Gluconeogenesis

Gluconeogenesis Authors: Sara Gunter and Brionna Buckman

//1. Overview// //2. Location// //3. Pathway// //4. Control// //5. Clinical//

1. Overview

Gluconeogenesis is an anabolic process by which glucose is made from smaller, simpler molecules, such as lactate and pyruvate. It functions in the liver and kidneys to keep blood glucose levels within normal limits. Due to the fact that glucose is the major energy source for most tissues like the brain, gluconeogenesis is very important for organisms to survive. It is essential that they have the ability to synthesize glucose from a noncarbohydrate source, in the case that not enough glucose is consumed in the diet. For example, in humans when muscles consume large amounts of glucose through glycolysis, a large quantity of pyruvate is produced. In very strenuous exercise when muscles become anaerobic, pyruvate is converted into lactate. Gluconeogenesis uses this lactate and pyruvate to make more glucose. Gluconeogenesis can be very generally viewed as the reverse of glycolysis. In glycolysis, glucose is catabolized to produce ATP, while in gluconeogenesis, ATP is consumed to produce glucose. Although, gluconeogenesis is more than just the mere reversal of glycolysis. It is not strongly endergonic, as the reversal of glycolysis would be. Gluconeogenesis must occur spontaneously when needed, and thus it can not be strongly endergonic. Also, glycolysis and gluconeogenesis do not have a completely shared pathway, because of a highly regulated control which inhibits gluconeogenesis when glycolysis is active and visa versa. The route of gluconeogenesis has unique reactions to differentiate it from glycolysis. There are four different reactions and four catalyzed enzymes in the process of gluconeogenesis than in glycolysis. First is the conversion of pyruvate to oxaloacetate with pyruvate carboxylase. Second is the conversion of oxaloacetate to PEP with PEP carboxykinase. Next is the hydrolysis of Fructose-1,6-Bisphosphate to fructose-6-phosphate with fructose-1,6-biphosphatase. Lastly is the conversion of glucose-6-phosphate to glucose by the action of glucose-6-phosphatase.

2. Location

The sites in which gluconeogenesis occurs is in the liver and kidneys. About 90% of gluconeogenic activity takes place in the liver and about 10% takes place in the kidneys. The formation of oxaloacetate from pyruvate occurs in the mitochondria, while the enzymes that convert PEP to glucose are located in the cytosol. The conversion of glucose-6-phosphate occurs through a series of transporter proteins, T1, T2, and T3, located in the endoplasmic reticulum. This process of gluconeogenesis is not located in muscle or brain cells because of the lack of the enzyme glucose-6-phosphatase in the endoplasmic reticulum. After glucose is synthesized in the liver and kidneys, it travels via the blood to be absorbed by the brain, muscle, and heart to meet necessary metabolic needs. After this, the pyruvate and lactate produced in these areas are returned to the liver and kidneys and used as substrates for more gluconeogenic processing.

