Glycolysis and Gluconeogenesis MCQs

Although most of the reactions of glycolysis are freely reversible, three are markedly exergonic and must therefore be considered to be physiologically irreversible. These reactions, catalyzed by hexokinase (and glucokinase), phosphofructokinase, and pyruvate kinase, are the major sites of regulation of glycolysis. All the Kinases in glycolysis are catalysing irreversible reactions, except 1, 3 Bisphosphoglycerate Kinase, which is catalysing reversible reaction. Reference: Harper's 32 nd edition, Page no. 172

Kinases require Mg2+, meaning 4 enzymes in glycolysis are dependent on presence of this ion. These enzymes include: Hexokinase/Glucokinase PFK-1 3-PG Kinase Pyruvate Kinase Enolase is also dependent on the presence of either Mg2+ or Mn2+ ions. Reference: Harper's 32 nd edition, Page no.171

Hexokinase, phosphofructokinase and pyruvate kinase are all catalysing irreversible reactions and are the regulated steps in glycolysis. The conversion of glucose to lactate (anaerobic glycolysis) is a process that does not involve a net oxidation or reduction and, thus, oxygen is not required. Glucokinase (not hexokinase) is important in hepatic glucose metabolism only in the absorptive period following consumption of a carbohydrate-containing meal. Fructose 2,6-bisphosphate is a potent activator (not inhibitor) of phosphofructokinase. The conversion of glucose to lactate yields two ATP but no net production of NADH. Reference: Lippincoatt SAE/Page no. 193-195 Reference: Harper's 32 nd edition

Warburg effect: In 1924, the biochemist Otto Warburg and his colleagues made the discovery that cancer cells take up large amounts of glucose and metabolize it to lactic acid, even in the presence of oxygen. This observation was termed the Warburg effect. Based on these data, Warburg made two hypotheses: first, that the increased ratio of glycolysis to aerobic respiration was likely due to defects in the mitochondrial respiratory chain; and second, that this enhanced glycolysis enabled cancer cells to preferentially proliferate in the reduced oxygen tension often seen in tumors. Furthermore, Warburg argued that the switch from aerobic to anaerobic glucose metabolism was the driver of tumor genesis. Reference: Harper's 32 nd edition, Page no. 738

Acetyl CoA can never be substrate for gluconeogenesis! Lactate from muscle and RBC are converted to glucose in the liver (Coris Cycle) Lactate and Alanine is converted to Pyruvate which can enter into Gluconeogenesis. Oxaloacetateis converted to Phosphoenolpyruvate by PEPCK and enter into Gluconeogenesis. Reference: Harper's 32 nd edition, Page no.185,186

Gluconeogenesis is the process of synthesizing glucose or glycogen from noncarbohydrate precursors. The major substrates are the glucogenic amino acids, lactate, glycerol, and propionate. Liver and kidney are the major gluconeogenic tissues; the kidney may contribute up to 40% of total glucose synthesis in the fasting state and more in starvation. Gluconeogenesis occur in liver and Kidney Fructose2, 6 Bisphosphate activate PFK-I, hence stimulate Glycolysis Reference: Harper's 32 nd edition, Page no.185,186

Gluconeogenesis takes place in cytosol and mitochondria The Mitochondrial step is Pyruvate Carboxylase Pathways taking place in two compartments are Heme Synthesis Urea Cycle Reference: Harper's 32 nd edition, Page no.188, 189

Subsequent reaction of pyruvate oxidation require mitochondria & erythrocyte lack mitochondria. Tissues That Function Under Hypoxic Conditions Produce Lactate This is true of skeletal muscle, particularly the white fibers, where the rate of work output, and hence the need for ATP formation, may exceed the rate at which oxygen can be taken up and utilized. Glycolysis in erythrocytes always terminates in lactate, because the subsequent reactions of pyruvate oxidation are mitochondrial, and erythrocytes lack mitochondria. Reference: Harper's 32 nd edition, page no. 171