Glycolysis and gluconeogenesis (Breakdown and synthesis of glucose)

Summary

Glycolysis and gluconeogenesis are metabolic processes responsible for glucose degradation or glucose synthesis respectively. In glycolysis, the breakdown of glucose molecules generates two net adenosine triphosphate (ATP) molecules, which provide a readily available source of energy for various reactions in the cell, and two pyruvate molecules, which can be further converted into lactate (used in gluconeogenesis), acetyl-CoA (used in citric acid cycle), oxaloacetate (used in citric acid cycle), and alanine (used in transamination reactions). Glycolysis, which occurs exclusively in the cytoplasm, is the sole source of ATP in cells that lack mitochondria (e.g., red blood cells). In gluconeogenesis, glucose, which ensures euglycemia during fasting, is synthesized from noncarbohydrate precursors such as glucogenic amino acids (mainly alanine and glutamine), odd-chain fatty acids, glycerol, pyruvate, and lactate. While the glycolytic pathway occurs in all cells, gluconeogenesis occurs almost exclusively in the liver. Phosphofructokinase-1 is the rate-limiting enzyme for glycolysis, while fructose 1,6-bisphosphatase is the rate-limiting enzyme for gluconeogenesis. The metabolism of glucose is mainly controlled by hormones such as insulin, which stimulates glycolysis, and glucagon, which stimulates gluconeogenesis. Glucose can also be shunted to the pentose phosphate pathway (also known as the hexose monophosphate shunt), which is a metabolic pathway that generates nicotinamide adenine dinucleotide phosphate (NADPH) and ribose 5-phosphate from glucose 6-phosphate. The pentose phosphate pathway occurs exclusively in the cytosol and is highly active in the adrenal cortex, liver, and red blood cells (RBCs). Ribose 5-phosphate is required for nucleotide synthesis, while NADPH is required for cholesterol synthesis, steroid synthesis, reduction of glutathione, and respiratory burst. Glucose 6-phosphate dehydrogenase deficiency results in hemolytic anemia due to insufficient production of NADPH, which is required for the reduction of the antioxidant glutathione to prevent excess hydrogen peroxide and free radicals from damaging RBC membranes.

Glycolysis versus gluconeogenesis

Glucose breakdown and synthesis are an essential process in the human body. Glucose provides the required substrates for aerobic and anaerobic metabolism. Glycolysis is the main route of metabolism for most carbohydrates (e.g., galactose, and fructose). Red blood cells, which lack mitochondria, even depend entirely on metabolizing glucose for energy and normal function. The metabolism of glucose is primarily controlled by hormones such as insulin and glucagon. Insulin is released in the postprandial state for anabolic metabolism, in which glucose is broken down to be transformed into storage forms (e.g., glycogen, fat). Conversely, glucagon predominates in the fasting state for catabolic metabolism, in which stored products are broken down (e.g., fats, amino acids) into glucose to be used as an energy source. The following table provides an overall comparison between glycolysis and gluconeogenesis.

Glycolysis Gluconeogenesis
Location

Rate limiting enzyme

Stimulation
  • Glucagon
Inhibition
Occurrence
  • Stimulated in postprandial state
  • Occurs independent of postprandial state in some cells (e.g., erythrocytes)
  • Fasting state

Gluconeogenesis is more than just the reversal of glycolysis: The reactions of the key enzymes of glycolysis are irreversible due to thermodynamics and must therefore be reversed by different enzymes that are only active in gluconeogenesis.

