22.7 Gluconeogenesis: Glucose Synthesis Glucose is synthesized in the tissues of the liver and kidneys. Tissues that use glucose as their main energy source are the brain, skeletal muscles, and red blood cells. Learning Goal Describe how glucose is synthesized from noncarbohydrate molecules.

Utilization of Glucose Glucose is the primary energy source for the brain, skeletal muscle, and red blood cells. Glucose deficiency can impair the brain and nervous system. If our glycogen stores are depleted, liver cells synthesize glucose by gluconeogenesis. glucose is synthesized in the cytosol of the liver cells, and some is synthesized in the kidneys.

Glycogen Glycogen can supply us with about one day’s requirement of glucose. is made from glucose, most of which is synthesized in the cytosol of liver cells. To begin gluconeogenesis, carbon atoms from noncarbohydrate food sources are converted to pyruvate.

Pyruvate to Phosphoenolpyruvate To start the synthesis of glucose from pyruvate, two catalyzed reactions are needed to replace reaction 10 in glycolysis. pyruvate carboxylase uses the energy of ATP hydrolysis to catalyze the addition of CO2 to pyruvate and produce oxaloacetate. phosphoenolpyruvate carboxykinase converts oxaloacetate to phosphoenolpyruvate. phosphoenolpyruvate molecules now use enzymes to form fructose-1,6-bisphosphate.

Pyruvate to Phosphoenolpyruvate

Fructose-1,6-Bisphosphate to Fructose-6-Phosphate The second irreversible reaction in glycolysis is bypassed when fructose-1,6-bisphosphatase cleaves a phosphate group from fructose-1,6-bisphosphate. The product fructose-6-phosphate undergoes the reversible reaction 2 of glycolysis to yield glucose-6-phosphate.

Glucose-6-Phosphate to Glucose In the final irreversible reaction, the phosphate group of glucose-6-phosphate is hydrolyzed by a different enzyme, glucose-6-phosphatase, to form glucose.

Glycolysis and Gluconeogenesis

Energy Cost of Gluconeogenesis The pathway consists of seven reversible reactions of glycolysis and four new reactions that replace the three irreversible reactions. Overall, glucose synthesis requires 4 ATPs, 2 GTPs, and 2 NADHs.

Lactate and the Cori Cycle is the flow of lactate and glucose between the muscles and the liver. occurs when anaerobic conditions occur in active muscle, and glycolysis produces lactate. operates when lactate moves through the blood stream to the liver, where it is oxidized back to pyruvate. converts pyruvate to glucose, which is carried back to the muscles.

Lactate and the Cori Cycle

Regulation of Gluconeogenesis Gluconeogenesis is not utilized when the diet is high in carbohydrates. very active when the diet is low in carbohydrates. When conditions in a cell favor glycolysis, there is no synthesis of glucose. When the cell requires the synthesis of glucose, glycolysis is turned off.

Regulation, Glycolysis, and Gluconeogenesis

Metabolic Reactions of Glucose Glycolysis involves the breakdown of glucose, and gluconeogenesis involves the synthesis of glucose from noncarbohydrates. The pentose phosphate pathway converts glucose to ribose and provides NADPH. Glycogenolysis degrades glycogen to glucose whereas glycogenesis produces glycogen from glucose.

Study Check Identify each process. 1) glycolysis 2) glycogenesis 3) glycogenolysis 4) gluconeogenesis the synthesis of glucose from noncarbohydrates the breakdown of glycogen into glucose the oxidation of glucose to two pyruvate the synthesis of glycogen from glucose

Solution Identify each process. 1) glycolysis 2) glycogenesis 3) glycogenolysis 4) gluconeogenesis the synthesis of glucose from noncarbohydrates 4) gluconeogenesis the breakdown of glycogen into glucose 3) glycogenolysis the oxidation of glucose to two pyruvate 1) glycolysis the synthesis of glycogen from glucose 2) glycogenesis

Concept Map