Transcript (in liver).
L2 Glycolysis The resultant glucose and other simple carbohydrates are transported across the intestinal wall to the hepatic portal vein and then to liver parenchymal cells and other tissues. There they are converted to fatty acids, amino acids, and glycogen, or else oxidized by the various catabolic pathways of cells. Oxidation of glucose is known as glycolysis. Glucose is oxidized to either lactate or pyruvate. Under aerobic conditions, the product in most tissues is pyruvate and the pathway is known as aerobic glycolysis. When oxygen is depleted, as for instance during prolonged vigorous exercise, and in RBCs the glycolytic product in many tissues is lactate and the process is known as anaerobic glycolysis Aerobic glycolysis of glucose to pyruvate, requires two equivalents of ATP to activate the process (-2 ATP) with the subsequent production of four equivalents of ATP and two equivalents of NADH. ( +4 ATP & +2 NADH) Thus, conversion of one mole of glucose to two moles of pyruvate is accompanied by the net production of (+2ATP and +2 NADH. ) Glucose + 2 ADP + 2 NAD+ + 2 Pi ------> 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ The NADH generated during glycolysis is used to fuel mitochondrial ATP synthesis via oxidative phosphorylation, producing either two or three equivalents of ATP depending upon whether the glycerol phosphate shuttle or the malate-aspartate shuttle is used to transport the electrons from cytoplasmic NADH into the mitochondria. The net yield from the oxidation of 1 mole of glucose to 2 moles of pyruvate is either 6 or 8 moles of ATP. Complete oxidation of the 2 moles of pyruvate, through the TCA Cycle, yeilds an additional 30 moles of ATP the total yield, therefore being either 36 or 38 moles of ATP from the complete oxidation of 1 mole of glucose to CO2 and H2O. The ATP-dependent phosphorylation of glucose to form glucose 6-phosphate (G6P)is the first reaction of glycolysis, and is catalyzed by tissue-specific isoenzymes known as hexokinases. Four mammalian isozymes of hexo - kinase are known (Types I - IV), with the Type IV isozyme often referred to as glucokinase (in liver). The high Km of glucokinase for glucose means that this enzyme is saturated only at very high concentrations of substrate (Glucose). • Comparison of the activities of hexokinase and glucokinase. The Km for hexokinase is significantly lower (0.1mM) than that of glucokinase (10mM). This difference ensures that non-hepatic tissues (which contain hexokinase) rapidly and efficiently trap blood glucose within their cells by converting it to glucose-6-phosphate. One major function of the liver is to deliver glucose to the blood and this in ensured by having a glucose phosphorylating enzyme (glucokinase) whose Km for glucose is sufficiently higher that the normal circulating concentration of glucose (5mM). This feature of hepatic glucokinase allows the liver to buffer blood glucose. e.g. After meals, when postprandial blood glucose levels are high, liver glucokinase is significantly active, which causes the liver to trap and to store circulating glucose. When blood glucose falls to very low levels, tissues such as liver and kidney, which contain glucokinases, do not continue to use the glucose and instead can use alternative sources such as FFAs and ketone bodies. At the same time, tissues such as the brain, which are dependent on glucose, continue to use blood glucose by their low Km hexokinases Under various conditions of glucose deficiency, such as long periods between meals, the liver is stimulated to supply the blood with glucose through the pathway of gluconeogenesis. Due to: 1st: the liver unlike other tissues has the enzyme G6Pase which converts G6P to free Glucose. 2nd: The levels of glucose produced during gluconeogenesis are insufficient to activate glucokinase, allowing the glucose to pass out of hepatocytes into the blood. Under aerobic conditions, pyruvate in most cells is further metabolized via the TCA cycle. Under anaerobic conditions and in erythrocytes under aerobic conditions, pyruvate is converted to lactate by the enzyme lactate dehydrogenase (LDH), and the lactate is transported out of the cell into the circulation. Why muscle cells derive almost all of the ATP consumed during exertion from anaerobic glycolysis? A/// Although aerobic glycolysis generates more ATP per mole of glucose oxidized than does anaerobic glycolysis but the rate of ATP production from glycolysis is 100X faster than from oxidative phosphorylation. Pyruvate is the branch point molecule of glycolysis. The ultimate fate of pyruvate depends on the oxidation state of