Steps Of Glycolysis

Phases of Glycolysis

Here we are going to discuss the step involved in glycolysis by taking the reference of EMP pathway. There are mainly ten steps of reactions in glycolysis. General glycolytic process can be represented as following skeletal flow diagram:

_{^{Source:wiki book/micro note/google}}

These ten steps of glycolytic process are mainly divided into two phases:

1. Preparatory phase or energy investment phase:

This phase includes first five steps in which the 6 carbon molecule, glucose is broken down to yield two molecules of 3 carbon molecule, Glyceraldehyde-3 phosphates. This phase is called energy investment phase as there is the consumption of energy in the form of ATP.

Steps of Reactions In preparatory phase:

• Phosphorylation of glucose:

This is where glycolysis initiates. The glucose molecule is at first phosphorylated by an ATP molecule donating a terminal phosphate group to glucose molecule to energize the molecule. The phosphate group attaches at the sixth carbon of glucose thus forming a glucose-6 phosphate. The ATP after phosphorylating glucose gets reduced to ADP. Enzyme Hexokinase catalyzes the reaction. The reaction in this step of glycolysis is as given below:

_{^{source:wikibook/micronote/google}}

• Isomerism of glucose-6 phosphate:

Glucose-6 phosphate then needs to get converted to its keto isomer i.e. Fructose-6 phosphate. Fructose derivatives readily undergo further reactions as it is more flexible than Glucose derivatives for chain reactions of this pathway. This reaction is catalyzed by phosphogluco isomerase with the ionic support of Mg­++­ ions. As there is the only change in isomeric form , this reaction is reversible in nature. The reaction in this step of glycolysis is as given below:

_{^{Source:wiki book/micro note/google}}

• Phosphorylation of Fructose-6 phosphate:

Fructose-6 phosphate is again phosphorylated to give fructose-1 , 6 bi - phosphate, which is more energetic molecule and is ready for splitting into half sized molecules. Again as usual ATP helps in phosphorylation in which ATP loses one terminal phosphate group which is attached at the first carbon of Fructose-6 phosphate thus giving Fructose-1 , 6 biphosphate, an unstable molecule. The reaction in this step of glycolysis is as given below:

_{^{Source:wiki book/micro note/google}}

• Cleavage of Fructose-1 , 6 biphosphate:

Fructose- 1 , 6 biphosphate is highly unstable molecule and thus it gets broken down into two equal carbon numbered molecules, i.e. glyceraldehyde- 3 phosphate and Di- hydroxyacetone phosphate. The reaction spontaneously occurs in the catalytic action of enzyme Aldolase. There is neither involvement of energy nor the yield of it. The reaction is irreversible in nature. The reaction in this step of glycolysis is as given below:

_{^{Source:wiki book/micro note/google}}

• Inter conversion of triose phosphates:

Among the two triose phosphates, only one of them can be degraded directly in the upcoming or subsequent steps of glycolysis i.e. only Glyceraldehyde- 3 phosphate can degenerate in further glycolytic steps. Therefore dihydroxy acetonephosphate should be converted into Glyceraldehyde- 3 phosphate. Enzyme Triose phosphate isomerase catalyzes the reaction. The reaction is reversible in nature. This is an eventual or an end step of preparatory phase. The reaction in this step of glycolysis is as given below:

_{^{Source:wiki book/micro note/google}}

2. Oxidative phase or energy-yielding phase:

This phase covers the rest of the steps i.e. from glyceraldehyde-3 phosphate to pyruvate. In this phase energy is yielded in the form of ATP and the reducing potential, NADH + H⁺. Four molecules of ATP is gained by the occupied phosphorylation of four molecules of ADP to ATP. Two molecules of reducing potentials i.e. NADH + H⁺ are also produced in this phase which after involvement in electron transport chain yields three molecules of ATP each (i.e. six molecules of ATP from two molecules of NADH).

• Oxidation of glyceraldehyde-3 phosphate:

This is the initiation step of oxidative phase or energy harvesting phase of glycolysis. Here Glyceraldehyde-3 phosphate is oxidized with the gain of an inorganic phosphate group to get 1, 3 - biphospho glycerate. This reaction is catalyzed by the enzyme Glyceraldehyde-3 phosphate dehydrogenase complex. There is the involvement of oxidizing potential i.e. NAD­­⁺ which gets reduced to NADH with a free proton (H⁺). NADH molecule is highly energetic and yields three ATPs after it's involvement in oxidative phosphorylation. The reaction is reversible in nature. The reaction in this step of glycolysis is as given below:

_{^{Source:wiki book/micro note/google}}

(Note: there is always the involvement of substrate dehydrogenase enzyme in the involvement of oxidizing potentials like NAD­­­­­­­­­⁺, FAD⁺, and NADP⁺. For example in above reaction, there is the involvement of NAD⁺ and the substrate is glyceraldehyde-3 phosphate^, so the enzyme involved is glyceraldehyde-3 phosphate dehydrogenase.)

• Phosphoryl transfer from 1, 3-biphospho glycerate to ADP :

This is where the first actual ATP from substrate level phosphorylation is obtained. The phosphate group from carbon number one of 1, 3-biphospho glycerate is transferred to an ADP molecule thus yielding an ATP molecule as energy conservator. This reaction is catalyzed by the enzyme, Phospho glycerate kinase that requires Mg⁺⁺ to catalyze. The product thus obtained is 3-phospho glycerate. The reaction in this step of glycolysis is as given below:

_{^{Source:wiki book/micro note/google}}

(Note: In case of involvement of ADP or ATP, enzyme substrate kinase catalyzes the reaction. For example: in above reaction, ADP is involved with the substrate phospho glycerate, so the acting enzyme is phospho glycerate kinase)

• Isomeric conversion of 3-phospho glycerate to 2-phospho glycerate:

For the instantaneous stability, 3-phospho glycerate is converted to 2-phospho glycerate by the positional isomerism of phosphate group from position 3 to position 2 of glycerate molecule. The reaction is catalyzed by the enzyme, phospho glycertae mutase. This is also a reversible reaction as there is only the occurrence of positional isomerism. The reaction in this step of glycolysis is as given below:

_{^{Source:wiki book/micro note/google}}

• Dehydrogenation of 2-phospho glycerate to phosphoenol pyruvate:

Again there is the occurrence of another reversible reaction which follows the dehydrogenation of 2-phospho glycerate. Dehydrogenation means the loss of water molecule from the substrate. Here one molecule of H₂O is lost from 2-phospho glycerate to form phospho enol pyruvate. This reaction is catalyzed by the enzyme, Enolase. The reaction can be reversed if water is added to phospho enol pyruvate in the presence of the same enzyme. Reaction involved can be shown as below:

_{^{Source:wiki book/micro note/google}}

• Formation of pyruvate:

This is the eventual (last) step of glycolysis. Pyruvate is formed by the phosphoryl transfer of phosphate group from phospho enol pyruvate to ADP again giving an ATP. Enzyme pyruvate kinase catalyzes the reaction in the presence of Mg⁺⁺ ion. Pyruvate thus formed is a very versatile molecule that can follow any path of upcoming metabolic pathways i.e. aerobic or anaerobic pathways. This way glycolysis is completed.

_{^{Source:wiki book/micro note/google}}

Calculation of net ATP production in Glycolysis:

ATP consumed:

ATP yielded:

Now,

Net ATP yield = Total ATP yield - Total ATPs consumed

= (10 - 2) ATPs = 8 ATPs