Glycolysis

Overview

  • Definition: : A metabolic pathway that breaks down glucose by substrate-level phosphorylation and oxidation, yielding two pyruvate molecules and two ATP per one glucose molecule. Under anaerobic conditions glycolysis yields two lactate molecules and two ATP per one glucose molecule.
  • Location: cytosol of cells
  • Enzymes
    • Rate-limiting enzyme: phosphofructokinase-1 (PFK-1)
    • There are three irreversible steps in the pathway of glycolysis, executed by the following enzymes:
      • Glucokinase (hexokinase IV)
        • The glucokinase of the liver and β-cells of the pancreas has a lower affinity for glucose than hexokinases of other tissues.
        • This causes the liver to store glucose in the form of glycogen only at high glucose concentrations in the portal blood.
        • At low concentrations, the supply of extrahepatic tissue with glucose is guaranteed.
      • PFK-1
      • Pyruvate kinase
  • Net reaction: glucose + 2 Pi + 2 ADP + 2 NAD+ 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Pyruvate kinase deficiency in erythrocytes causes chronic hemolytic anemia due to impaired glycolysis and a lack of ATP in the RBCs.

Sequence of reactions

Glucose is composed of a 6-carbon skeleton (C6H12O6). Each glucose molecule produces 2 pyruvate molecules, which are composed of a 3-carbon skeleton.

  1. Glucose → glucose 6-phosphate (G6P)
  2. G6P → fructose 6-phosphate (F6P)
  3. F6P → fructose 1,6-biphosphate
    • Enzyme: PKF-1
    • Requires ATP
  4. Fructose 1,6-biphosphate → glyceraldehyde 3-phosphate (GAP)
    • Enzyme: aldolase
  5. GAP → 1,3-Biphosphoglycerate (1,3-BPG)
  6. 1,3-BPG → 3-phosphoglycerate
    • Enzyme: phosphoglycerate kinase
    • Produces ATP
  7. 3-phosphoglycerate → 2-phosphoglycerate
    • Enzyme: phosphoglycerate mutase
  8. 2-phosphoglycerate → phosphoenolpyruvate (PEP)
    • Enzyme: enolase
  9. PEP → pyruvate
    • Enzyme: pyruvate kinase
    • Produces ATP
    • Stimulated by fructose 1,6-biphosphate
    • Inhibited by ATP and alanine

The net reaction for glycolysis is as follows: glucose + 2 Pi + 2 ADP + 2 NAD+ → 2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

In glycolysis, 2 ATP are being invested to gain 4 ATP, so in total, a net gain of 2 ATP per 1 molecule of glucose!

Glycolysis regulation

The regulation of glycolysis is determined by the activity of the enzymes hexokinase, phosphofructokinase-1, and pyruvate kinase, which catalyze essentially irreversible reactions in glycolysis and are therefore the main sites of control.

Enzyme Hexokinase Phosphofructokinase-1 Pyruvate kinase
Reaction
  • Converts glucose to G6P
  • Converts F6P to fructose 1,6-biphosphate
Stimulation
  • AMP (indicates that energy is required → glycolysis is activated)
  • Fructose 2,6-bisphosphate
  • Insulin (in postprandial state): indirect stimulation by increasing levels of fructose 2,6-bisphosphate (see detailed mechanism below table)
  • Fructose-1,6-bisphosphate (feed-forward stimulation)
Inhibition
  • G6P (feedback inhibition)
  • Glucagon
  • ATP (indicates that energy is plentiful → glycolysis is slowed down)
  • Citrate (indicates plentiful supply of intermediates for energy production)
  • Low pH
  • Only in the liver: glucagon indirectly inhibits glycolysis by lowering levels of fructose 2,6-bisphosphate during a fasting state (see detailed mechanism below table)

Fructose-2,6-bisphosphate (feed-forward regulation)

Pyruvate metabolism

Pyruvate characteristics

LDH is found in almost every cell of the body. Elevated LDH levels without exercise may indicate cell injury due to cancer (e.g., germ cell tumors), hemolytic anemia, myocardial infarction, infection, kidney, or liver disease.

Products of pyruvate metabolism

Product Reaction Location Function Regulation
Lactate Cytosol
Acetyl-CoA Mitochondrion
Oxaloacetate Mitochondrion
Alanine Cytosol of myocytes

The five cofactors of the pyruvate dehydrogenase complex: Tender (Thiamine) Loving (lipoic acid) Care (CoA) For (FAD) Nancy (NAD+).

