Answer- Gluconeogenesis is the process of converting noncarbohydrate precursors to glucose or glycogen. The major substrates (non carbohydrate precursors ) are the glucogenic amino acids, lactate, glycerol, propionate and the intermediates of TCA cycle.
Liver and kidney are the major gluconeogenic tissues. Gluconeogenesis meets the needs of the body for glucose when sufficient carbohydrate is not available from the diet or glycogen reserves. A supply of glucose is necessary especially for the nervous system and erythrocytes. Failure of gluconeogenesis is usually fatal.
Gluconeogenesis involves Glycolysis, Citric Acid Cycle, Plus Some Special Reactions –
Three nonequilibrium reactions in glycolysis , catalyzed by hexokinase, phosphofructokinase and pyruvate kinase, prevent simple reversal of glycolysis for glucose synthesis. These are considered thermodynamic barriers of gluconeogenesis and are circumvented by alternative reactions-
1) Pyruvate to Phosphoenolpyruvate
Reversal of the reaction catalyzed by pyruvate kinase in glycolysis involves two endothermic reactions. Mitochondrial pyruvate carboxylase catalyzes the carboxylation of pyruvate to oxaloacetate, an ATP-requiring reaction in which the vitamin biotin is the coenzyme. (Figure-1). A second enzyme, phosphoenolpyruvate carboxykinase, catalyzes the decarboxylation and phosphorylation of oxaloacetate to phosphoenolpyruvate using GTP as the phosphate donor. In liver and kidney, the reaction of succinate thiokinase in the citric acid cycle produces GTP (rather than ATP as in other tissues), and this GTP is used for the reaction of phosphoenolpyruvate carboxykinase, thus providing a link between citric acid cycle activity and gluconeogenesis, to prevent excessive removal of oxaloacetate for gluconeogenesis, which would impair citric acid cycle activity.
Fructose 1,6-Bisphosphate to Fructose 6-Phosphate
The conversion of fructose 1,6-bisphosphate to fructose 6-phosphate, for the reversal of glycolysis, is catalyzed by fructose 1,6-bisphosphatase. Its presence determines whether a tissue is capable of synthesizing glucose (or glycogen) not only from pyruvate, but also from triose phosphates.
Glucose 6-Phosphate & Glucose
The conversion of glucose 6-phosphate to glucose is catalyzed by glucose 6-phosphatase. It is present in liver and kidney, but absent from muscle and adipose tissue, which therefore, cannot export glucose into the bloodstream.
Figure-1-Showing an overview of gluconeogenesis, the thermodynamic barriers and the alternative reactions, the entry of major substrates in to the main pathway of gluconeogenesis
Entry of substrates of gluconeogenesis in to the main pathway-
A) Glucogenic amino acids- Amino acids are derived from the dietary proteins, tissue proteins or from the breakdown of skeletal muscle proteins during starvation. After transamination or deamination, glucogenic amino acids yield either pyruvate or intermediates of the citric acid cycle. Amino acids that are degraded to acetyl CoA or Acetoacetyl CoA are termed ketogenic amino acids because they can give rise to ketone bodies or fatty acids. Amino acids that are degraded to pyruvate, α-ketoglutarate, succinyl CoA, fumarate, or oxaloacetate are termed glucogenic amino acids (Figure-2).The net synthesis of glucose from these amino acids is feasible because these citric acid cycle intermediates and pyruvate can be converted into phosphoenolpyruvate. The entry of each of the glucogenic amino acids in to the pathway of gluconeogenesis is as follows-
1) Pyruvate is the point of entry for alanine, serine, cysteine, glycine, threonine, and tryptophan (Figure-2). The transamination of alanine directly yields pyruvate.
2) Oxalo acetate- Aspartate and asparagine are converted into oxaloacetate, a citric acid cycle intermediate. Aspartate, a four-carbon amino acid, is directly transaminated to oxaloacetate.
3) α-Ketoglutarate is the point of entry of several five-carbon amino acids that are first converted into glutamate.(Figure-2)
4) Succinyl CoA is a point of entry for some of the carbon atoms of methionine, isoleucine, and valine. Propionyl CoA and then Methylmalonyl CoA are intermediates in the breakdown of these three nonpolar amino acids.
