Glycolysis is the process by which one molecule of glucose is converted into two molecules of pyruvate, two hydrogen ions and two molecules of water. This leads to the synthesis of the ‘high energy’ intermediate molecules of ATP and NADH. Pyruvate molecules then proceed to the link reaction, where acetyl-CoA is produced and acetyl-CoA then proceeds to the TCA cycle.
In this article, we will look at the steps of glycolysis, and its regulation and consider some clinical conditions related to glycolysis.
Glycolysis is the metabolism of glucose into two pyruvate molecules, with the net generation of two molecules of ATP and two molecules of NADH.
It is regulated at the entry to the pathway and at the irreversible steps (1, 3, and 10). This will be discussed in more detail below.
Glycolysis is an anaerobic reaction, and in low oxygen conditions it is the cell’s sole source of ATP. You can read more about anaerobic respiration here.
Substrates can enter the glycolysis pathway via three different ways, which are referred to as ‘entry points’. These are:
- Dietary glucose – glucose is directly absorbed into the bloodstream from the gastrointestinal tract and enters the pathway.
- Glycogenolysis – glucose is released from hepatic stores of glycogen and enters the pathway.
- Other monosaccharides – galactose and fructose enter the glycolysis pathway at various levels via common intermediates.
Glycogen in skeletal muscle cannot be fully broken down into glucose. This means it cannot leave the cell, therefore can only feed into glycolysis within the individual skeletal muscle cells it is stored in.
Transport into Cells
In order for circulating glucose to be used by cells, it needs to pass from the extracellular space (bloodstream) into the intracellular space. Various transporters (GLUT 1-4) transport glucose into cells. They have different kinetics and methods of regulation depending on the purpose of glycolysis in that cell.
N.B. For an explanation of the term Km, please see our article on enzyme kinetics.
|GLUT transporter||Key feature||Location||Reason|
|GLUT-1||Low Km, so highly active||All cells||Regulates basal uptake even at low glucose levels, ensures a constant supply of energy for survival|
|GLUT-2||Concentration-dependent, due to high Km||Liver, pancreatic islets||Acts as a glucose sensor – increases uptake in high glucose levels for storage. Also regulates insulin release from the pancreas|
|GLUT-4||Insulin-dependent||Muscle, adipose, heart||Increases uptake in presence of insulin (i.e. after meals) for storage|
Phases of Glycolysis
Glycolysis can be considered as a two-part process. Firstly, energy is consumed to generate high-energy intermediates, which then go on to release their energy during the second phase.
- Energy investment phase – requires two ATP molecules to produce high-energy intermediates.
- Energy payout phase – The intermediate is metabolised, producing four ATP molecules and two NADH molecules.
Energy Investment Phase
Glucose is phosphorylated by hexokinase to form glucose-6-phosphate (G6P). The negative charge effectively traps G6P in the cell as it cannot pass through the membrane.
This reaction consumes a molecule of ATP, so is spontaneous and irreversible. It is regulated by product inhibition; higher concentrations of G6P inhibit hexokinase and slow the reaction.
In the liver, glucokinase also catalyses this reaction. It has a higher Km than hexokinase, and therefore works at greater concentrations of serum glucose.
Galactose can enter glycolysis here through its conversion into G6P, via galactose-1-phosphate and glucose-1-phosphate.
In reaction two, G6P is converted into fructose-6-phosphate by glucose isomerase.
This provides an entry point for fructose into glycolysis.
Fructose-6-phosphate is phosphorylated by phosphofructokinase into fructose-1,6-bisphosphate. This creates an unstable molecule that will split spontaneously to form two 3-carbon molecules and consumes the second molecule of ATP.
This is a key regulatory step of glycolysis. It is allosterically inhibited by ATP and activated by AMP. Furthermore, phosphofructokinase is inhibited by glucagon, whilst insulin activates the enzyme. This ensures that when there is high blood glucose, and therefore high circulating insulin, the speed of glycolysis increases.
This is also the step of commitment to glycolysis. Once fructose-1,6-bisphosphate has been formed glycolysis has to occur, as the molecule cannot enter other metabolic pathways.
By reaction 4, the energy consumption of the investment phase is complete and two ATP molecules have been consumed.
Here, fructose-1,6-bisphosphate is converted into two triose sugars by fructose-bisphosphate aldolase. These triose sugars are glyceraldehyde-3-phosphate (GA3P) and dihydroxyacetone phosphate (DHAP).
Here, DHAP is converted into a second molecule of GA3P.
Both molecules of GA3P then enter the second stage of glycolysis, the payout phase.
Energy Payout Phase
In the payout phase, a molecule of NADH and two molecules of ATP are produced for each molecule of GA3P entering the pathway. As our first molecule of glucose has generated two molecules of GA3P, the total payout from the payout phase is 2 NADH + 4 ATP.
As we used 2 ATP in the investment phase, the net gain from our first molecule of glucose is 2 NADH and 2 ATP.
In reaction 6, GA3P is converted into 1,3-bisphosphoglycerate (1,3-BPG) by glyceraldehyde phosphate dehydrogenase.
This yields a molecule of NADH, formed by the reduction of NAD+.
Here, 1,3-BPG is converted into 3-phosphoglycerate (3PG) by phosphoglycerate kinase.
This generates a molecule of ATP.
3PG is converted into 2PG by phosphoglycerate mutase.
2PG is converted into phosphoenolpyruvate by enolase.
Phosphoenolpyruvate is converted into pyruvate by pyruvate kinase, which yields our second molecule of ATP. This is irreversible and is therefore another key regulatory step.
All the steps of glycolysis are laid out above. You are very unlikely to need to memorise all of these, but it is important to note the following:
- The net effect is that 2 ATP and 2 NADH are produced.
- Reactions 1, 3, and 10 are unidirectional and are therefore key regulatory steps.
- Molecules are able to enter glycolysis mid-way through the pathway via the intermediates produced.
Fates of Pyruvate
Pyruvate is a versatile molecule that feeds into numerous pathways. After glycolysis, it can be converted to acetyl-CoA, which has numerous metabolic destinations, including the TCA cycle. It can also be converted into lactate, which enters the Cori cycle in absence of mitochondria or oxygen.
Other Important Pathway Interactions
DHAP, an intermediate of glycolysis, can be converted to glycerol phosphate in the liver and adipose tissue. This can feed into biosynthetic pathways, such as triglyceride and phospholipid biosynthesis, which also recycles NADH.
1,3-BPG can also be converted to 2,3-BPG in red blood cells to alter the affinity of haemoglobin for O2.
Excessive anaerobic glycolysis produces large quantities of lactic acid. This can exit the cell and enter the bloodstream, and in sufficient amounts can cause lactic acidosis. At this point, serum pH is reduced which can lead to organ dysfunction if severe and untreated.
Cancer and Glycolysis
Tumour cells have a very high rate of glycolysis. This is advantageous to the tumour if it outgrows its blood supply, as it can quickly produce energy from anaerobic glycolysis, allowing fast tumour growth.
It is also clinically advantageous in the detection and treatment of cancer. PET scans depict radioactive glycolytic intermediates in cancer cells, allowing visual detection of metastases. Drugs targeted at glycolysis are also used in the treatment of cancer in chemotherapy. An example is imatinib (Gleevec) which reduces the synthesis of hexokinase so that less glucose is trapped in the cell.