How are competing catabolic and anabolic pathways regulated, so that each is active only when required? As an example, we will examine gluconeogenesis, the synthesis of glucose from small molecule precursors such as pyruvate We will see that the anabolic pathway (gluconeogenesis) is not simply the reverse of the catabolic pathway (glycolysis), though some reactions are shared
http://sportsillustrated.cnn.com/2006/writers/austin_murphy/07/19/stage.16/
Why is gluconeogenesis necessary? • Humans require ~160 g glucose/day, most is used by the brain • Body fluids carry ~20 g of free glucose, ~200 g is available as glycogen (marathon runners ‘hit the wall’, and cyclists ‘bonk’ when all glycogen is used up) • So the body carries a ~1 day supply of glucose. If insufficient is obtained from diet then synthesis of new glucose from noncarbohydrate precursors is necessary. Fat catabolism does not generate glucose • Pyruvate and lactate produced during exercise can be recycled to glucose by gluconeogenesis
• Pyruvate, lactate, most amino acids, glycerol and citric acid cycle intermediates can all be converted to glucose by gluconeogenesis • Brain and muscle are the major sites of glucose consumption. But liver and kidney account for almost all gluconeogenesis. Glucose is then transported to where it is needed via bloodstream • Gluconeogenesis appears to be the reverse of glycolysis. Glucose is made, ATP and NADH are consumed. But it cannot be the simple reverse of glycolysis because: • Glycolysis is exergonic, so the reverse pathway cannot happen spontaneously • Different enzymes are required to allow separate regulation of the two pathways
Seven of the reactions of glycolysis are shared The three steps that are highly exergonic are replaced by four different reactions and enzymes in gluconeogenesis Pyruvate carboxylase converts pyruvate to oxaloacetate PEP carboxykinase makes PEP from oxaloacetate
Fig. 22-1, p. 664
Conversion of fructose-1,6bisphosphate to fructose-6phosphate requires a specific phosphatase, fructose-1,6bisphosphatase Glucose-6-phosphatase dephosphorylates glucose-6phosphate
Fig. 22-1, p. 664
Pyruvate carboxylase contains biotin covalently linked to a lysine residue Biotin functions as a carboxyl group carrier
p. 665
Fig. 22-3, p. 708
Fig. 22-2, p. 708
ATP activates the bicarbonate group, which is then transferred to biotin
Fig. 22-3, p. 665
Acetyl CoA is an allosteric activator of pyruvate carboxylase. So if [acetyl-CoA] is low, carbon is chanelled towards the citric acid cycle and gluconeogenesis is inhibited. The same is true if [ATP] is low, since pyruvate carboxylase requires ATP So, high levels of acetyl CoA and ATP indicate high energy status and favor gluconeogenesis
Fig. 19-1, p. 609
Pyruvate carboxylase is located in the mitochondrion Compartmentalization may help to prevent simultaneous glycolysis and gluconeogenesis Transport of oxaloacetate to the cytoplasm for continuing gluconeogenesis requires conversion to malate, by NADH-linked malate dehydrogenase (reverse of TCA cycle reaction)
Fig. 22-4, p. 666
• Decarboxylation is energetically favorable and helps drive the PEP carboxykinase reaction forwards • Hydrolysis of GTP also provides a driving force for reaction, and is equivalent to consumption of ATP, since ATP is required to phosphorylate GDP • The overall ΔG for formation of PEP from pyruvate is -22.6 kJ/ mol
p. 666
Fig. 22-1, p. 664
Fructose-1,6-bisphosphatase
Phosphate ester hydrolysis is thermodynamically favorable (ΔG = -8.6 kJ/mol) Enzyme is activated by citrate (which inhibits PFK) and is inhibited by fructose-2,6-bisphosphate and AMP (which activate PFK). So there is reciprocal regulation of glycolysis and gluconeogenesis, at the steps which are not in common
p. 667
Synergistic inhibition by fructose-2,6-bisphosphate and AMP concentration of fructose-2,6-bisphosphate
-AMP
+ 25 mM AMP
Fig. 22-9, p. 672
F-6-P allosterically activates PFK-2 F-6-P +
Phosphorylation of PFK-2 by cAMP dependent protein kinase converts it into fructose-2,6-bisphosphatase
Fig. 22-10, p. 672
GLYCOLYSIS
PFK-1
fructose-6-phosphate
GLUCONEOGENESIS
Fruc-1,6bisphophatase
fructose-1,6-bisphosphate
