specific substance, its substrate, invariably transforming it into a specific product. That is, an enzyme binds only certain compounds, and then, only a specific reaction ensues. Some enzymes show absolute specificity, catalyzing the transformation of only one specific substrate to yield a unique product. Other enzymes carry out a particular reaction but act on a class of compounds. For example, hexokinase (ATP hexose-6-phosphotransferase) will carry out the ATP-dependent phosphorylation of a number of hexoses at the 6-posi-tion, including glucose. 

In the kidney and in muscle tissues, fructose is readily phosphorylated by hexokinase, which, as pointed out above, can utilize several different hexose substrates. The free energy of hydrolysis of ATP drives the reaction forward ... 

A somewhat more complicated route into glycolysis is followed by galactose, another simple hexose sugar. The process, called the Leloir pathway after Luis Leloir, its discoverer, begins with phosphorylation from ATP at the C-1 position by galactokinase ... 

In extrahepatic tissues, hexokinase catalyzes the phosphorylation of most hexose sugars, including fruc-... 

Hexokinase is of great biological interest since it would appear that not only in yeast cells but in most, if not all, plant and animal cells phosphorylation at C6 of the common hexoses D-glucose, D-fructose and D-mannose initiates sugar utilization. Since on solution in water the crystalline hexoses quickly undergo mutarotation, resulting in an equilibrium mixture of various tautomeric modifications, the fermentability... 

The natural substrate for the dehydrogenase, glyceraldehyde-3-phosphate (G-3-P), had been synthesized earlier by Hermann Fischer, Emil Fischer s son, and Baer in 1932. In 1934 Meyerhof and Lohmann synthesized hexose diphosphate, establishing it to be fructose 1,6 bisphosphate (F-l, 6 bis P). With F-1,6 bisP as substrate and hydrazine to trap the aldehydic and ketonic products of the reaction, G-3-P was identified in the mixture of G-3-P and dihydroxyacetone phosphate which resulted. Triose phosphate isomerase was then isolated and the importance of phosphorylated 3C derivatives established. 

This enzyme [EC 2.7.1.61] catalyzes the reaction of an acyl phosphate with a hexose to produce an acid and hexose phosphate. If the sugar is D-glucose or D-man-nose, phosphorylation is on 06. If the sugar is D-fructose, phosphorylation is on 01 or 06. 

This enzyme [EC 3.1.3.9] catalyzes the hydrolysis of o-glucose 6-phosphate to yield o-glucose and orthophosphate. Some glucose phosphatases also catalyze transphosphorylation reactions from carbamoyl phosphate, hexose phosphates, pyrophosphate, phosphoenolpyru-vate and nucleoside di- and triphosphates, using D-glu-cose, D-mannose, 3-methyl-D-glucose, or 2-deoxy-D-glu-cose as phosphoryl acceptors. See Isotope Exchange (Reactions Away from Equilibrium)... 

This phosphotransferase [EC 2.7.1.1] catalyzes phosphorylation of D-glucose as well as many other aldo- and keto-hexoses (with the notable exception of o-galac-tose) MgATP is the active phosphoryl donor, and the overall reaction is highly favorable T eq = [ADP]... 

While this model explained the action of the brain enzyme on a number of hexose substrates and nonsubstrate inhibitory analogs, the mode had its weaknesses. It assumed that the other conformations of a hexose that are in equilibrium with the active conformer act as competitive inhibitors relative to this conformer. One cannot evaluate the effect of a competitive inhibitor which is present in a constant proportion relative to the active substrate by initial velocity measurements. Moreover, the use of apparent Michaelis constants may not provide accurate estimates of affinity, which is more directly related to a dissociation constant. The chief limitation of the model, however, is that an equally great number of experimental facts can be satisfactorily explained in terms of a simpler scheme involving the binding and phosphorylation of the Cl conformer. Furthermore, one can understand more directly how the enzyme can phosphorylate glucopyranose and fructofuranose equally well. 

It had been known from at least the time of Pasteur that the presence of sodium or potassium phosphate aided the progress of a yeast fermentation. Later intensive study showed that a complex group of enzymes (phosphatases and phosphorylases) was responsible for the phosphorylation, dephosphorylation and interconversion of D-glucose 6-phosphate, D-fructose 6-phosphate, D-fructose 1,6-diphosphate and similar substances in various types of cells and muscle tissue. Detailed reviews of the field are available. - A further advance was made in 1936, when Cori and Cori noted that in certain circumstances well-washed frog muscle immersed in a sodium phosphate buffer utilized the inorganic phosphate to produce a new hexose phosphate (the Cori ester). This compound was later shown to be a-D-glucopyranose-l-phosphate and yielded crystalline dipotassium and brucine salts. The Cori ester arose because... 

This reaction, which is irreversible under intracellular conditions, is catalyzed by hexokinase. Recall that kinases are enzymes that catalyze the transfer of the terminal phosphoryl group from ATP to an acceptor nucleophile (see Fig. 13-10). Kinases are a subclass of transferases (see Table 6-3). The acceptor in the case of hexokinase is a hexose, normally D-glucose, although hexokinase also catalyzes the phosphorylation of other common hexoses, such as D-fructose and D-mannose. 

In most organisms, hexoses other than glucose can undergo glycolysis after conversion to a phosphorylated... 

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