Gluconeogenesis Synthesis of "new glucose" from common metabolites •Humans use ~160 g of glucose per day •75% is used by the brain •Body fluids contain only 20 g of glucose •Glycogen stores yield 180-200 g of glucose •So the body must be able to make its own glucose • 90% of gluconeogenesis occurs in the liver and kidneys Figure 18.1 The Glycolysis Pathway Figure 18.1 The Glycolysis Pathway Why is gluconeogenesis not just the reverse of glycolysis? The reverse of glycolysis is 2 Pyruvate + 2ATP + 2 NADH + 2H+ + 2H20 a glucose +2ADP +2Pi + 2 NAD + (DG = +74 kJ/mol) This is thermodynamically unfavorable, so energetically unfavorable steps in the reverse glyolysis reaction are replaced and energy is added in the form of GTP and ATP to give: The actual equation for gluconeogenesis of a 2Pyruvate + 4ATP +2GTP+ 2NADH + 2H+ + 6H20 glucose +4ADP +2GDP +6Pi + 2 NAD + (DG = -38 kJ/mol) Notice the extra ATPs and GTPs drive the process Glycolysis vs Gluconeogenesis Glycolosis Gluconeogenesis Glucose (6C) to 2 pyruvates (3C) Creates energy 2ATP Reduces 2 NAD+ to 2 NADH Active when energy in cell low 10 steps from glucose to pyruvate Pyruvate to AcCoA before Krebs 2 pyruvates (3C) to Glucose (6C) Consumes energy 4ATP+2GTP Oxidizes 2NADH to 2 NAD+ Active when energy in cell high 11 steps from pyruvate to glucose AcCoA isn’t used in gluconeogenesis Gluconeogenesis uses 7 of the 10 enzymatic reactions of glycolysis but in the reverse direction. The 3 not used are the ones requiring ATP in glycolysis. First Reaction of Gluconeogenesis - recall that pyruvate is the final product of glycolysis. The pyruvate carboxylase reaction. (Simplified) Biotin is an essential cofactor in most carboxylation reactions. It is an essential vitamin in the human diet, but deficiencies are rare. Avidin, a protein found in egg white binds tightly to biotin and excessive consumption of raw egg white can lead to biotin deficiency. ATP Carbonyl phosphate oxaloacetate Pyruvate is converted to oxaloacetate in the mitochondria Oxaloacetate cannot be transported directly across the mitochondrial membrane so it is converted to malate, then transported, then oxidized back to oxaloacetate. The PEP carboxykinase reaction. Nucleotide diphosphate kinases Both glycolysis and Oxidative phosphorylation produce ATP with its high energy phoshoanhydride bonds: How does GTP get made from GDP? Directly from a single step in the Krebs cycle AND from the following reaction GDP + ATP → GTP + ADP This is carried out in the cell by an enzyme called Nucleotide diphosphate kinase which carries out the general reaction NDP + ATP → NTP + ADP (where N is T, G, or C) Enolase Reaction glycolysis gluconeogenesis Fig. 18-26, p. 595 The Phosphoglycerate Mutase Reaction glycolysis gluconeogenesis Fig. 18-23, p. 594 Isomerase: An enzyme that catalyzes the transformation of compounds into their positional isomers. In the case of sugars this usually involves the interconversion of an aldose into a ketose, or vice versa. Kinase: An enzyme that catalyzes the phosphorylation (or dephosphorylation) of a molecule using ATP (or ADP). Mutase: An enzyme that catalyzes the transposition of functional groups, such as phosphates, sulfates, etc. glycolysis gluconeogenesis Phospoglycerate kinase Fig. 18-20, p. 593 The glyceraldehyde-3-phosphate dehydrogenase reaction glycolysis gluconeogenesis glycolysis gluconeogenesis Triose phosphate isomerase Fig. 18-14, p. 589 Aldolase 4th reaction of glycolysis (7th reaction of gluconeogenesis). Reversible reaction also used in gluconeogenesis. An aldol cleavage reaction (the reverse of an aldol condensation). glycolysis gluconeogenesis Fig. 18-4, p. 584 Glucose-6-phosphatase -enzyme unique to liver and kidney allowing them to supply glucose to other tissues. Found in ER The Cori Cycle Regulation of Gluconeogenesis Glucose-6-phosphatase is subject to substrate level control. - at higher G6P concentrations reaction rate increases - recall, this happens in the liver. Other tissues do not hydrolyze their G6P, thereby trapping it in the cells. Glycolysis and gluconeogenesis are reciprocally regulated. - regulatory molecules that inhibit gluconeogenesis often activate glycolysis, and vise versa. A potent allosteric regulatory molecule. - activates phosphofructokinase. - inhibits fructose-1,6-bisphosphatase. - its synthesis and degradation are catalyzed by the same bifunctional enzyme. Fructose-2,6-bisphosphate activates glycolysis and inhibits gluconeogenesis, so its level is very important. F2,6 BP PFK-1 F2,6 BP ATP Pi ADP PFK-2 F2,6 BP 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase Low glucose High glucagon P Increased phosphorylation Phosphorylation of the enzyme results in the inactivation of the phosphofructokinase-2 activity and activation of the fructose-2,6bisphosphatase activity. This results in a down regulation of glycolysis and increased gluconeogenesis. Substrates for gluconeogenesis: Pyruvate Lactate TCA cycle intermediates Most amino acids Not substrates for gluconeogenesis: Acetyl-CoA Fatty acids Lysine Leucine Plants and bacteria can make glucose from acetate.