the cell. In aerobic pathway: Pyruvate enters the TCA cycle in the form of acetyl-CoA which is the product of the pyruvate dehydrogenase reaction. During anaerobic glycolysis : The fate of pyruvate is reduction to lactate. Erythrocytes and skeletal muscle (under conditions of exertion) derive all of their ATP needs through anaerobic glycolysis. The large quantity of NADH produced is oxidized by reducing pyruvate to lactate. This reaction is carried out by lactate dehydrogenase, (LDH). The lactate produced during anaerobic glycolysis diffuses from the tissues and is transproted to highly aerobic tissues such as cardiac muscle and liver. The lactate is then oxidized to pyruvate in these cells by LDH and the pyruvate is further oxidized in the TCA cycle. If the energy level in these cells is high the carbons of pyruvate will be diverted back to glucose via the gluconeogenesis pathway. note: in liver cells contain two distinct types of LDH, termed M and H. The H type subunit predominates in aerobic tissues such as heart muscle (as the H4 tetramer) while the M subunit predominates in anaerobic tissues such as skeletal muscle as the M4 tetramer). H4 LDH has a low Km for pyruvate and also is inhibited by high levels of pyruvate. The M4 LDH enzyme has a high Km for pyruvate and is not inhibited by pyruvate. This suggests that the H-type LDH is utilized for oxidizing lactate to pyruvate and the M-type the reverse. Animal cells (hepatocytes) contain the cytosolic enzyme alcohol dehydrogenase (ADH) which oxidizes ethanol to acetaldehyde. Acetaldehyde then enters the mitochondria where it is oxidized to acetate by acetaldehyde dehydrogenase (AcDH). Acetaldehyde forms adducts with proteins, nucleic acids and other compounds, the results of which are the toxic side effects (the hangover) that are associated with alcohol consumption. The ADH and AcDH catalyzed reactions also leads to the reduction of NAD+ to NADH. (cellular imbalance in the NADH/NAD+). This has various metabolic effects: 1st : Rate of TCA cycle in the mitochondria is being impacted by the NADH produced by the AcDH reaction. 2nd : The reduction in NAD+ impairs the flux of glucose through glycolysis at the glyceraldehyde-3-phosphate dehydrogenase reaction, thereby limiting energy production. 3rd: there is an increased rate of hepatic lactate production, This reverseral of the LDH reaction in hepatocytes diverts pyruvate from gluconeogenesis leading to a reduction in the capacity of the liver to deliver glucose to the blood. 4th : Fatty acid oxidation is also reduced as this process requires NAD+ as a cofactor. 5th: the opposite is true, fatty acid synthesis is increased and there is an increase in triacylglyceride production by the liver. (HOW????) and WHAT is the clinical effect???????????????? In the mitocondria, the production of acetate from acetaldehyde leads to increased levels of acetyl-CoA. Since the increased generation of NADH also reduces the activity of the TCA cycle, the acetyl-CoA is diverted to fatty acid synthesis. The reduction in cytosolic NAD+ leads to reduced activity of glycerol-3-phosphate dehydrogenase (in the glcerol 3-phosphate to DHAP direction) resulting in increased levels of glycerol 3phosphate which is the backbone for the synthesis of the triacylglycerides. Both of these two events lead to fatty acid deposition in the liver leading to fatty liver syndrome. Clinical Examples of Glycolysis Defects Genetic diseases Glycolytic mutations are generally rare due to importance of the metabolic pathway, this means that the majority of occurring mutations result in an inability for the cell to respire, and therefore cause the death of the cell at an early stage. However, some mutations are seen with one notable example being pyruvate kinase deficiency, leading to chronic hemolytic anemia. Glycolysis and Disease Cancer Malignant rapidly growing Tumor cells typically have glycolytic rates that are up to 200 times higher than those of their normal tissues of origin. One theory suggests that the increased glycolysis is a normal protective process of the body and that malignant change could be primarily caused by energy metabolism. This high glycolysis rate has important medical applications, as high aerobic glycolysis by malignant tumors is utilized clinically to diagnose and monitor treatment responses of cancers by uptake of Fludeoxyglucose (18F)(FDG) by positron emission tomography (PET). There is ongoing research to affect mitochondrial metabolism and treat cancer by reducing glycolysis and thus starving cancerous cells in various new ways, including a ketogenic diet. LDH is a marker of malignancy ? How????