Glycolysis – Cellular Respiration (Part II)

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The breakdown of glucose to pyruvic acid in a multi step manner is called as Glycolysis. It takes place in cytoplasm of the cell.
It is common pathway to both Anaerobic and Aerobic respiration.
The Glycolysis pathway is a very ancient pathway present in majority of organisms from every branch of the bacteria, archaea, and eukaryotes. This clearly indicates the existence of glycolysis pathway in the ancestor of all current life forms.
This also suggests us that glycolysis pathway evolved in an anaerobic world. However, the pathway works equally efficient in aerobic respiration of glucose.
The difference between glycolysis in anaerobic and aerobic conditions is the final product of the pathway. It is pyruvic acid in aerobic settings and lactate in anaerobic conditions.
The term glycolysis has originated from Greek words glycos for sugar and lysis for splitting.
Glycolysis was deciphered by Gustav Embden, Otto Meyerhof and J. Parnas and hence it is often referred as EMP pathway.
Glycolysis takes place in 10 enzyme catalyzed steps in a linear manner, five of which are in the preparatory phase (ATP or energy is used ) and five are in the pay-off phase (ATP is produced more than what is consumed in first phase)
It can takes place in absence of oxygen.

Steps involved in Glycolysis

In Plants, the end product of photosynthesis i.e., Glucose or sucrose and in non-photosynthetic organisms – the food they consume, forms the starting point for glycolysis pathway.

Glycolysis can be separated into two phases of five reaction each : Preparatory phase ( energy requiring phase) and Pay-off phase ( energy is generated).

Preparatory Phase :

Step 1 : Glucose is converted glucose into glucose-6-phosphate using one ATP, which is converted to ADP. This first reaction of glycolysis is catalyzed by Hexokinase.

Addition of a phosphate to Glucose molecule makes it more reactive and traps it inside the cell, as glucose with a phosphate can’t readily cross the membrane.

Step 2: Phosphoglucose isomerase converts glucose-6-phosphate into fructose-6-phosphate.

Step 3. Again a phosphate group is added from ATP to form ADP. This time the phosphate is added to fructose-6-phosphate to form fructose-1,6-bisphosphate. This step is catalyzed by the enzyme phosphofructokinase, which enzyme is under strict regulation of cell.

This allows the cell to either increase or slow down the glycolysis pathway depending on the availability of glucose or any other parameters affecting glycolysis pathway in the cell.

Step 4: fructose-1,6-bisphosphate formed is split into two Dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate, both three carbon sugar molecules.

DHAP and glyceraldehyde-3-phosphate are isomers of each other. Out of these two, only glyceraldehyde-3-phosphate can enter second phase of glycolysis.

Step 5 : Eventually all DHAP is converted to glyceraldehyde-3-phosphate to enter glycolysis.

In the first phase of glycolysis, 2 ATP’s are used up to form, two 3 carbon sugar molecules from one glucose molecule.

***Steps involved in preparatory phase of Glycolysis***

Pay-off Phase :

In this phase of glycolysis, glyceraldehyde-3-phosphate will be subjected to further conversion to finally yield 4 ATP’s ( 2 from each molecule of glyceraldehyde-3-phosphate and since two 3-carbon molecules are produced from one glucose molecule in the preparatory phase – the pay off happens twice for each glucose molecule, so totally 4 ATP’s are produced ) and two higher-energy NADH molecules.

Hence this phase is termed as pay-off phase as it is compensating for the two lost ATP’s from earlier phase and in addition, it also generates 2 more ATP’s, so the net yield of glycolysis is 2 ATP’s and 2 NADH.

Step 6: glyceraldehyde-3-phosphate is oxidized by releasing electrons, which are picked up by the electron carrier NAD⁺, producing NADH and H⁺. The glyceraldehyde-3-phosphate is phosphorylated by the addition of a second phosphate group ( here ATP is not required) to form 1,3-bisphosphoglycerate. The enzyme used in this reaction is Glyceraldehyde 3-phosphate dehydrogenase.

Step 7: 1,3-bisphosphoglycerate is converted to 3- Phosphoglycerate in presence of phosphoglycerate kinase enzyme. The phosphate group from 1,3-bisposphoglycerate is donated to ADP to form ATP.

Step 8: 3-phosphoglycerate is converted into its isomer, 2-phosphoglycerate by phosphoglycerate mutase enzyme.

Step 9: 2-Phosphoglycerate molecules loses a water molecule n a dehydration step to form phosphoenolpyruvate (PEP). in presence of enolase.

Step 10: PEP is finally converted to end product of glycolysis to form Pyruvate or Pyruvic acid. The enzyme which catalyzes this reaction is pyruvate kinase. In this step also an ATP is formed by receiving phosphate from PEP.

The metabolic fate of Pyruvic acid depend upon the presence or absence of oxygen.

In anaerobic conditions, cells convert Pyruvic acid into lactic acid or ethanol by means of Fermentation as observed in many prokaryotes and unicellular eukaryotes.

In presence of oxygen, pyruvic acid acid is further oxidized through kreb’s cycle, oxidative phosphorylation and this called aerobic respiration which will form many ATP’s from single molecule of glucose.

Fate of NADH formed during pay-off phase of Glycolysis

NADH (reduced form) formed during pay-off phase can be reconverted to NAD⁺ (oxidized form), so that it can be ready to accept electrons again for glycolysis to continue. In absence of NAD+, Glycolysis will come to a halt.

Again presence or absence of oxygen will decide the fate of NADH formed during pay-off pathway.

In absence of oxygen, NADH donates its electrons to an acceptor molecule to regenerate NAD⁺. In this reaction, no ATP is formed, but ensures enough NAD⁺ are available for glycolysis to continue. This process is called fermentation.

When oxygen is present, NADH can pass its electrons into the electron transport chain to regenerate NAD⁺ for use in glycolysis and in addition it will generate excess amount of ATP during oxidative phosphorylation step of aerobic respiration.

Link to Introduction to Cellular Respiration Part I