Phosphofructokinase (EC 2.7.1.11) catalyses the reverse conversion of fructose 6-phosphate to fructose-1,6-bisphosphate, but this is not just the reverse reaction, because the co-substrates are different (and so thermodynamic requirements are not violated). The two enzymes each catalyse the conversion in one direction only, and are regulated by metabolites such as fructose 2,6-bisphosphate so that high activity of one of them is accompanied by low activity of the other. More specifically, fructose 2,6-bisphosphate allosterically inhibits fructose 1,6-bisphosphatase, but activates phosphofructokinase-I. Fructose 1,6-bisphosphatase is involved in many different metabolic pathways and found in most organisms. FBPase requires metal ions for catalysis (Mg2+ and Mn2+ being preferred) and the enzyme is potently inhibited by Li+.
Structure
The fold of fructose-1,6-bisphosphatase from pigs was noted to be identical to that of inositol-1-phosphatase (IMPase).[3]Inositol polyphosphate 1-phosphatase (IPPase), IMPase and FBPase share a sequence motif (Asp-Pro-Ile/Leu-Asp-Gly/Ser-Thr/Ser) which has been shown to bind metal ions and participate in catalysis. This motif is also found in the distantly-related fungal, bacterial and yeast IMPase homologues. It has been suggested that these proteins define an ancient structurally conserved family involved in diverse metabolic pathways, including inositol signalling, gluconeogenesis, sulphate assimilation and possibly quinonemetabolism.[4]
Species distribution
Three different groups of FBPases have been identified in eukaryotes and bacteria (FBPase I-III).[5] None of these groups have been found in Archaea so far, though a new group of FBPases (FBPase IV) which also show inositol monophosphatase activity has recently been identified in Archaea.[6]
A new group of FBPases (FBPase V) is found in thermophilic archaea and the hyperthermophilic bacteriumAquifex aeolicus.[7] The characterised members of this group show strict substrate specificity for FBP and are suggested to be the true FBPase in these organisms.[7][8] A structural study suggests that FBPase V has a novel fold for a sugarphosphatase, forming a four-layer alpha-beta-beta-alpha sandwich, unlike the more usual five-layered alpha-beta-alpha-beta-alpha arrangement.[8] The arrangement of the catalyticside chains and metal ligands was found to be consistent with the three-metal ion assisted catalysis mechanism proposed for other FBPases.
Fructose 1,6-bisphosphatase also plays a key role in hibernation, which requires strict regulation of metabolic processes to facilitate entry into hibernation, maintenance, arousal from hibernation, and adjustments to allow long-term dormancy.[11][12][13] During hibernation, an animal's metabolic rate may decrease to around 1/25 of its euthermic resting metabolic rate.[12][13][14] FBPase is modified in hibernating animals to be much more temperature sensitive than it is in euthermic animals.[11][13][14] FBPase in the liver of a hibernating bat showed a 75% decrease in Km for its substrate FBP at 5 °C than at 37 °C.[11] However, in a euthermic bat this decrease was only 25%, demonstrating the difference in temperature sensitivity between hibernating and euthermic bats.[11] When sensitivity to allosteric inhibitors such as AMP, ADP, inorganic phosphate, and fructose-2,6-bisphosphate were examined, FBPase from hibernating bats was much more sensitive to inhibitors at low temperature than in euthermic bats.[11][15][16]
During hibernation, respiration also dramatically decreases, resulting in conditions of relative anoxia in the tissues. Anoxic conditions inhibit gluconeogenesis, and therefore FBPase, while stimulating glycolysis, and this is another reason for reduced FBPase activity in hibernating animals.[17] The substrate of FBPase, fructose 1,6-bisphosphate, has also been shown to activate pyruvate kinase in glycolysis, linking increased glycolysis to decreased gluconeogenesis when FBPase activity is decreased during hibernation.[13]
In addition to hibernation, there is evidence that FBPase activity varies significantly between warm and cold seasons even for animals that do not hibernate.[18]
In rabbits exposed to cold temperatures, FBPase activity decreased throughout the duration of cold exposure, increasing when temperatures became warmer again.[18] The mechanism of this FBPase inhibition is thought to be digestion of FBPase by lysosomalproteases, which are released at higher levels during colder periods.[18] Inhibition of FBPase through proteolytic digestion decreases gluconeogenesis relative to glycolysis during cold periods, similar to hibernation.[18]
Fructose 1,6-bisphosphate aldolase is another temperature dependent enzyme that plays an important role in the regulation of glycolysis and gluconeogenesis during hibernation.[14] Its main role is in glycolysis instead of gluconeogenesis, but its substrate is the same as FBPase's, so its activity affects that of FBPase in gluconeogenesis. Aldolase shows similar changes in activity to FBPase at colder temperatures, such as an upward shift in optimum pH at colder temperatures. This adaptation allows enzymes such as FBPase and fructose-1,6-bisphosphate aldolase to track intracellular pH changes in hibernating animals and match their activity ranges to these shifts.[14] Aldolase also complements the activity of FBPase in anoxic conditions (discussed above) by increasing glycolytic output while FBPase inhibition decreases gluconeogenesis activity.[19]
Diabetes
AMP
Fructose 1,6-bisphosphatase is also a key player in treating type 2 diabetes. In this disease, hyperglycemia causes many serious problems, and treatments often focus on lowering blood sugar levels.[20][21][22] Gluconeogenesis in the liver is a major cause of glucose overproduction in these patients, and so inhibition of gluconeogenesis is a reasonable way to treat type 2 diabetes. FBPase is a good enzyme to target in the gluconeogenesis pathway because it is rate-limiting and controls the incorporation of all three-carbon substrates into glucose but is not involved in glycogen breakdown and is removed from mitochondrial steps in the pathway.[20][21][22] This means that altering its activity can have a large effect on gluconeogenesis while reducing the risk of hypoglycemia and other potential side effects from altering other enzymes in gluconeogenesis.[20][21]
Drug candidates have been developed that mimic the inhibitory activity of AMP on FBPase.[20][22] Efforts were made to mimic the allosteric inhibitory effects of AMP while making the drug as structurally different from it as possible.[22] Second-generation FBPase inhibitors have now been developed and have had good results in clinical trials with non-human mammals and now humans.[20][23]
^Marcus F, Harrsch PB (May 1990). "Amino acid sequence of spinach chloroplast fructose-1,6-bisphosphatase". Archives of Biochemistry and Biophysics. 279 (1): 151–7. doi:10.1016/0003-9861(90)90475-E. PMID2159755.
