Fructose phosphorylation

The kinetics of fructose phosphorylation catalyzed by permeabilized membranes [48],... 

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. 

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... 

Non-sulfur, purple, photosynthetic bacteria, Rho do spirillum rub-rum and Rhodopseudomonas spheroides172 also possess a PEP-de-pendent D-fructose phosphotransferase. Two protein fractions are required for D-fructose phosphorylation. In contrast to PEP-depend-ent, phosphotransferase systems isolated from other bacteria, the aforementioned two organisms have one active protein fraction tightly associated with the membrane fraction, while another in the crude extract is solubilized by extraction with water, and has a molecular weight of about 200,000. There is no evidence for the presence of a phosphate-carrier protein of low molecular weight like HPr.171,173 The... 

Other tissues also have the capacity to metabolize fructose but do so much more slowly. The hexokinase isoforms present in muscle, adipose tissue, and other tissues can convert fructose to fructose 6-phosphate, but react so much more efficiently with glucose. As a result, fructose phosphorylation is very slow in the presence of physiologic levels of intracellular glucose and glucose 6-phosphate. 

It has been suggested that ATP is utilized for fructose phosphorylation in preference to protein synthesis, and that this results in an interference with uric acid metabolism. 

Figure 6.24 The function of the enzyme phosphofructokinase. (a) Phosphofructokinase is a key enzyme in the gycolytic pathway, the breakdown of glucose to pyruvate. One of the end products in this pathway, phosphoenolpyruvate, is an allosteric feedback inhibitor to this enzyme and ADP is an activator, (b) Phosphofructokinase catalyzes the phosphorylation by ATP of fructose-6-phosphate to give fructose-1,6-bisphosphate. (c) Phosphoglycolate, which has a structure similar to phosphoenolpyruvate, is also an inhibitor of the enzyme.

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). 

Pyruvate kinase possesses allosteric sites for numerous effectors. It is activated by AMP and fructose-1,6-bisphosphate and inhibited by ATP, acetyl-CoA, and alanine. (Note that alanine is the a-amino acid counterpart of the a-keto acid, pyruvate.) Furthermore, liver pyruvate kinase is regulated by covalent modification. Flormones such as glucagon activate a cAMP-dependent protein kinase, which transfers a phosphoryl group from ATP to the enzyme. The phos-phorylated form of pyruvate kinase is more strongly inhibited by ATP and alanine and has a higher for PEP, so that, in the presence of physiological levels of PEP, the enzyme is inactive. Then PEP is used as a substrate for glucose synthesis in the pathway (to be described in Chapter 23), instead... 

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 ... 

Another simple sugar that enters glycolysis at the same point as fructose is mannose, which occurs in many glycoproteins, glycolipids, and polysaccharides (Chapter 7). Mannose is also phosphorylated from ATP by hexokinase, and the mannose-6-phosphate thus produced is converted to fructose-6-phosphate by phosphomannoisomerase. 

Fructose 6-phosphate is phosphorylated by reaction with ATP to yield fructose 1,6-bisphosphate. 

Step 3 of Figure 29.7 Phosphorylation Fructose 6-phosphate is converted in step 3 to fructose 1,6-bisphosphate (FBP) by a phosphofmctokinase-catalyzed reaction with ATP (recall that the prefix bis- means two). The mechanism is similar to that in step 1, with Mg2+ ion again required as cofactor. Interestingly, the product of step 2 is the tv anomer of fructose 6-phosphate, but it is the (3 anomer that is phos-phorylated in step 3, implying that the two anomers equilibrate rapidly through the open-chain form. The result of step 3 is a molecule ready to be split into the two three-carbon intermediates that will ultimately become two molecules of pyruvate. 

Transfer of the phosphoryl group to ADP in step 10 then generates ATP and gives enolpyruvate, which undergoes tautomerization to pyruvate. The reaction is catalyzed by pyruvate kinase and requires that a molecule of fructose 1,6-bis-phosphate also be present, as well as 2 equivalents of Mg2+. One Mg2+ ion coordinates to ADP, and the other increases the acidity of a water molecule necessary for protonation of the enolate ion. 

Functionally related to FruA is the novel class I fructose 6-phosphate aldolase (FSA) from E. coli, which catalyzes the reversible cleavage of fructose 6-phosphate (30) to give dihydroxyacetone (31) and d-(18) [90]. It is the only known enzyme that does not require the expensive phosphorylated nucleophile DHAP for synthetic purpose. 

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

Fructose 2,6-bisphosphate is formed by phosphorylation of fructose 6-phosphate by phosphofructoki-nase-2. The same enzyme protein is also responsible for its breakdown, since it has fructose-2,6-hisphos-phatase activity. This hifrmctional enzyme is under the allosteric control of fructose 6-phosphate, which stimulates the kinase and inhibits the phosphatase. Hence, when glucose is abundant, the concentration of fructose 2,6-bisphosphate increases, stimulating glycolysis by activating phosphofructokinase-1 and inhibiting... 

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