Glucose is a highly adaptable metabolite found in many organisms, offering a free energy of -2830kJ mol when fully metabolised.
This energy is released in small portions via ATP, the body’s universal energy currency. A molecule of ATP holds approx 30kJ mol.
The diagram above shows the process and points where ATP is released or consumed during Glycolysis. It is important to remember that this pathway is only the first section of a larger process (metabolism), as Pyruvate from this chain is used later in Krebs cycle etc.
On this diagram we see the steps again, and highlighted in green are the molecules which differ from the next. I’ll come back and edit this later but for now you’ll have to compare it with the first diagram for enzyme names etc.
- Glucose –> Glucose-6-Phosphate (-1 ATP)
The hydrogen on the alcohol on carbon 6 of glucose is replaced by a phosphate group from the ATP by Hexokinase.
- Glucose-6-phosphate –> Fructose-6-phosphate
Phosphoglucose isomerase changes the glucose structure to fructose by swapping the C=O and alcohol groups on carbons 1&2.
- Fructose-6-phosphate –> Fructose-1,6-bisphosphate (-1 ATP)
Phosphofructokinase replaces the hydrogen on the alcohol group of C1 with another phosphate group.
- Fructose-1,6-bisphosphate –> GLAP + DHAP
Aldolase splits the fructose-1,6-bisphosphate into two 3 carbon molecules, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.
- DHAP –> GLAP
Triose phosphate isomerase converts DHAP into GLAP by changing the structural configuration.
From here on there are two molecules at a time (2 x 3 carbon rather than 1 x 6 carbon) and so all ATP & NADH figures have been doubled.
- GLAP –> 1,3-bisphosphoglycerate (-2 Pi) (+2 NADH)
Glyceraldehyde 3-phosphate dehydrogenase replaces a H on C1 with an O and phosphate group.
- 1,3-bisphosphoglycerate –> 3-phosphoglycerate (+2 ATP)
Phosphoglycerate kinase removes the phosphate group from C1.
- 3-phosphoglycerate –> 2-phosphoglycerate
Phosphoglycerate mutase switches C2 & C3.
- 2-phosphoglycerate –> 2-phosphoenolpyruvate (+2 H2O)
Enolase removes the alcohol on C3, forming a C=C between C2 & C3.
- 2-phosphoenolpyruvate –> Pyruvate (+2 ATP)
Pyruvate kinase removes the phosphate group from C2, double bond C=O alters structure below C2.
Balancesheet: 2 ATP + 2 NADH – however 1 NADH produces 3 ATP when oxidised by the electron transport chain so glycolysis indirectly produces another 6 ATP. This means glycolysis has a net ATP production of 8 ATP.
In anaerobic conditions we find only 2 ATP’s are produced for every glucose molecule converted to 2 lactate molecules.
This is because the cell needs to reoxidise the NADH, and one such way of doing this is reducing the pyruvate by lactate dehydrogenase with the NADH, producing lactate. All pyruvate must be converted to lactate to allow ATP synthesis to continue; and the lack of oxygen means no energy is gained from the oxidation of NADH.