Glycolysis Steps

Glycolysis is the systematic breakdown of glucose and other sugars to power the process of cellular respiration. It is a universal biochemical reaction that occurs in every living unicellular or multicellular organism which respires aerobically and anaerobically. There are many metabolic pathways through which this process occurs.

The glycolysis steps presented here, refer to a particular process called the Embden-Meyerhof-Parnus pathway. This process is a small part of the cellular respiration cycle and overall body metabolism, directed towards creating ATP (Adenosine Triphosphate), which is the energy currency of the body.

Glycolysis literally means glucose breakdown or decomposition. Through this process, one glucose molecule is completely broken down to yield two molecules of pyruvic acid, two molecules of ATP, and two NADH (Reduced Nicotinamide Adenine Dinucleotide) radicals, carrying electrons.

It took years of painstaking research in biochemistry, to reveal the glycolysis steps that make cellular respiration possible. 
Here are the various steps, presented in the order of occurrence, beginning with glucose as the prime raw material.

The process involves 10 steps, with a product forming at every stage and every one of them, regulated by a different enzyme. The production of various compounds at every step, offers different entry points into the process. This process may directly start from an intermediate stage, if compound that is the reactant at that stage, is directly made available.

The first step is the phosphorylation of glucose (adding of a phosphate group). This reaction is made possible by the enzyme hexokinase, which separates one phosphate group out of ATP (Adenosine Triphsophate) and adds it to glucose, transforming it to glucose 6-phosphate.

In the process, one ATP molecule, which is the energy currency of the body, is used up and gets transformed to ADP (Adenosine Diphosphate), due to the separation of one phosphate group. The entire reaction can be summarized as follows:

Glucose (C₆H₁₂O₆) + ATP + Hexokinase → Glucose 6-Phosphate (C₆H₁₁O₆P₁) + ADP

The second step is the production of fructose 6-phosphate. It is made possible by the action of the enzyme phosphoglucoisomerase. It acts on the product of the earlier step, glucose 6-phosphate, and transforms it into fructose 6-phosphate, which is its isomer (Isomers are different molecules with same molecular formula but different arrangement of atoms).

The entire reaction is summarized as follows:

Glucose 6-Phosphate (C₆H₁₁O₆P₁) + Phosphoglucoisomerase (Enzyme) → Fructose 6-Phosphate (C₆H₁₁O₆P₁)

In the next step, the isomer, Fructose 6-Phosphate, is converted to Fructose 1, 6-Diphosphate by the addition of another phosphate group. This conversion is made possible by the enzyme phosphofructokinase, which utilizes one more ATP molecule in the process.

The reaction is summarized as follows: 

Fructose 6-Phosphate (C₆H₁₀O₆P₁) + phosphofructokinase (Enzyme) + ATP → Fructose 1, 6-diphosphate (C₆H₁₀O₆P₂)

In the fourth step, the enzyme adolase brings about the splitting of Fructose 1, 6-diphosphate
into two different sugar molecules, that are both isomers of each other. The two sugars formed are glyceraldehyde phosphate and dihydroxyacetone phosphate.

The reaction goes as follows:

Fructose 1, 6-diphosphate (C₆H₁₀O₆P₂) + Aldolase (Enzyme) → Glyceraldehyde Phosphate (C₃H₅O₃P₁) + Dihydroxyacetone phosphate (C₃H₅O₃P₁)

Dihydroxyacetone phosphate is a short-lived molecule. As soon as it is created, it gets converted into Glyceraldehyde phosphate by the enzyme called triose phosphate. So in totality, the fourth and fifth steps of glycolysis yield two molecules of Glyceraldehyde phosphate.

Dihydroxyacetone phosphate (C₃H₅O₃P₁) + Triose Phosphate → Glyceraldehyde phosphate (C₃H₅O₃P₁)

The sixth step involves two important reactions. First is the formation of NADH from NAD+ (nicotinamide adenine dinucleotide) by the use of the enzyme triose phosphate dehydrogenase and second is the creation of 1,3-diphoshoglyceric acid from the two glyceraldehyde phosphate molecules produced in the earlier step.