Arsenic inhibits lipoic acid, thereby preventing the production of acetyl-CoA and inhibiting the TCA cycle!

Pyruvate dehydrogenase complex deficiency results in impaired conversion of pyruvate to acetyl-CoA, a reduced production of citrate, and thus an impaired TCA cycle, leading to severe energy deficits (especially in the CNS). Long-term treatment includes a ketogenic diet (high fat, low carbohydrate, glucogenic amino acids, e.g. valine) and cofactor supplementation with thiamine and lipoic acid.

References:[1]

Gluconeogenesis

Overview

Primary substrates

All amino acids, except for leucine and lysine, can be used as substrates for gluconeogenesis.

Gluconeogenesis reactions and regulation

  • Gluconeogenesis is inhibited when there is an excess of energy available (i.e., large ATP/AMP ration) and activated if energy is required (i.e., low ATP/AMP ratio).
  • Gluconeogenesis is also stimulated by glucagon and inhibited by insulin (see phosphofructokinase-2 for the mechanism).
  • The following shows the key steps of gluconeogenesis that are irreversible and need to be bypassed with special enzymes. The other steps are merely the opposite reactions of glycolysis that are carried out by bidirectional enzymes (see glycolysis and this illustration ).
Enzyme 1. Pyruvate carboxylase → 2. Phosphoenolpyruvate carboxykinase → 3. Fructose-1,6-bisphosphatase → 4. Glucose-6-phosphatase
Reaction
  • Converts fructose-1,6-bisphosphate to F6P
  • Converts G6P to glucose
Location
Stimulated by:
  • Citrate
  • ATP
  • Fructose-1,6-bisphosphate (feed-forward regulation)
  • G6P (feed-forward regulation)
Inhibited by:
  • ADP
  • Fructose-2,6-bisphosphate
  • AMP

Clinical significance

References:[2]

Pentose phosphate pathway

Overview

In the pentose phosphate pathway, no ATP is produced or used up.

G6PD deficiency is the most common human enzyme deficiency. It results in insufficient NADPH production, which is required for reduction of the antioxidant glutathione to prevent excess hydrogen peroxide and free radicals from damaging RBC membrane (and causing hemolytic anemia).

Sequence of reactions (two phases)

Oxidative phase (irreversible)

Net reaction in 3 steps: G6P + 2 NADP+ + H2O → ribulose 5-phosphate + 2 NADPH + 2 H+ + CO2

  1. G6P6-phosphogluconolactone
  2. 6-phosphogluconolactone → 6-phosphogluconate
    • Enzyme: 6-phosphogluconolactonase
    • Requires 1 H2O
  3. 6-phosphogluconate → ribulose 5–phosphate

Nonoxidative phase (reversible)

Net reaction: 3 ribulose 5-phosphateribose 5-phosphate + 2 xylulose 5-phosphate ⇄ 2 fructose 6-phosphate + glyceraldehyde 3-phosphate

  • Function depends on the cell's needs:
  • Main reactions include:
    • Ribulose 5-phosphateribose 5-phosphate (isomerization reaction)
    • Ribulose 5-phosphate ⇄ xylulose 5-phosphate (epimerization reaction)
      • Enzyme: ribulose-5-phosphate epimerase (phosphopentose epimerase)
    • Ribose 5-phosphate + xylulose 5-phosphate ⇄ fructose 6-phosphate + glyceraldehyde 3-phosphate
  • 1. Anemaet IG, González JD, Salgado MC, et al. Transactivation of cytosolic alanine aminotransferase gene promoter by p300 and c-Myb. J Mol Endocrinol. 2010; 45(3): pp. 119–132. doi: 10.1677/jme-10-0022.
  • 2. van Schaftingen E, Gerin I. The glucose-6-phosphatase system. Biochem J. 2002; 362(Pt 3): pp. 513–32. pmid: 11879177.
last updated 07/08/2020
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