Glycolysis and gluconeogenesis (Breakdown and synthesis of glucose)

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
  • Insulin (in the liver): indirect stimulation
  • Adenosine monophosphate (AMP)
  • Fructose 2,6-biphosphate
  • 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 2 pyruvate molecules and 2 ATP.
  • Location: cytosol of cells
  • Enzymes

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)
    • Enzyme: GAP dehydrogenase
    • Produces NADH + H+
  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)

  • Fructose 2,6-bisphosphate is synthesized by phosphofructokinase-2.
  • Phosphofructokinase-2 is a bifunctional enzyme with a phosphatase (FBPase-2) and a kinase (PFK-2) domain
    • Regulation ;:
      • In the liver
        • Fasting state: low blood glucose → increased circulating glucagon levels → increased levels of cAMP → increased protein kinase A (PKA) activity → stimulation of FBPase-2 and inhibition of PFK-2 domain → decreased production of F-2,6-P2 → less glycolysis + more gluconeogenesis
        • Postprandial state: high blood glucose → increased circulating insulin levels (indicate a high abundance of blood glucose available for glycolysis) → decreased levels of cAMP → decreased PKA activity → inhibition of FBPase-2 and stimulation of PFK-2 domain → increased production of F-2,6-P2 → F-2,6-P2 activates PFK-1more glycolysis + less gluconeogenesis
      • In the heart: epinephrine and/or insulin → stimulation of PFK-2 domain → increases production of F-2,6-P2 → F-2,6-P2 activates PFK-1more glycolysis provides quick energy, e.g., in the case of stress

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
  • Pyruvatelactate (reversible)
    • Enzyme: lactate dehydrogenase (LDH)
      • Present in the heart, RBCs, and muscle, among others
      • Only occurs in anaerobic glycolysis
    • NADH is oxidized to NAD+ in the process
Cytosol
  • Stimulated by: NADH/NAD+ ratio
  • Inhibited by: high concentrations of lactate (feedback inhibition)
Acetyl-CoA Mitochondrion
  • Stimulated by: ADP, NADH/NAD+ ratio, ↑ Ca2+
  • Inhibited by: ↑ acetyl-CoA/CoA ratio, NADH/NAD+ ratio, and ATP/ADP ratio
Oxaloacetate Mitochondrion
Alanine Cytosol of myocytes
  • Stimulated by: high protein intake, fasting, cortisol, epinephrine, and glucagon


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.

Gluconeogenesis

Overview

  • Definition: A series of metabolic events that allows for the production of glucose from noncarbohydrate precursors.
  • Purpose: During fasting, gluconeogenesis becomes the main source of glycemia maintenance after glycogen stores are depleted (after 1–3 days of normal activity)
  • Cell location: : Responsible enzymes are located in cytosol and mitochondria.
  • Sites of gluconeogenesis
  • Rate-limiting enzyme: fructose-1,6-bisphosphatase
  • Noncarbohydrate precursors: Glucogenic amino acids (mainly alanine and glutamine), lipids, glycerol, pyruvate, and lactate can all be converted to glucose in an attempt to preserve serum glucose levels. These reactions are energy intensive, as they rely on the consumption of high energy molecules (GTP, ATP).

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

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. G6P → 6-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-phosphate → 1 ribose-5-phosphate + 2 xylulose-5-phosphate → 2 fructose-6-phosphate + glyceraldehyde-3-phosphate

last updated 11/07/2018
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