Editing Entner–Doudoroff pathway (section)

== Catalyzing enzymes ==

=== Conversion of glucose to glucose-6-phosphate   ===
The first step in ED is phosphorylation of glucose by a family of enzymes called [[hexokinase]]s to form [[glucose 6-phosphate]] (G6P). This reaction consumes ATP, but it acts to keep the glucose concentration low, promoting continuous transport of glucose into the cell through the plasma membrane transporters. In addition, it blocks the glucose from leaking out – the cell lacks transporters for G6P, and free diffusion out of the cell is prevented due to the charged nature of G6P. Glucose may alternatively be formed from the [[phosphorolysis]] or [[hydrolysis]] of intracellular starch or glycogen.

In [[animal]]s, an [[isozyme]] of hexokinase called [[glucokinase]] is also used in the liver, which has a much lower affinity for glucose (K<sub>m</sub> in the vicinity of normal glycemia), and differs in regulatory properties. The different substrate affinity and alternate regulation of this enzyme are a reflection of the role of the liver in maintaining blood sugar levels.

''Cofactors:'' Mg<sup>2+</sup>

=== Conversion of glucose-6-phosphate to 6-phosphogluconolactone ===
The G6P is then converted to 6-[[6-Phosphogluconolactone|phosphogluconolactone]] in the presence of enzyme [[glucose-6-phosphate dehydrogenase]] ([[Oxidoreductase|an oxido-reductase]]) with the presence of  [[co-enzyme]] [[Nicotinamide adenine dinucleotide|nicotinamide adenine dinucleotide phosphate]] (NADP<sup>+</sup>). which will be reduced to nicotinamide adenine dinucleotide phosphate hydrogen along with a free hydrogen atom H<sup>+</sup>.

=== Conversion of 6-phosphogluconolactone to 6-phosphogluconic acid ===
The 6PGL is converted into 6-phosphogluconic acid in the presence of enzyme [[hydrolase]].

=== Conversion of 6-phosphogluconic acid to 2-keto-3-deoxy-6-phosphogluconate ===
The 6-phosphogluconic acid is converted to 2-keto-3-deoxy-6-phosphogluconate (KDPG) in the presence of enzyme 6-phosphogluconate dehydratase; in the process, a water molecule is released to the surroundings.

=== Conversion of 2-keto-3-deoxy-6-phosphogluconate to pyruvate and glyceraldehyde-3-phosphate ===
The KDPG is then converted into pyruvate and glyceraldehyde-3-phosphate in the presence of enzyme KDPG aldolase. For the pyruvate, the ED pathway ends here, and the pyruvate then goes into further metabolic pathways (TCA cycle, ETC cycle, etc).

The other product (glyceraldehyde-3-phosphate) is further converted by entering into the [[glycolysis]] pathway, via which it, too, gets converted into pyruvate for further metabolism.

=== Conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate ===
The G3P is converted to 1,3-bisphosphoglycerate in the presence of enzyme glyceraldehyde-3-phosphate dehydrogenase (an oxido-reductase).

The aldehyde groups of the triose sugars are [[oxidised]], and [[inorganic phosphate]] is added to them, forming [[1,3-bisphosphoglycerate]].

The hydrogen is used to reduce two molecules of [[NAD+|NAD<sup>+</sup>]], a hydrogen carrier, to give NADH '''+''' H<sup>+</sup> for each triose.

Hydrogen atom balance and charge balance are both maintained because the phosphate (P<sub>i</sub>) group actually exists in the form of a [[Phosphoric acid#Orthophosphoric acid chemistry|hydrogen phosphate]] anion (HPO<sub>4</sub><sup>2−</sup>), which dissociates to contribute the extra H<sup>+</sup> ion and gives a net charge of -3 on both sides.

=== Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate ===
This step is the enzymatic transfer of a phosphate group from [[1,3-bisphosphoglycerate]] to ADP by [[phosphoglycerate kinase]], forming ATP and [[3-phosphoglycerate]].

=== Conversion of 3-phosphoglycerate to 2-phosphoglycerate ===
[[Phosphoglycerate mutase]] isomerises [[3-phosphoglycerate]] into [[2-phosphoglycerate]].

=== Conversion of 2-phosphoglycerate to phosphoenolpyruvate ===
[[Enolase]] next converts [[2-phosphoglycerate]] to [[phosphoenolpyruvate]]. This reaction is an elimination reaction involving an [[E1cB-elimination reaction|E1cB]] mechanism.

''Cofactors:'' 2 Mg<sup>2+</sup>: one "conformational" ion to coordinate with the carboxylate group of the substrate, and one "catalytic" ion that participates in the dehydration

=== Conversion of phosphoenol pyruvate to pyruvate ===
A final [[substrate-level phosphorylation]] now forms a molecule of [[pyruvate]] and a molecule of ATP by means of the enzyme [[pyruvate kinase]]. This serves as an additional regulatory step, similar to the phosphoglycerate kinase step.

''Cofactors:'' Mg<sup>2+</sup>