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Core
All tissues in the body can metabolise glucose (use glucose to produce the ATP they need for cellular processes).
However, some cells can only metabolise glucose (they have an absolute requirement for glucose) because they cannot perform stage 3 or stage 4 of metabolism (the stages of metabolism are introduced here). These cells include red blood cells (which lack mitochondria) and neutrophils, which rely predominantly on glycolysis for ATP production. This absolute requirement is because only glucose can enter into glycolysis, and only glycolysis can happen without mitochondria.
This also means that the body must be excellent at absorbing monosaccharides like glucose from the gut. This can be done either through active co-transport with sodium (via SGLT1) or passive facilitated diffusion via GLUT transporters, particularly GLUT2, within the intestinal epithelium.
Glycolysis is the first stage in aerobic respiration and is the basis for anaerobic respiration. It is made up of ten progressive chemical reactions, split into two phases. It is done in so many small stages because it gives versatility (one of those intermediates might be able to slot into another reaction), fine control (with ten steps it is easy to slow down/speed up/stop at a particular point through the inhibition of a certain enzyme), and it conserves energy (multiple small actions require smaller amounts of activation energy per reaction).
Glycolysis acts as the central pathway of carbohydrate metabolism, and it occurs in all tissues as a cytosolic process. It is exergonic (releases energy), oxidative (causes the reduction of NAD+ to NADH), and with the action of LDH (lactate dehydrogenase), it can be carried out anaerobically.
Functions of glycolysis include:
- Oxidation of glucose
- Production of NADH (2)
- ATP synthesis (2 net per cycle)
- Production of 3C and 6C intermediates for other uses
Glycolysis has 10 stages, 3 of which are irreversible (stages 1, 3 and 10). The irreversible stages of glycolysis are key to know and are mediated by the following enzymes:
Stage 1 is regulated by hexokinase (or glucokinase in the liver)
Stage 3 is regulated by Phosphofructokinase (PFK)
Stage 10 is regulated by Pyruvate Kinase
Diagram - Provides a summary of the reactions in glycolysis
SimpleMed original by Dr. Maddie Swannack
Phase 1 of Glycolysis Includes Reactions 1-3
This involves the phosphorylation of glucose, making it negatively charged. This prevents passage of glucose back across the plasma membrane and increases the reactivity of the glucose. It uses up 2 moles of ATP. Reactions 1 and 3 have large, negative, delta G values, meaning they are irreversible.
Phase 2 of Glycolysis Includes Reactions 4-10
Reaction 4 splits the 6C up into two 3C molecules, meaning reactions 5-10 happen twice per glucose molecule.
Reaction 6 captures one molecule of NADH.
Reaction 10 also has a negative delta G, so is irreversible. It is the site of ATP synthesis in glycolysis, meaning that 2 ATP is captured per reaction, meaning 4 in total per glucose. However, there is only a net gain of 2 ATP per cycle, as 2 ATP were used earlier in the pathway during the energy investment phase.
Important Intermediates of Glycolysis
Each component of glycolysis can be removed from the cycle at any point to be used elsewhere. Each of the substrates in black, involved in the irreversible reactions of glycolysis, have their own unique alternative uses.
- Glucose-6-Phosphate (G-6-P) - the phosphorylated intermediate of glucose that allows glycolysis to occur. Also, an important common intermediate in the digestion of galactose - this means galactose can be used for glycolysis.
- Fructose-6-P - even though it contains the word “fructose” this is not a common intermediate in the metabolism of fructose.
- Glyceraldehyde-3-P - this is the common intermediate for glycolysis and fructose metabolism.
- 1,3-bisphosphoglycerate - in red blood cells, this is converted to 2,3-bisphosphoglycerate (2,3-BPG), an important regulatory mechanism for modifying red blood cell oxygen affinity for haemoglobin.
- 3-phosphoglycerate - an intermediate that can be diverted into other pathways, for example amino acid synthesis. Lipid synthesis uses glycerol-3-phosphate, which is produced in the liver from DHAP, meaning that lipid synthesis in the liver needs glycolysis to occur. This is very important in relation to the metabolism of alcohol, covered in a later lesson.
- Pyruvate - the end point of the glycolysis reaction. It is the reactant for the Link Reaction, which leads to Krebs Cycle (see the next article on the oxygen-dependent pathway).
Diagram - The relationships between glycolysis and the metabolism of other sugars in the liver and how they link together
SimpleMed original by Dr. Maddie Swannack
Step 6 produces 2NADH per glucose, meaning that to happen, step 6 needs a supply of NAD+.
Since levels of NAD+ in the cell are constant, the body needs a mechanism to regenerate NAD+ from NADH. In most cells in the body, this is the electron transport chain (the final stage of aerobic respiration that occurs in the mitochondria, part of oxidative phosphorylation).
However, some cells don’t have mitochondria, meaning they cannot perform the electron transport chain to regenerate their NAD+, so they need another method for NAD+ regeneration. This alternative method is also used in anaerobic respiration like exercise or in pathological situations like heart failure (because the electron transport chain needs oxygen to be able to regenerate NAD+, thus anaerobic conditions can lead to the build up of NADH, but no NAD+).
This issue can be resolved via the conversion of pyruvate to lactate via the enzyme lactate dehydrogenase. This process takes in NADH and converts it back to NAD+. The lactate is then transported to the liver for breakdown back into pyruvate.
