Gluconeogenesis

Gluconeogenesis

Process that converts non-carbohydrate precursors to carbohydrates (generally to glucose)

Two Sites of Gluconeogenesis in Mammals

1. Liver (the predominant site)

2. Kidney cortex

Functions of Gluconeogenesis

1. A source of glucose independent of diet - Starvation

2. Cori Cycle – conversion of lactate to glucose during or after heavy exercise

Maintaining Levels of Glucose

The end products of many biochemical pathways are salvaged to synthesize glucose in gluconeogenesis.

• The brain depends on glucose as its primary fuel and red blood cells use glucose as their only fuel.

Glycolysis and gluconeogenesis have some enzymes in common:

• The highly exergonic, irreversible steps of glycolysis are bypassed in gluconeogenesis with reactions that render gluconeogenesis exergonic under cellular conditions.

• The two pathways are reciprocally regulated so that glycolysis and gluconeogenesis do not take place in the same cell at the same time to a significant extent.

Major Noncarbohydrate Precursors of Glucose

Lactate, amino acids, and glycerol:

• They are converted into pyruvate or later intermediates for glycolysis.

• Skeletal muscle forms lactate through lactic acid fermentation.

• The liver converts lactate into pyruvate by lactate dehydrogenase

• Amino acids are derived from proteins in the diet and, during starvation, from the breakdown of proteins in skeletal muscle.

• Hydrolysis of triacylglycerols in fat yields glycerol and fatty acids. Glycerol is a glucose precursor that may enter the gluconeogenic or glycolytic pathway at dihydroxyacetone phosphate.

Pathway of Gluconeogenesis

• In glycolysis, glucose is converted into pyruvate.

• In gluconeogenesis, pyruvate is converted into glucose.

• However, gluconeogenesis is not a reversal of glycolysis.

• Most of the decrease in free energy in glycolysis takes place in the three irreversible steps catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase.

• In gluconeogenesis, these irreversible reactions of glycolysis must be bypassed.

Conversion of Pyruvate into Phosphoenolpyruvate

• The first step in gluconeogenesis is the carboxylation of pyruvate to form oxaloacetate.

• This is at the expense of a molecule of ATP.

• The reaction is catalyzed by pyruvate carboxylase.

• This reaction occurs in the mitochondria.

Biotin

• Also called vitamin B7

• Used in CO2 transfer and carboxylation reactions

• deficiency is characterized by muscle pain, lethargy, anorexia, and depression

• This vitamin is synthesized by microflora in the intestinal tract and can be obtained in the diet from liver, soybeans, nuts, and many other sources

Oxaloacetate Is Converted into Phosphoenolpyruvate

Oxaloacetate must be transported to the cytoplasm to complete the synthesis of phosphoenolpyruvate

• Oxaloacetate is first reduced to malate by malate dehydrogenase

• Malate is then transported across the mitochondrial membrane and reoxidized to oxaloacetate by a cytoplasmic NAD+ - linked malate dehydrogenase

• The formation of oxaloacetate from malate also provides NADH for use in subsequent steps in gluconeogenesis

• Finally, oxaloacetate is simultaneously decarboxylated and phosphorylated by phosphoenolpyruvate carboxykinase (PEPCK) to generate phosphoenolpyruvate.

• The phosphoryl donor is GTP.

• The CO2 that was added to pyruvate by pyruvate carboxylase comes off in this step.

• The sum of the reactions catalyzed by pyruvate carboxylase and phosphoenolpyruvate carboxykinase is:

Pyruvate + ATP + GTP + H2O → phosphoenolpyruvate + ADP + GDP + Pi + 2 H+

• This pair of reactions bypasses the irreversible reaction catalyzed by pyruvate kinase in glycolysis.

Conversion of Fructose 1,6-Bisphosphate
into Fructose 6-Phosphate and
Orthophosphate

• The hydrolysis of fructose 1,6-bisphosphate to fructose 6-phosphate and Pi occurs after the formation of phosphoenolpyruvate.

• The enzyme responsible for this step is fructose 1,6-bisphosphatase (FBPase).

Fructose 1,6-bisphosphatase (FBPase)

• It is an allosteric enzyme that is the primary regulatory point of gluconeogenesis.

