Phases of Glycolysis

Glycolysis - a 10 step biochemical pathway where a glucose molecule (6 C) is split into 2 molecules of pyruvate (3 C). To begin the process 2 ATP must be invested. The energy released from the reactions is captured in the form of 4 molecules of ATP molecules and high energy electrons are trapped in the reduction of 2 molecules NAD to NADH.

Preparatory Phase

Preparatory phase is the stage in which there is consumption of ATP and is also known as the investment phase. The pay-off phase is where ATP is produced. The first five steps of the glycolysis reaction are known as the preparatory or investment phase. This stage consumes energy to convert the glucose molecule into two molecules three-carbon sugar molecule.

Step 1

The step one in glycolysis is phosphorylation. This step glucose is phosphorylated by the enzyme hexokinases. In this process, ATP molecule is consumed. A phosphate group from the ATP is transferred to the glucose molecules to produce glucose-6-phosphate.

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

Step 2

The second stage of glycolysis is an isomerization reaction. In this reaction, the glucose-6-phosphate is rearranged into fructose-6-phosphate by the enzyme glucose phosphate isomerase. This is a reversible reaction under normal conditions of the cell.

Glucose-6-phosphate (C6H11O6P1) + Phosphoglucoisomerase → Fructose-6-phosphate (C6H11O6P1)

Step 3

In the third step of glycolysis is a phosphorylation reaction. In this step, the enzyme phosphofructokinase is transferred to a phosphate group to form fructose 1,6-bisphosphate. Another ATP molecule is used in this step.

Fructose 6-phosphate (C6H11O6P1) + phosphofructokinase + ATP → Fructose 1,6-bisphosphate (C6H10O6P2) + ADP

Step 4

This step in glycolysis is a destabilization step, where the action of the enzyme aldolase splits fructose 1,6-bisphosphate into two sugars. These sugars are isomers of each other, they are Dihydroxyacetone phosphate and glyceraldehyde phosphate.

Fructose 1,6-bisphosphate (C6H10O6P2) + aldolase → Dihydroxyacetone phosphate (C3H5O3P1) + Glyceraldehyde phosphate (C3H5O3P1)

Step 5

Step 5 of glycolysis is an interconversion reaction. Here, the enzyme triose phosphate isomerase interconverts the molecules Dihydroxyacetone phosphate and glyceraldehyde phosphate.

Dihydroxyacetone phosphate (C3H5O3P1) → Glyceraldehyde phosphate (C3H5O3P1)

Pay-off Phase

The second phase of glycolysis is known as the pay-off phase of glycolysis. This phase is characterized by a gain of the energy-rich molecules ATP and NADH.

Step 6

This step of glycolysis is a dehydrogenation step. The enzyme triose phosphate dehydrogenase dehydrogenates glyceraldehyde 3-phosphate and adds an inorganic phosphate to form 1,3-bisphosphoglycerate. Firstly, the enzyme action transfers an H- (hydrogen) from glyceraldehyde phosphate to the NAD+ which is an oxidizing agent to form NADH. The enzyme also adds an inorganic phosphate from the cytosol to the glyceraldehyde phosphate to form 1,3-bisphosphoglycerate. This reaction occurs with both the molecules produced in the previous step.

2 Glyceraldehyde phosphate (C3H5O3P1) + Triose phosphate dehydrogenase + 2H- + 2P + 2NAD+ → two 1,3-bisphosphoglycerate (C3H4O4P2) + 2NADH + 2H+

Step 7

Step 7 of glycolysis is a substrate-level phosphorylation step, where the enzyme phosphoglycerokinase transfers a phosphate group from 1,3-bisphosphoglycerate. The phosphate is transferred to ADP to form ATP. This process yields two molecules of 3-phosphoglycerate molecules and two molecules of ATP. There are two molecules of ATP synthesized in this step of glycolysis.

2 molecules of 1,3 bisphosphoglycerate (C3H4O4P2)+ phosphoglycerokinase + 2 ADP → 2 molecules of 3-phosphoglycerate (C3H5O4P1) + 2 ATP

Step 8

This step of glycolysis is a mutase step, occurs in the presence of the enzyme phosphoglycerate mutase. This enzyme relocates the phosphate from the 3-phosphoglycerate molecule are third carbon position to the second carbon position, this results in the formation of 2-phosphoglycerates.

