Copyright © 2000-2003 Mark Brandt, Ph.D.39Glucose-6-phosphate in Metabolic ProcessesGlucose-6-phosphate acts as a branch point for a wide variety of pathways. It can beused: 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 reducingequivalents and intermediates via the hexose monophosphate shunt, or 5) (in liverand 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 havealready covered the glycolytic and gluconeogenic pathways; we will now turn toother 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-syntheticenzymatic reactions. In addition, it is used for the production of ribose for nucleotideand nucleic acid synthesis. The hexose monophosphate pathway also allows theentry of some carbohydrates into the glycolytic pathway (especially ribose, but alsosome others), and therefore acts as a connection route between different pathways.Steroidogenic tissues, red blood cells, and the liver are the major sites of hexosemonophosphate pathway. Muscle has small amounts of some of the hexosemonophosphate pathway enzymes, because it has little need for synthetic reactions,and therefore, little need for NADPH. The muscle, however, like all tissues, needsto be able to synthesize ribose in order to make nucleotides and nucleic acids.Features of the hexose monophosphate pathwayLike the glycolytic enzymes, the enzymes of the hexose monophosphate pathway arefound in the cytoplasm.Copyright © 2000-2003 Mark Brandt, Ph.D.40Although the hexose monophosphate pathway has fewer enzymes than glycolysis, itis somewhat more complicated. The net reaction for the pathway is somewhatvariable, depending on the entry and exit points of the metabolites. The fullpathway is shown below; it is important to remember that not all of these reactionsoccur in all tissues.Hexose monophosphate pathway reactions (NADPH-generating steps)Glucose-6-phosphate dehydrogenaseGlucose-6-phosphate dehydrogenase is the rate-limiting step, and primarycontrol point of the NADPH-generating portion of the hexose monophosphatepathway. Glucose-6-phosphate dehydrogenase is stimulated by NADP and inhibitedby NADPH. The levels of glucose-6-phosphate dehydrogenase are increased byCopyright © 2000-2003 Mark Brandt, Ph.D.41insulin (insulin stimulates anabolic processes, and many anabolic reactions requireNADPH).Glucose-6-phosphate dehydrogenase catalyzes the formation of the first NADPH ofthe pathway. The reaction is drawn as reversible, but is effectively irreversibleunder physiological conditions due to the instability of the product and the presenceof gluconolactone hydrolase.Gluconolactone hydrolaseThe 6-phosphogluconolactone produced by glucose-6-phosphate dehydrogenase isunstable, and will undergo spontaneous hydrolysis. The hydrolase merelyaccelerates the process. The hydrolase reaction is effectively irreversible underphysiological conditions.6-Phosphogluconate dehydrogenase6-Phosphogluconate dehydrogenase catalyzes the formation of the second NADPH.It has a reaction mechanism similar to that of isocitrate dehydrogenase, and likeisocitrate dehydrogenase, yields an unstable product (3-keto-6-phosphogluconate)that rapidly decarboxylates to release ribulose-5-phosphate. Due to the loss ofcarbon dioxide in this step, the 6-phosphogluconate dehydrogenase reaction isirreversible under physiological conditions.Note that 6-phosphogluconate contains six carbons, while ribulose-5-phosphatecontains 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 amuch higher free energy than NADP, both as a result of the concentrationdifference, and of the fact that, like NADH, NADPH is a more energetic moleculethan its oxidized counterpart. While most dehydrogenase reactions are reversible,net synthesis of NADPH is possible because the products of both dehydrogenasereactions in this pathway are rapidly and irreversibly converted to compounds thatcannot act as substrates for the reverse reactions. Therefore, the glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenasereactions are irreversible because their product concentrations are neverhigh 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-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, all tissuescontain the remaining enzymes in the pathway. These remaining enzymes arenecessary for the synthesis of nucleotides, and for the conversion of ribose intoglycolytic intermediates. All of the remaining hexose monophosphate pathwayenzymes catalyze reversible reactions. Flow of carbon compounds through theremainder of the pathway is dependent on the availability of substrates. Otherpathways regulate the availability of all of these substrates; regulation of the otherpathways (especially nucleotide synthesis and breakdown, and glycolysis andgluconeogenesis) therefore indirectly regulates the flow of substrates through thenon-NADPH synthesizing part of the hexose monophosphate pathway.Copyright © 2000-2003 Mark Brandt, Ph.D.42The ribulose-5-phosphate formed in the 6-phosphogluconate dehydrogenase reactionacts as a substrate for two different enzymes.Ribulose-5-phosphate isomeraseThe isomerase converts the ketopentose ribulose-5-phosphate to the aldopentoseribose-5-phosphate; in other words, it rearranges the molecule to move the carbonylfrom the 2-position to the 1-position. The product, ribose-5-phosphate is theprecursor for all nucleotide synthesis, and its production is an importantfunction of the hexose monophosphate pathway.Ribulose-5-phosphate epimeraseEpimers are carbohydrates with