The two reaction are as follows: 

Triose phosphate dehydrogenase (Enzyme) + 2 NAD+ + 2 H- → 2NADH (Reduced Nicotinamide Adenine Dinucleotide) + 2 H+

Triose phosphate dehydrogenase + 2 Glyceraldehyde phosphate (C₃H₅O₃P₁) + 2P (from cytoplasm) → 2 molecules of 1,3-diphoshoglyceric acid (C₃H₄O₄P₂)

The seventh step involves the creation of 2 ATP molecules, along with two molecules of 3-phosphoglyceric acid, from the reaction of phosphoglycerokinase on the two product molecules of 1,3-diphoshoglyceric acid, yielded from the previous step.

2 molecules of 1,3-diphoshoglyceric acid (C₃H₄O₄P₂) + 2ADP + phosphoglycerokinase → 2 molecules of 3-Phosphoglyceric acid (C₃H₅O₄P₁) + 2ATP (Adenosine Triphosphate)

Step eight is a very subtle rearrangement reaction, which involves the relocation of the Phosphorus atom in 3-phosphoglyceric acid, from the third carbon in the chain, to the second carbon and creates 2- phosphoglyceric acid.

The entire reaction is summarized as follows: 

2 molecules of 3-Phosphoglyceric acid (C₃H₅O₄P₁) + phosphoglyceromutase (enzyme) → 2 molecules of 2-Phosphoglyceric acid (C₃H₅O₄P₁)

The enzyme enolase comes into play and removes a water molecule from 2-phosphoglyceric acid to form another acid called phosphoenolpyruvic acid (PEP). This reaction converts both the molecules of 2-Phosphoglyceric acid that form in the previous step.

2 molecules of 2-Phosphoglyceric acid (C₃H₅O₄P₁) + Enolase (Enzyme) -> 2 molecules of phosphoenolpyruvic acid (PEP) (C₃H₃O₃P₁) + 2 H₂O

This step involves the creation of two ATP molecules, along with two molecules of pyruvic acid, from the action of the enzyme pyruvate kinase, on the two molecules of phosphoenolpyruvic acid, produced in the previous step. This is made possible by the transfer of a Phosphorus atom from phosphoenolpyruvic acid (PEP) to ADP (Adenosine triphosphate).

2 molecules of phosphoenolpyruvic acid (PEP) (C₃H₃O₃P₁) + 2ADP + Pyruvate kinase (Enzyme) → 2ATP + 2 molecules of pyruvic acid.

As you can see, all steps mostly involve the manipulation of the phosphate group and then the phosphorus atom, which is made possible by the various enzymes in the cytoplasm. Enzymes are like catalysts which make a reaction possible and then disengage.

Let me summarize all the steps in the end, in a concise form. The whole process involves the breakdown of one glucose molecule and it yields 2 molecules of NADH, 2 molecules of ATP, 2 molecules of water, and 2 molecules of pyruvic acid. The products of glycolysis are further used in the citric acid or Krebs cycle, which is a part of cellular respiration.

Glucose (C₆H₁₂O₆) + 2 [NAD]+ + 2[ADP (Adenosine Diphosphate)] + 2 [P]i ---> 2 [C₃H₃O₃]-(Pyruvate) + 2 [NADH] (Reduced Nicotinamide Adenine Dinucleotide) + 2H+ + 2 [ATP] (Adenosine Triphosphate) + 2 H₂O

Each of these steps are subtle energy changes made possible by the various enzymes present in the cytoplasm, which work in coordination. The precision with which each of these reactions go ahead in a synchronized fashion is simply amazing. As you go deeper into biochemistry, you can increasingly appreciate the miracle that life is.