2 molecules of 3-phosphoglycerate (C3H5O4P1) + phosphoglyceromutase → 2 molecules of 2-Phosphoglycerate (C3H5O4P1)

Step 9

This step of glycolysis is a lyase reaction, which occurs in the presence of enolase enzyme. In this reaction, the enzyme removes a molecule of water from 2-phosphoglycerate to form a phosphoenolpyruvic acid (PEP)

2 molecules of 2-phosphoglycerate (C3H5O4P1) + enolase → 2 molecules of phosphoenolpyruvic acid (PEP) (C3H3O3P1) + H2O

Step 10

This is the final stage of glycolysis which is a substrate-level phosphorylation step. In the presence of the enzyme pyruvate kinase, there is a transfer of an inorganic phosphate molecule from phosphoenol pyruvate molecule to ADP to form pyruvic acid and ATP. This reaction yields 2 molecules of pyruvic acid and two molecules of ATP.

2 molecules of PEP (C3H3O3P1) + pyruvate kinase + 2 ADP → 2 molecules of pyruvic acid (C3H4O3) + 2 ATP This reaction marks the end of glycolysis, hereby producing two ATP molecules per glucose molecule

Link reaction

This links glycolysis to the Krebs cycle.Pyruvate molecules are decarboxylated (they lose a molecule of carbon dioxide) in the mitochondria. Pyruvate molecules are oxidized and converted to acetyl coenzyme A, usually abbreviated to acetyl CoA.

2CH3COCOO- + 2NAD+ + 2H2O 2CH3COO- + 2NADH + 2H+ + 2CO2

The oxidized form of nicotinamide adenine dinucleotide, NAD+, is reduced to its reduced form NADH (Link Reaction)Pyruvate oxidation - In a single step, a carbon is removed from pyruvate (3 C) as CO2, leaving 2 of the original carbons attached to Coenzyme A The complex is called Acetyl Co A. Attached to Coenzyme-A. The complex is called Acetyl Co-A. In this process, one NADH molecule is produced.

Krebs cycle

Krebs cycle - A 8 step biochemical pathway that converts all of the remaining carbons from the original glucose into CO2, and yields 1 ATP, and traps high energy electrons in 3 NADH, and 1 FADH per Acetyl Co-A.

Acetyl CoA + 3 NAD + FAD + ADP + HPO4-2 —————> 2 CO2 + CoA + 3 NADH+ + FADH+ + ATP

Reaction 1: Formation of Citrate

The first reaction of the cycle is the condensation of acetyl-CoA with oxaloacetate to form citrate, catalyzed by citrate synthase.

Once oxaloacetate is joined with acetyl-CoA, a water molecule attacks the acetyl leading to the release of coenzyme A from the complex.

Reaction 2: Formation of Isocitrate

The citrate is rearranged to form an isomeric form, isocitrate by an enzyme aconitase.

In this reaction, a water molecule is removed from the citric acid and then put back on in another location. The overall effect of this conversion is that the –OH group is moved from the 3' to the 4' position on the molecule. This transformation yields the molecule isocitrate.

Reaction 3: Oxidation of Isocitrate to a-Ketoglutarate

In this step, isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to form a-Ketoglutarate.

In the reaction, generation of NADH from NAD is seen. The enzyme isocitrate dehydrogenase catalyzes the oxidation of the –OH group at the 4' position of isocitrate to yield an intermediate which then has a carbon dioxide molecule removed from it to yield alpha-ketoglutarate.

Reaction 4: Oxidation of a-Ketoglutarate to Succinyl-CoA

Alpha-ketoglutarate is oxidized, carbon dioxide is removed, and coenzyme A is added to form the 4-carbon compound succinyl-CoA.

During this oxidation, NAD+ is reduced to NADH + H+. The enzyme that catalyzes this reaction is alpha-ketoglutarate dehydrogenase.

Reaction 5: Conversion of Succinyl-CoA to Succinate

CoA is removed from succinyl-CoA to produce succinate.