^Marcus F, Gontero B, Harrsch PB, Rittenhouse J (Mar 1986). "Amino acid sequence homology among fructose-1,6-bisphosphatases". Biochemical and Biophysical Research Communications. 135 (2): 374–81. doi:10.1016/0006-291X(86)90005-7. PMID3008716.
^Zhang Y, Liang JY, Lipscomb WN (Feb 1993). "Structural similarities between fructose-1,6-bisphosphatase and inositol monophosphatase". Biochemical and Biophysical Research Communications. 190 (3): 1080–3. doi:10.1006/bbrc.1993.1159. PMID8382485.
^Stec B, Yang H, Johnson KA, Chen L, Roberts MF (Nov 2000). "MJ0109 is an enzyme that is both an inositol monophosphatase and the 'missing' archaeal fructose-1,6-bisphosphatase". Nature Structural Biology. 7 (11): 1046–50. doi:10.1038/80968. PMID11062561. S2CID7617099.
^ abcdeStorey KB (December 1997). "Metabolic regulation in mammalian hibernation: enzyme and protein adaptations". Comparative Biochemistry and Physiology A. 118 (4): 1115–24. doi:10.1016/S0300-9629(97)00238-7. PMID9505421.
^ abHeldmaier G, Ortmann S, Elvert R (August 2004). "Natural hypometabolism during hibernation and daily torpor in mammals". Respiratory Physiology & Neurobiology. 141 (3): 317–29. doi:10.1016/j.resp.2004.03.014. PMID15288602. S2CID32940046.
^ abcdBrooks SP, Storey KB (January 1992). "Mechanisms of glycolytic control during hibernation in the ground squirrel Spermophilus lateralis". Journal of Comparative Physiology B. 162 (1): 23–28. doi:10.1007/BF00257932. S2CID1881399.
^ abcdMacDonald JA, Storey KB (December 2002). "Purification and characterization of fructose bisphosphate aldolase from the ground squirrel, Spermophilus lateralis: enzyme role in mammalian hibernation". Archives of Biochemistry and Biophysics. 408 (2): 279–85. doi:10.1016/S0003-9861(02)00579-9. PMID12464282.
^Ekdahl KN, Ekman P (February 1984). "The effect of fructose 2,6-bisphosphate and AMP on the activity of phosphorylated and unphosphorylated fructose-1,6-bisphosphatase from rat liver". FEBS Letters. 167 (2): 203–9. doi:10.1016/0014-5793(84)80127-1. PMID6321241. S2CID22515761.
^ abcdFischer EH, Krebs EG, Neurath H, Stadtman ER, eds. (1974). Metabolic Interconversion of Enzymes 1973 Third International Symposium held in Seattle, June 5-8, 1973. Berlin, Heidelberg: Springer. ISBN978-3-642-80817-3.
^ abcdeDang Q, Van Poelje PD, Erion MD (2012). "Chapter 11: The Discovery and Development of MB07803, a Second-Generation Fructose-1,6-bisphosphatase Inhibitor with Improved Pharmacokinetic Properties, as a Potential Treatment of Type 2 Diabetes". In Jones RM (ed.). New Therapeutic Strategies for Type 2 Diabetes: Small Molecule Approaches. Cambridge: The Royal Society of Chemistry. doi:10.1039/9781849735322-00306. ISBN978-1-84973-414-1.
^ abcArch JR (2011). "Thermogenesis and Related Metabolic Targets in Anti-Diabetic Therapy". In Schwanstecher M (ed.). Diabetes - Perspectives in Drug Therapy (1st ed.). Berlin, Heidelberg: Springer. p. 203. ISBN978-3-642-17214-4.
^ abcdvan Poelje PD, Potter SC, Chandramouli VC, Landau BR, Dang Q, Erion MD (June 2006). "Inhibition of fructose 1,6-bisphosphatase reduces excessive endogenous glucose production and attenuates hyperglycemia in Zucker diabetic fatty rats". Diabetes. 55 (6): 1747–54. doi:10.2337/db05-1443. PMID16731838. S2CID19832318.
^Kaur R, Dahiya L, Kumar M (December 2017). "Fructose-1,6-bisphosphatase inhibitors: A new valid approach for management of type 2 diabetes mellitus". European Journal of Medicinal Chemistry. 141: 473–505. doi:10.1016/j.ejmech.2017.09.029. PMID29055870.
Further reading
Berg JM, Tymoczko JL, Stryer L (2002). "Glycolysis and Gluconeogenesis". In Susan Moran (ed.). Biochemistry (5th ed.). New York: W. H. Freeman and Company. ISBN0-7167-3051-0.