5) Fumarate is the point of entry for Aspartate, Phenyl alanine and Tyrosine.
Figure- 2- Amino acids forming Acetyl co A or Acetoacetyl co A are not considered glucogenic, they are called ketogenic amino acids since acetyl co A is a precursor for ketone bodies. All other amino acids which form pyruvate or intermediates of TCA cycle are considered glucogenic.
B) Lactate- Lactate is formed by active skeletal muscle when the rate of glycolysis exceeds the rate of oxidative metabolism. Lactate is readily converted into pyruvate by the action of lactate dehydrogenase (Figure-3)
Figure-3- Reaction showing the inter conversion of lactate and pyruvate
Lactate is a major source of carbon atoms for glucose synthesis by gluconeogenesis. During anaerobic glycolysis in skeletal muscle, pyruvate is reduced to lactate by lactate dehydrogenase (LDH). This reaction serves two critical functions during anaerobic glycolysis. First, in the direction of lactate formation the LDH reaction requires NADH and yields NAD+ which is then available for use by the glyceraldehyde-3-phosphate dehydrogenase reaction of glycolysis. These two reactions are, therefore, intimately coupled during anaerobic glycolysis. Secondly, the lactate produced by the LDH reaction is released to the blood stream and transported to the liver where it is converted to glucose. The glucose is then returned to the blood for use by muscle as an energy source and to replenish glycogen stores. This cycle is termed the Cori cycle (Figure-4)
Figure-4- Cori cycle, the interchange of lactate and glucose between skeletal muscle and Glucose
C) Propionate- Propionate is a major precursor of glucose in ruminants; it enters gluconeogenesis via the citric acid cycle. After esterificaton with CoA, Propionyl-CoA is carboxylated to D-Methylmalonyl-CoA, catalyzed by Propionyl-CoA carboxylase, a biotin-dependent enzyme (Figure-5). Methylmalonyl-CoA racemase catalyzes the conversion of D-Methylmalonyl-CoA to L-Methylmalonyl-CoA, which then undergoes isomerization to succinyl-CoA catalyzed by Methylmalonyl-CoA mutase. In non-ruminants, including humans, propionate arises from the Beta -oxidation of odd-chain fatty acids that occur in ruminant lipids, as well as the oxidation of isoleucine and the side-chain of cholesterol, and is a (relatively minor) substrate for gluconeogenesis. Methylmalonyl CoA Isomerase/ mutase is a vitamin B12 dependent enzyme, and in deficiency methylmalonic acid is excreted in the urine (methylmalonic aciduria). (Figure-5)
Figure-5- showing the fate of Propionyl co A
D) Glycerol-The hydrolysis of triacylglycerols in fat cells yields glycerol and fatty acids. Glycerol may enter either the gluconeogenic or the glycolytic pathway at Dihydroxyacetone phosphate; however, the carbons of the fatty acids cannot be utilized for net synthesis of glucose. In the fasting state glycerol released from lipolysis of adipose tissue triacylglycerol is used solely as a substrate for gluconeogenesis in the liver and kidneys.This requires phosphorylation to glycerol-3-phosphate by glycerol kinase and dehydrogenation to Dihydroxyacetone phosphate (DHAP) by glyceraldehyde-3-phosphate dehydrogenase (G3PDH). The G3PDH reaction is the same as that used in the transport of cytosolic reducing equivalents into the mitochondrion for use in oxidative phosphorylation. This transport pathway is called the glycerol-phosphate shuttle.
Figure-6- showing the conversion of glycerol to dihydroxy acetone phosphate
Glycerol kinase is absent in adipose tissue, so glycerol released by hydrolysis of triglycerides can not be utilized for re esterificaton, it is a waste product, It is carried through circulation to the liver and is used for gluconeogenesis or glycolysis as the need may be. In fact adipocytes require a basal level of glycolysis in order to provide them with DHAP as an intermediate in the synthesis of triacylglycerols.Please help "Biochemistry for Medics" by CLICKING ON THE ADVERTISEMENTS above!