• It is an example of a phosphatase, an enzyme that catalyzes the hydrolysis of a phosphate to form inorganic phosphate and the reverse reaction

The Generation of Free Glucose

Occurs essentially only in the liver and is the final step in gluconeogenesis

• Glucose 6-phosphate is transported into the lumen of
  the endoplasmic reticulum

• Glucose 6-phosphatase, an integral membrane on the
  inner surface of the endoplasmic reticulum, catalyzes the formation of glucose from glucose 6-phosphate.

• The fructose 6-phosphate generated by fructose 1,6-bisphosphatase is readily
  converted into glucose 6-phosphate.

• In most tissues, gluconeogenesis ends with the formation of glucose 6-phosphate.

• The glucose 6-phosphate is commonly converted into glycogen or used for the biosynthesis of other molecules like nucleotides.

Glucose 6-phosphate

transported into the lumen of the endoplasmic reticulum of liver cells, where it is hydrolyzed to glucose by glucose 6-phosphatase

• a pair of transporters then shuttle glucose and Pi back to the cytoplasm

Six High-Transfer-Potential Phosphoryl
Groups of Gluconeogenesis

• The formation of glucose from pyruvate is energetically unfavorable unless it is coupled to favourable reactions

The stoichiometry of gluconeogenesis is:

2 Pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 6 H2O → glucose + 4 ADP + 2 GDP + 6 Pi + 2 NAD+

ΔG°’ = −48 kJ mol−1(−11 kcal mol −1)

• Six nucleoside triphosphate molecules are hydrolyzed to synthesize glucose from pyruvate in gluconeogenesis, whereas only two molecules of ATP are generated in glycolysis.

Gluconeogenesis and glycolysis are reciprocally regulated:

• Gluconeogenesis and glycolysis are coordinated so that, within a cell, one pathway is relatively inactive while the other one is highly active

• The basic premise of the reciprocal regulation is that when glucose is abundant, glycolysis predominates. When glucose is scarce, gluconeogenesis takes over.

• The rate of glycolysis is regulated by the concentration of glucose

• The rate of gluconeogenesis is regulated by the concentrations of lactate and other precursors of glucose.

Energy Charge Determines Whether Glycolysis or Gluconeogenesis Will Be More Active

• The interconversion of fructose 1,6-bisphosphate and fructose 6-phosphate is a key regulatory site.

• Additionally, glycolysis and gluconeogenesis are reciprocally regulated at the interconversion of phosphoenolpyruvate and pyruvate.

• If ATP is needed, glycolysis predominates. If glucose is needed, gluconeogenesis is favored.

Glycolysis and Gluconeogenesis Are Regulated by Metabolic Intermediates

• The key regulatory site in the gluconeogenesis pathway is the interconversion of fructose 6-phosphate and fructose 1,6-bisphosphate.

• Glycolysis and gluconeogenesis are also reciprocally regulated at the interconversion of phosphoenolpyruvate and pyruvate in the liver.

The Balance between Glycolysis and Gluconeogenesis in the Liver Is Sensitive to Blood-Glucose Concentration

• In the liver, the rates of glycolysis and gluconeogenesis are adjusted to maintain blood-glucose levels.

• The key regulator of glucose metabolism in the liver is fructose 2,6-bisphosphate.

Fructose 2,6-bisphosphate

stimulates phosphofructokinase and inhibits fructose 1,6-bisphosphatase.

When blood glucose is high…

Insulin is secreted:

• Insulin stimulates glycolysis.

• Insulin normally inhibits gluconeogenesis.

• However, in type 2 diabetes, insulin fails to act, a condition called insulin resistance (type-2-diabetes).

• The treatment of type 2 diabetes includes weight loss, a healthy diet, exercise, and drug treatment to enhance sensitivity to insulin.

When blood glucose is low…

The hormone glucagon is secreted.

Cori Cycle

• Muscle and liver display inter-organ cooperation in a series of reactions

• Lactate produced by muscle during contraction is released
  into the blood

• Liver removes the lactate and converts it into glucose,
  which can be released into the blood

The Bifunctional Enzyme PFK2/FBPase

• has two distinct domains with
  opposing enzymatic activities

• the kinase domain is fused to the phosphatase domain

• determines F-2,6-BP levels
  that are controlled by hormones

The synthesis and degradation of fructose 2,6-bisphosphate is hormonally controlled:

Insulin accelerates the formation of fructose 2,6-bisphosphate by facilitating the dephosphorylation of the bifunctional enzyme