Diagram - The equation of pyruvate to lactate using lactate dehydrogenase
SimpleMed original by Dr. Maddie Swannack
If however, anaerobic respiration happens for too long and lactate is allowed to build up, this can lead to a number of worrying pathological situations:
- Hyperlactatemia - lactate levels in the blood of 2-5mM. There is no change in blood pH at this point due to the buffering capacity of the blood.
- Lactic acidosis - blood lactate of >5mM. Blood pH is lowered at this point due to overcoming the blood’s buffering capacity. This is a marker of severe illness.
Allosteric Regulation of Glycolysis Enzymes
Allosteric regulation of enzymes occurs when an activator or inhibitor binds to the enzyme at a site that is not the active site (called the allosteric site), or when the enzyme has covalent modifications made to it like phosphorylation or dephosphorylation. The irreversible steps in glycolysis (1, 3 and 10) are sites of allosteric regulation because high concentrations of the products of those reactions can allosterically inhibit the reaction from taking place.
For example phosphofructokinase exhibits both allosteric and hormonal regulation:
- Allosteric regulation - it is inhibited by high concentrations of ATP as ATP is a high energy signal (if there’s lots of ATP produced, there is plenty enough energy) and therefore glycolysis needs to slow down.
- Hormonal regulation - PFK activity is increased in the presence of insulin, reflecting plentiful glucose availability and promoting glycolysis.
Diagram - The metabolism pathway for galactose, the monosaccharide which (when paired with glucose) makes the disaccharide lactose
SimpleMed original by Dr. Maddie Swannack
Deficiency in any of the three enzymes on the diagram (listed in blue) can lead to Galactosaemia, a condition where galactose or its intermediates build up in the blood. Galactosaemia can lead to cataracts and liver damage, as well as extensive reactive oxygen species damage due to a lack of active glutathione. Galactosemia follows an autosomal recessive mode of inheritance that confers a deficiency in these enzymes.
If galactose is in excess it enters a different pathway to normal and is converted to galactitol using NADPH - this reduces NADPH available. NADPH is needed for the biosynthesis of lipids, the reduction of inappropriate disulphide bonds (absence of which leads to protein cross-linking, which will cause cataracts), as well as glutathione (GSH) regeneration (covered in the article on oxidative stress). Glutathione is used in the clearance of reactive oxygen species (ROS), which are known to damage lipid bilayers and DNA.
The treatment for galactosemia is removal of lactose from diet - galactose is produced from the breakdown of lactose into glucose and galactose.
Lactose is a disaccharide commonly found in dairy products in the diet. It is broken down into its monosaccharide components, galactose and glucose, via lactase on the brush border of the small intestine, before being absorbed into the body.
In the case of lactose intolerance, the enzyme lactase is not able to function correctly. If lactose cannot be digested and absorbed in the small intestine, it remains in the lumen, lowering the water potential and causing water to be drawn in. This causes diarrhoea (due to the large amount of water) and flatulence (produced by the bacteria in the gut digesting the excess unabsorbed carbohydrate).
Primary lactase deficiency is caused by the absence of the lactase persistence allele. This means that the patients produce lactase as babies (when their diet is almost exclusively milk), but once dairy becomes less of a part of the diet (normally around 2 years old) their body stops producing as much lactase.
Secondary lactase deficiency is caused by damage to the small intestine, for example through gastroenteritis or prolonged antibiotic use. It can resolve if given time without the offending stimuli, but it can also persist for years.
Congenital lactase deficiency is a very rare, autosomal recessive disease where the patient has no lactase produced at all, leaving them unable to digest breast milk.
Diagram - The Pentose Phosphate pathway, involving the turning of glucose-6-Phosphate into 5 carbon sugars through the action of Glucose-6-Phosphate Dehydrogenase
SimpleMed original by Dr. Maddie Swannack
The Pentose Phosphate Pathway converts G-6-P from glycolysis to 5C sugar phosphates, providing NADPH, a vital molecule that helps the antioxidant molecule glutathione (GSH) to remain in its active state. It requires Glucose-6-Phosphate dehydrogenase.
Glutathione is capable of preventing damage to important cellular components caused by reactive oxygen species such as free radicals, peroxides, lipid peroxides, and heavy metals.
Glutathione reduces disulphide bonds formed within cytoplasmic proteins to cysteines by serving as an electron donor. In the process, glutathione is converted to its oxidized form, glutathione disulphide (GSSG). To be converted back to GSH, NADPH is required as an electron donor.
Glutathione helps to maintain proteins in their correct structure by keeping the SH groups in their reduced (hydrogenated) state.
If there is reduced NADPH production (such as in G-6-P dehydrogenase deficiency), inappropriate disulphide bonds can form within cells, causing proteins to aggregate. One mechanism contributing to aggregation is damage caused by reactive oxygen species in red blood cells. Damage to the RBC causes Heinz Bodies (denatured haemoglobin) to form, leading them to be removed by the spleen.
The 5C sugar phosphates produced in this pathway can either then be used in RNA and DNA synthesis (which require a 5C sugar backbone) or can re-enter glycolysis by their conversion to G-3-P.
Edited by: Dr. Ben Appleby and Dr. Marcus Judge
In this article
Glycolysis is a ten step process that turns glucose into pyruvate. It releases a small amount of ATP (2 net ATP per glucose) and captures 2 NADH per…
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