Glucose 6 phosphate in Metabolic Processes Glucose 6 phosphate acts as a branch point for a wide variety of pathways It can be used 1 as a glycolytic substrate 2 as a substrate for other synthetic reactions 3 as a substrate for glycogen synthesis 4 as a source of biosynthetic reducing equivalents and intermediates via the hexose monophosphate shunt or 5 in liver and kidney only be converted back to glucose and released into the bloodstream The flow of glucose through each of these processes is tightly regulated We have already covered the glycolytic and gluconeogenic pathways we will now turn to other possible fates of glucose 6 phosphate Hexose monophosphate shunt Pentose phosphate pathway Use and Importance The hexose monophosphate pathway is used for production of NADPH from NADP The NADPH is required for biosynthetic reactions such as fatty acid synthesis cholesterol synthesis drug reduction and as a cofactor for some non synthetic enzymatic reactions In addition it is used for the production of ribose for nucleotide and nucleic acid synthesis The hexose monophosphate pathway also allows the entry of some carbohydrates into the glycolytic pathway especially ribose but also some others and therefore acts as a connection route between different pathways Steroidogenic tissues red blood cells and the liver are the major sites of hexose monophosphate pathway Muscle has small amounts of some of the hexose monophosphate pathway enzymes because it has little need for synthetic reactions and therefore little need for NADPH The muscle however like all tissues needs to be able to synthesize ribose in order to make nucleotides and nucleic acids Features of the hexose monophosphate pathway Like the glycolytic enzymes the enzymes of the hexose monophosphate pathway are found in the cytoplasm Copyright 2000 2003 Mark Brandt Ph D 39 Although the hexose monophosphate pathway has fewer enzymes than glycolysis it is somewhat more complicated The net reaction for the pathway is somewhat variable depending on the entry and exit points of the metabolites The full pathway is shown below it is important to remember that not all of these reactions occur in all tissues Hexose monophosphate pathway reactions NADPH generating steps Glucose 6 phosphate dehydrogenase Glucose 6 phosphate dehydrogenase is the rate limiting step and primary control point of the NADPH generating portion of the hexose monophosphate pathway Glucose 6 phosphate dehydrogenase is stimulated by NADP and inhibited by NADPH The levels of glucose 6 phosphate dehydrogenase are increased by Copyright 2000 2003 Mark Brandt Ph D 40 insulin insulin stimulates anabolic processes and many anabolic reactions require NADPH Glucose 6 phosphate dehydrogenase catalyzes the formation of the first NADPH of the pathway The reaction is drawn as reversible but is effectively irreversible under physiological conditions due to the instability of the product and the presence of gluconolactone hydrolase Gluconolactone hydrolase The 6 phosphogluconolactone produced by glucose 6 phosphate dehydrogenase is unstable and will undergo spontaneous hydrolysis The hydrolase merely accelerates the process The hydrolase reaction is effectively irreversible under physiological conditions 6 Phosphogluconate dehydrogenase 6 Phosphogluconate dehydrogenase catalyzes the formation of the second NADPH It has a reaction mechanism similar to that of isocitrate dehydrogenase and like isocitrate dehydrogenase yields an unstable product 3 keto 6 phosphogluconate that rapidly decarboxylates to release ribulose 5 phosphate Due to the loss of carbon dioxide in this step the 6 phosphogluconate dehydrogenase reaction is irreversible under physiological conditions Note that 6 phosphogluconate contains six carbons while ribulose 5 phosphate contains five carbons as mentioned above one carbon is lost as carbon dioxide In most cells NADPH levels are much higher than NADP levels NADPH has a much higher free energy than NADP both as a result of the concentration difference and of the fact that like NADH NADPH is a more energetic molecule than its oxidized counterpart While most dehydrogenase reactions are reversible net synthesis of NADPH is possible because the products of both dehydrogenase reactions in this pathway are rapidly and irreversibly converted to compounds that cannot act as substrates for the reverse reactions Therefore the glucose 6phosphate dehydrogenase and 6 phosphogluconate dehydrogenase reactions are irreversible because their product concentrations are never high enough to allow the reverse reactions to proceed Hexose monophosphate pathway reactions Later reversible steps Although some tissues e g skeletal muscle contain small amounts of glucose 6phosphate dehydrogenase and 6 phosphogluconate dehydrogenase all tissues contain the remaining enzymes in the pathway These remaining enzymes are necessary for the synthesis of nucleotides and for the conversion of ribose into glycolytic intermediates All of the remaining hexose monophosphate pathway enzymes catalyze reversible reactions Flow of carbon compounds through the remainder of the pathway is dependent on the availability of substrates Other pathways regulate the availability of all of these substrates regulation of the other pathways especially nucleotide synthesis and breakdown and glycolysis and gluconeogenesis therefore indirectly regulates the flow of substrates through the non NADPH synthesizing part of the hexose monophosphate pathway Copyright 2000 2003 Mark Brandt Ph D 41 The ribulose 5 phosphate formed in the 6 phosphogluconate dehydrogenase reaction acts as a substrate for two different enzymes Ribulose 5 phosphate isomerase The isomerase converts the ketopentose ribulose 5 phosphate to the aldopentose ribose 5 phosphate in other words it rearranges the molecule to move the carbonyl from the 2 position to the 1 position The product ribose 5 phosphate is the precursor for all nucleotide synthesis and its production is an important function of the hexose monophosphate pathway Ribulose 5 phosphate epimerase Epimers are carbohydrates with reversed stereochemistry at one of the carbons Ribulose 5 phosphate epimerase converts ribulose 5 phosphate to its epimer xylulose 5 phosphate Note that for D ketopentoses only two epimers are possible reversal of the stereochemistry at the 4 position would result in an Lmonosaccharide Because the isomerase and epimerase reactions are reversible excess ribose 5phosphate