# 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.
* 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.
* Glu 6-P, under the activity of the enzyme phosphoglucose isomerase, becomes another isomer, Fructose 6-Phosphate (Fru 6-P).
* 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.
* Fruc 1,6-P (after ring opening), after binding to the enzyme aldolase, produces two new molecules known as:
** Glyceraldehyde 3-phosphate (GAP)
** Dihydroxyacetone phosphate (DHAP)
** Note that GAP can isomerize to DHAP and vice versa in the reaction catalyzed by the enzyme Triose Phosphate Isomerase.
* 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.
* Next, 1,3-bisphosphate glycerate undergoes group-transfer reaction, catalyzed by phosphoglycerate kinase, with ADP to produce 3-phosphoglycerate and ATP.
* Then, 3-phosphoglycerate, catalyzed by phosphoglycerate mutase, becomes 2-phosphoglycerate.
* Catalyzed by the enzyme enolase, 2-phosphoglycerate produces phosphoenolpyruvate (PEP) and water.
* 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 O$_2$, will undergo either aerobic reaction (with O$_2$) to continue the metabolic pathway into the nitric acid cycle (also known as Kreb's cycle), or anaerobic reaction (without O$_2$) to start a new process known as fermentation to produce lactic acid (mostly in human's muscular cells) or ethanol and carbon dioxide (CO$_2$) in microbes such as yeasts.

## Post-glycolysis processes

After glycolysis happens, 2 pyruvate molecules are used in the following processes:
* Anaerobic Respiration (Fermentation)
* Aerobic Respiration
** Pyruvate Dehydrogenase Complex (PDC)
** Krebs Cycle (or Nitric Acid Cycle)
** 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