3. Pathway

Three or four carbon substrates may enter the gluconeogenesis pathway. Lactate from anaerobic respiration may enter the pathway when it is converted into pyruvate in liver cells during the Cori cycle. The first step in the pathway involves the conversion of a 3 carbon pyruvate to a 4 carbon oxaloacetate intermediate. In this step, one ATP is converted to ADP. This reaction is catalyzed by the enzyme pyruvate carboxylase, which requires a biotin cofactor. Pyruvate carboxylase is found only in the matrix of the mitrochondria and is activated by acetyl-CoA. Oxaloacetate is then reduced to malate by the malate dehydrogenase. This step is required for transport out of the mitochondria and into the cytosol. A molecule of NADH is also required in this step. Once in the cytosol, malate is oxidized back to oxaloacetate using NAD+ and malate dehydrogenase. Oxaloacetate could also be transported across the membrane by conversion to PEP or by transamination to aspartate. Oxaloacetate is converted to phosphoenolpyruvate (PEP) using the unique enzyme phosphoenolpyruvate carboxykinase. This is an energy requiring step that converts GTP to GDP. In this step, CO 2 from the oxaloacetate is removed. The decarboxylation of oxaloacetate drives the reaction toward the formation of PEP and drives a reaction that would otherwise be endergonic. The reversal of glycolysis is followed as PEP is converted to 2-phosphoglycerate, then 3-phosphoglycerate. The conversion of 3-phosphoglycerate to 1,3-bisphosphoglycerate requires the reduction of ATP to ADP. By oxidizing NADH to NAD +, 1.3-bisphosphoglycerate is converted to glyceraldehyde-3-phosphate and dihydroxyacetone-phosphate. These steps all use the same enzymes as glycolysis. From this, fructose-1,6-bisphosphate is formed and then hydrolyzed to fructose-6-phosphate. This phosphate ester hydrolysis uses the enzyme fructose-1.-6-bisphosphatase and is an exergonic reaction. Fructose-6-phosphate is converted to glucose-6-phosphate through the enzyme phosphoglucose isomerase. In the final step, glucose-6-phosphate is converted to glucose by glucose-6-phosphatase. This enzyme, glucose-6-phosphatase is located in the endoplasmic reticulum (ER) of kidney and liver cells. There are 3 transporter proteins, T1, T2, and T3, used in the glucose-6-phosphatase system. T1 transports glucose-6-phosphate into the ER and hydrolyzes it to glucose and Pi. Glucose and Pi are then exported into the cytosol by the T2 and T3 transporters, respectively. The GLUT2 then transport glucose into circulation.

Overall reaction: 2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 2H + + 6H20 > glucose + 4 ADP + 2 GDP + 6Pi + 2 NAD +

4. Control

Reciprocal control takes place in the cell to ensure that glycolysis and gluconeogenesis do not take place at the same time. This way, the cell does not over-consume ATP. Through this control, glycolysis does not take place at the same time as gluconeogenesis, and visa versa. When the body has a low amount of energy, glucose is rapidly used in glycolysis. However, when energy levels are high, pyruvate is used to synthesize and store glucose. Glyconeogenesis is also activated when blood glucose levels are low. Three enzymes catalyze the strongly exergonic reactions of glyconeogenesis. These include hexokinase (glucokinase), phosphofructokinase, and pyruvate kinase. Three sites are important for regulation of gluconeogenesis including glucose-6-phosphatase, fructose-1,6-bisphosphatase, and pyruvate carboxylase-PEP carboxykinase pair. Gluconeogenesis is regulated by two control mechanisms; substrate-level and allosteric control. Substrate-level control involves the pyruvate carboxylate step in which high levels of acetyl CoA activate pyruvate carboxylate. A rise in acetyl CoA would indicate that cellular energy levels (ATP) are high so glucose synthesis and storage is promoted. When acetyl CoA levels are low, pyruvate is directed into the TCA cycle for the generation of ATP. Allosteric control involves fructose-2,6-phosphate as a inhibitor of fructose-1,6-bisphosphatase in the presence or absence of ATP.

5. Clinical

Inhibition of glucogenesis is now the target of diabetes therapy. Diabetes, especially type 2 diabetes, is associated with an insensitively to insulin. A new drug therapy, Metformin, may improve patient's sensitively to insulin for those with type 2 diabetes. This would stimulate the uptake of glucose by glucose transporters and may increase binding of insulin to insulin receptors. By doing so, the activity of tyrosine kinase at the insulin receptor would be increased and thus gluconeogenesis would be inhibited. Other drugs, such as 3-Mercaptopicolinate and hydrazine, are also being researched as diabetes therapy. These drugs may be able to inhibit gluconeogenesis without affecting glycolysis by inhibiting the transport activity of the glucose-6-phosphatase system. This, however, would not affect the glucose-6-phosphate enzyme activity.

References

Garrett, Reginald H., Charles M. Grisham. __Biochemistry__. Boston: Brooks/Cole, 2010.

"Gluconeogenesis." __Interactive Concepts in Biochemistry.__ Wiley. http://www.wiley.com/college/boyer/0470003790/animations/gluconeogenesis/gluconeogenesis.htm