The energy released is used to make guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and Pi by substrate-level phosphorylation. GTP can then be used to make ATP. The enzyme succinyl-CoA synthase catalyzes this reaction of the citric acid cycle.

Reaction 6: Oxidation of Succinate to Fumarate

Succinate is oxidized to fumarate.

During this oxidation, the FAD is reduced to FADH2. The enzyme succinate dehydrogenase catalyzes the removal of two hydrogens from succinate.

Reaction 7: Hydration of Fumarate to Malate

The reversible hydration of fumarate to L-malate is catalyzed by fumarase (fumarate hydrate). Fumarase continues the rearrangement process by adding Hydrogen and Oxygen back into the substrate that had been previously removed.

Reaction 8: Oxidation of Malate to Oxaloacetate

Malate is oxidized to produce oxaloacetate, the starting compound of the citric acid cycle by malate dehydrogenase. During this oxidation, NAD+ is reduced to NADH + H+.

Electron transport chain

Electron Transport Chain - the high energy electrons trapped in NADH and FADH in glycolysis, pyruvate oxidation, and the Krebs cycle are used to produce ATP through chemiosmosis. O2 is the final acceptor of high energy electrons. In eukaryotes, Glycolysis occurs in the cytoplasm, pyruvate oxidation, the Krebs cycle and the Electron Transport
System occur in the mitochondrion

This pathway is the most efficient method of producing energy. The initial substrates for this cycle are the end products obtained from other pathways. Pyruvate, obtained from glycolysis, is taken up by the mitochondria, where it is oxidized via the Krebs/citric acid cycle. The substrates required for the pathway are NADH (nicotinamide adenine dinucleotide), succinate, and molecular oxygen.

NADH acts as the first electron donor and gets oxidized to NAD+ by enzyme complex I, accompanied by the release of a proton out of the matrix. The electron is then transported to complex II, which brings about the conversion of succinate to fumarate. Molecular oxygen (O2) acts as an electron acceptor in complex IV and gets converted to a water molecule (H2O). Each enzyme complex carries out the transport of electrons accompanied by the release of protons in the intermembrane space.

The accumulation of protons outside the membrane gives rise to a proton gradient. This high concentration of protons initiates the process of chemiosmosis and activates the ATP synthase complex. Chemiosmosis refers to the generation of an electrical as well as a pH potential across a membrane due to the large difference in proton concentrations. The activated ATP synthase utilizes this potential and acts as a proton pump to restore concentration balance. While pumping the proton back into the matrix, it also conducts the phosphorylation of ADP (Adenosine Diphosphate) to yield ATP molecules.

Complex I - NADH-coenzyme Q oxidoreductase The reduced coenzyme NADH binds to this complex, and functions to reduce coenzyme Q10. This reaction donates electrons, which are then transferred through this complex using FMN (Flavin mononucleotide) and a series of Fe-S (Iron-sulfur) clusters. The transport of these electrons brings about the transfer of protons across the membrane into the intermembrane space.

Complex II - Succinate-Q oxidoreductase This complex acts on the succinate produced by the citric acid cycle and converts it to fumarate. This reaction is driven by the reduction and oxidation of FAD (Flavin adenine dinucleotide) along with the help of a series of Fe-S clusters. These reactions also drive the redox reactions of Quinone. These sets of reactions help in transporting the electrons to the third enzyme complex.

Complex III - Q-cytochrome c oxidoreductase This complex oxidizes ubiquinol and also reduces two molecules of cytochrome-c. The electron is transported via these reactions onto complex IV accompanied by the release of protons.

Complex IV - cytochrome c oxidase The received electron is received by a molecular oxygen to yield a water molecule. This conversion occurs in the presence of Copper (Cu) ions and drives the oxidation of the reduced cytochrome-c. Protons are pumped out during the course of this reaction.

ATP Synthase The protons produced from the initial oxidation of the NADH molecule, and their presence in the intermembrane space gives rise to a potential gradient. It is utilized by this complex to transport the protons back into the matrix. The transport itself also generates energy that is used to achieve phosphorylation of the ADP molecules to form ATP.