Biochemistry/Glycolysis

Introduction

Glycolysis is the process which glucose is divided into two molecules of pyruvate. In fact, glycolysis considered a linear pathway of ten enzyme-mediated steps. The pathway for glycolysis has two phases: the energy investment phase and energy generation phase. The first five steps in the glycolysis are the energy investment "preparatory phase", which produce glyceraldehyde 3-phosphate. Energy generation phase is the last five steps of glycolysis, which produce the final two pyruvate molecules product. Glycolysis occurs in cytoplasm and can be achieved in the absence of oxygen. In the body, the source of glucose for glycolysis comes from dietary disaccharides and monosaccharides.

Glycolysis is the first step in carbohydrate metabolism, the end result of which is the conversion of glucose to carbon dioxide and water. When total oxidation of glucose to carbon dioxide ended, a large amount of energy is converted into ATP.

During glycolysis, two molecules of NADH and a net two molecules of ATP are generated (two molecules of ATP are used to get the pathway started, but four molecules are then synthesized). Thus, there is a net production of two ATP molecules for each glucose molecule converted to two molecules of pyruvate.

At the end of the process, only a small part of all glucose energy is released and the rest of the potential energy stays in pyruvate molecules which are further oxidized to firstly to acetyl- CoA and later on carbon dioxide by TCA cycle. The NADH and FADH₂ gained from TCA cycle enter into ETC to produce ATP via oxidative phosphorylation. For this reason aerobic degradation is much more efficient than anaerobic metabolism. That is why the aerobic mechanism is now much more spread within living organisms, but nevertheless anaerobic pathways still take place even in animals under certain physiological circumstances.

Overview

Glycolysis is a central pathway for the catabolism of carbohydrates in which the six-carbon sugars are split to three-carbon compounds with subsequent release of energy used to transform ADP to ATP. Glycolysis can proceed under anaerobic (without oxygen) and aerobic conditions.

Glycolysis is a 10-step pathway which converts glucose to 2 pyruvate molecules. The overall Glycolysis can be written as a net equation:

Glucose + 2ADP + 2NAD⁺ → 2Pyruvate + 2ATP + 2NADH

Glycolysis enzymes:

Hexokinase (HK)
Phosphoglucoisomerase (PGI)
Phosphofructokinase (PFK)
Aldolase (ALDO)
Triose phosphate isomerase (TPI)
Glyceraldehyde-3-phosphate dehydrogenase (GAPDH)
Phosphoglycerate kinase (PGK)
Phosphoglycerate mutase (PGM)
Enolase (ENO)
Pyruvate kinase (PK)

Glycolysis compounds:

Glucose
Glucose 6-Phosphate (G6P)
Fructose 6-Phosphate (F6P)
Fructose 1,6-Bisphosphate (F1,6BP)
Glyceraldehyde 3-Phosphate (GADP)
Dihydroxyacetone Phosphate (DHAP)
1,3-Bisphosphoglyceric acid (1,3PG)
3-Phosphoglyceric acid (3PG)
2-Phosphoglyceric acid (2PG)
Phosphoenolpyruvate (PEP)
Pyruvate

Visual overview

Stage I: Energy Investment (step 1-5)

Stage II: Energy Harvesting (step 6-10)

Detailed description of the ten step

Step 1: Glucose phosphorylation at its 6^th carbon catalysed by glucokinase (hexokinase IV) or other hexokinases:

δG⁰ = -16.7 kJ/mol

Glucose that enters the cell has only one fate: it is converted to glucose-6-phosphate by a typical kinase reaction. When glucose is phosphorylated, it is trapped inside of the cell because phosphoryl at 6^th carbon pushes phospholipid membrane thus glucose-6-phosphate even could not close to plasma membrane.

ATP is used as phosphoryl group reservoir.

Mg²⁺ found as a cofactor of the enzyme. Presence of Mg²⁺ shields the charge on the phosphoryl of ATP. So compounds can come closer in an easier way.

At the reaction, a pyrophosphate bond is cleaved (-7.5 kcal/mole) and a phosphate ester bond is formed (+3 kcal/mole), resulting in a net -4.0 kcal/mole. Thus it can be concluded that product (glucose-6-phosphate) has less energy than the reactant (glucose). Less energy means that more stability. Therefore, product is more stable than the reactant. This condition makes the reaction irreversible.

Step 2: Isomerization of glucose-6-phosphate catalysed by means of phosphoglucoisomerase:

δG⁰ = +1.7 kJ/mol

This convertion is reversible. Via converting 6C cyclic compound to 5C cyclic compound which includes oxygen in the cyclic part as seen at 6C compound, too, it is aimed to produce a molecule that is able to be splited into two equal carbon contented (3C) compound.

Step 3: Second phosphorylation catalysed by phosphofructokinase:

δG⁰ = -18.5 kJ/mol

By the help of this step, division into two equal carbon contented (3C) compound also balanced in terms of phosphoryl group.

Again Mg²⁺ is found as a cofactor of the enzyme. It functions like seen at step 1.

This reaction is irreversible as it can be implied from its negative δG⁰

Step 4: Cleavage to two triose phosphates catalysed by aldolase:

δG⁰ = +28 kJ/mol

Now, two molecules of 3C compound is generated.

Step 5: Isomerization of dihydroxyacetone phosphate catalysed by triose phosphate isomerase:

δG⁰ = +7.6 kJ/mol

After this step each reaction occurs double time because 2 molecules of glyceraldehyde-3-phosphate exists for further steps.

Step 6: Generation of 1,3-bisphosphoglycerate catalysed by glyceraldehyde-3-phosphate dehydrogenase:

δG⁰ = +6.3 kJ/mol

Here, it is goaled to insert phosphoryl group which will be transferred to ADP to create ATP, later. Also production of NADH happens.

Step 7: Production of 3-phosphoglycerate via substrate-level phosphorylation catalysed by phosphoglycerate kinase:

δG⁰ = -18.8 kJ/mol

The first step which ATP is produced is step 7.

Step 8: Phosphate transfer from 3^rd carbon to 2^nd carbon of 3-Phosphoglycerate catalysed by phosphoglycerate mutase:

δG⁰ = +4.4 kJ/mol

Via the effect of this reaction, resonance at PEP (step 8 product) is achieved.

Step 9: Synthesis of phosphoenolpyruvate catalysed by enolase:

δG⁰ = +1.7 kJ/mol

Water exit occurs.

Step 10: Substrate-level phosphorylation. Pyruvate synthesis catalysed by pyruvate kinase:

δG⁰ = -31.4 kJ/mol

ATP is produced. Pyruvate tautomerizes rapidly and nonenzymatically to its keto form from its enol form. It is the last and one of the irreversible steps of glycolysis.

References

Nelson, D. L., & Cox, M. M. (2008). Glycolysis, gluconeogenesis, and the pentose phosphate pathway. Lehninger Principles of Biochemistry, 4, 521-559.
Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Chapter 16, Glycolysis and Gluconeogenesis.