low blood sugar
+
fructose-6-phosphate
glucagon fructose-6-phosphate +
phosphorylation
PFK-2
PFK-2-P
fructose-2,6-bisphosphate
Fig. 22-1, p. 664
The glucose-6-phosphatase reaction is also an exergonic phosphate ester hydrolysis Enzyme is absent from muscle and brain, which cannot make glucose Enzyme is associated with the endoplasmic reticulum
Fig. 22-6, p. 668
Fig. 22-5, p. 667
Six nucleoside triphosphates (4 ATP + 2 GTP) are hydrolyzed as 2 pyruvate are converted to glucose. This makes gluconeogenesis exergonic (ΔGo’ = -37.7 kJ/mol) and thermodynamically feasible The reverse of glycolysis would consume only 2 ATP and would be endergonic (ΔGo’ = +74 kJ/ mol)
Fig. 22-1, p. 664
It is the three exergonic reactions of glycolysis that are replaced in gluconeogenesis
Fig. 18-22, p. 554
Fig. 22-1, p. 664
von Gierke’s disease (Glycogen storage disease I) is caused by a hereditary defect in glucose-6-phosphatase Pathophysiology: • Problems with glycogen metabolism (production of glucose from glycogen is blocked; next lecture) • Hypoglycemia (inability to maintain blood glucose levels because of defect in gluconeogenesis) in periods between meals • Lactic acidosis (lactic acid build up due to defect in gluconeogenesis)
Lactate is oxidized to pyruvate by liver lactate dehydrogenase, and pyruvate used for gluconeogenesis Oxygen debt during exercise causes formation of lactate in muscle Liver shares the metabolic burden of exercise
Fig. 22-7, p. 669
• Glycolysis and gluconeogenesis are subject to reciprocal control, such that one is inhibited when the other is active. Without this control, there would be a futile cycle and massive ATP consumption • Reciprocal regulation is by the energy status of the cell • When energy status is low, glucose is degraded to provide ATP • When energy status is high, pyruvate is used for synthesis of glucose and, ultimately, glycogen
Glucose-6-phosphatase activity increases linearly with substrate concentration
Fig. 22-8, p. 671
Acetyl-CoA inhibits glycolysis and stimulates gluconeogenesis. High [acetylCoA] signals high energy status
Fig. 22-8, p. 671
High [AMP] signals low energy status, and stimulates glycolysis and inhibits gluconeogenesis
Fig. 22-8, p. 671
Reciprocal regulation by fructose-2,6-bisphosphate
Fig. 22-8, p. 671
Starvation and ketone bodies During starvation, fat is oxidized to fatty acids and then to acetyl CoA, which is then converted to ketone bodies (acetone, acetoacetate and ßhydroxybutyrate) in liver Ketone bodies are transported to target organs (eg brain & heart) through bloodstream and can be converted back to acetyl CoA and used as an energy source
Fig. 23-26, p. 718
Ketone bodies Brain can get ~70 % of its energy from ketone bodies when blood glucose is very low, but retains some requirement for glucose (in order to make oxaloacetate)
Fig. 23-26, p. 671
Ketone acidosis In diabetes, cells are biochemically starved and fat is oxidized. Especially in Type 1 diabetes, the liver can produce large amounts of ketone bodies, causing ketosis. In severe circumstances, ketosis is accompanied by acidosis, lowering of the blood pH caused by accumulation of acetoacetate and ß-hydroxybutyrate
Fig. 23-26, p. 671
Low carb diets Restriction of carbohydrate in the diet should switch the body from degrading glucose to degrading fat Low carb diet induces ketosis (which accounts for bad taste in mouth). Energy sources are ketone bodies and glucose (from glycogen and gluconeogenesis). Some fat should be lost as excreted ketone bodies (~20g/day)
Low carb diets Restriction of carbohydrate in the diet should switch the body from degrading glucose to degrading fat Low carb diet induces ketosis (which accounts for bad taste in mouth). Energy sources are ketone bodies and glucose (from glycogen and gluconeogenesis). Some fat should be lost as excreted ketone bodies (~20g/day)
But… Initial weight loss in early phase of diet is partly water, because glycogen is associated with 3X its weight of water Increased risk of heart disease associated with increased intake of animal fat Increased risk of cancer associated with reduced intake of fruit
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