Structural Biochemistry/Glycolysis and Gluconeogenesis/Glycolysis and Gluconeogenesis

Definition

Glycolysis and Gluconeogenesis: Glycolysis/Gluconeogenesis is the set of reactions that converts glucose into pyruvate. Glycolysis is a compilation of ten reactions (with 1 being an isomerization of 2 products into one another) of glycolysis take place in the cytoplasm.

1. One glucose molecule will go under group-transfer reaction with 1 ATP (Adenosine Triphosphate) - which is catalyzed by hexokinase - to become Glucose 6-phosphate (Glu 6-P) and produce 1 ADP (adenosine diphosphate) molecule.
2. Glu 6-P, under the activity of the enzyme phosphoglucose isomerase, becomes another isomer, Fructose 6-Phosphate (Fru 6-P).
3. Then, the group-transfer reaction between Fru 6-P and ATP, catalyzed by the enzyme Phosphofructokinase-1 (known as PFK-1), produces 1 fructose 1,6-biphosphate (Fru 1,6-P) molecule and another ADP molecule.
4. Fruc 1,6-P (after ring opening), after binding to the enzyme aldolase, produces two new molecules known as:
   1. Glyceraldehyde 3-phosphate (GAP)
   2. Dihydroxyacetone phosphate (DHAP)
   3. Note that GAP can isomerize to DHAP and vice versa in the reaction catalyzed by the enzyme Triose Phosphate Isomerase.
5. GAP, which is constantly supplemented by the isomerization of DHAP, continues the process of glycolysis by reacting with 2 inorganic phosphorus molecules and 2 nicotinamide adenine dinucleotide (NAD⁺, oxidized form) molecules, under oxidation-reduction reactions catalyzed by the enzyme Glyceraldehyde Phosphate Dehydrogenase, to produce 2 1,3-bisphosphate glycerate molecules, 2 protons, and 2 NADH molecules (reduced form of NAD⁺). Note that from this stage on, reactions are carried out by 2 molecules of the previous products to produce 2 new molecules. Therefore, it is assumed in the next stages, "1,3-biphosphate glycerate reacts with some compound A" means 2 1,3-bisphosphate glycerate molecules react with 2 molecules of compound A to produce 2 new molecules.
6. Next, 1,3-bisphosphate glycerate undergoes group-transfer reaction, catalyzed by phosphoglycerate kinase, with ADP to produce 3-phosphoglycerate and ATP.
7. Then, 3-phosphoglycerate, catalyzed by phosphoglycerate mutase, becomes 2-phosphoglycerate.
8. Catalyzed by the enzyme enolase, 2-phosphoglycerate produces phosphoenolpyruvate (PEP) and water.
9. In the last step, PEP reacts with ADP in a group-transfer reaction catalyzed by phosphopyruvate kinase, to produce pyruvate and ATP.

In total, glycolysis consumes 1 glucose molecule, 2 ATP molecules, and 2 NAD⁺ to produce 2 pyruvate molecules, 4 ATP molecules, and 2 NADH molecules.

Therefore, in overall, 1 Glu + 2 NAD⁺ → 2 Pyruvate + 2 NADH + 2 ATP

After glycolysis, the products, depending on the appearance of O₂, will undergo either aerobic reaction (with O₂) to continue the metabolic pathway into the nitric acid cycle (also known as Kreb's cycle), or anaerobic reaction (without O₂) to start a new process known as fermentation to produce lactic acid (mostly in human's muscular cells) or ethanol and carbon dioxide (CO₂) in microbes such as yeasts.

Post-glycolysis processes

After glycolysis happens, 2 pyruvate molecules are used in the following processes:

1. Anaerobic Respiration (Fermentation)
2. Aerobic Respiration
   1. Pyruvate Dehydrogenase Complex (PDC)
   2. Krebs Cycle (or Nitric Acid Cycle)
   3. Oxidative Phosphorylation Cycle (or known as Electron Transport Chain cycle which involves ATP synthesis)

References

Biochemistry 6th edition. Berg, Jeremy M; Tymoczko, John L; Stryer, Lubert. W.H. Freeman Company, New York