Formation of Covalent Intermediate

When you look at the NAD+ dependent oxidation mechanism it is not clear how an aldehyde is oxidized to an organic acid. (where does the "other" oxygen atom come from???) The formation of the covalent intermediate answers this.

Cysteine forms a covalent intermediate at the aldehyde of glyceraldhyde generating something that "looks more" like and alcohol. Bear in mind that there are still two bonds to "O" or "S" so it still qualifies an aldehyde according to the rules.

  1. The Cysteine is deprotonated by a nearby base to help it become a better nucleophile so that it can attack the aldehyde.
  2. This makes a pair of electrons on the C=O to move
  3. The easiest move is to capture a proton from the nearby Histidine which is in the acid state.

Oxidation of glyceceraldehyde

The results of the previous steps are shown. The substrate is covalently attached to the Cysteine, the Histidine is in the base state

  1. NAD+ enters the active site
  2. Histidine pulls on the proton of the -OH on C1
  3. The pair of electrons moves to reform the C=O again.
  4. The Hydrogen NAD a pair of electrons (H-) moves from C1 to the NAD+
  5. electron movement within NAD accommodate

Stable intermediate

The results of the previous steps are shown. The substrate is covalently attached to the Cysteine, the Histidine is in the acid state again and C1 has been oxidized - there are THREE bonds to "O" or "S" now (this group is called a thioester). and NADH is present

Phosphate Attacks instead ot water

NADH leaves the active site and Phosphate enters

  1. the phosphate attacks the thioester
  2. The pair of electrons from the S-C bond of the thioester move to grag a nearby proton - thus cutting the covalent attachment to the protein and regenerating the original cysteine

Done

the results of the previous step are shown

The product 1,3-bisphosphoglycerate is in the active site- the cyteine is free and NADH is in solution all amino acids are back to their original states.

Anaerobic Glucose Metabolism

Glyceraldehyde-3-Phosphate Dehydrogenase Information


Enzyme Name

Glyceraldehyde-3-Phosphate Dehydrogenase


Reaction Catalyzed

two step reaction:
Written in the direction of glycolysis
  1. Oxidation of Glyceraldehyde-3-phosphate to glycerate-3-phosphate
  2. addtion of phosphate to C1
This order reverses for gluconeogenesis, of course

Reaction Type

Two Step Reaction
  1. Oxidation/Redxution (REDOX)
  2. Hydrolysis type reaction (with a twist here since phosphate (instead of water) is used to attack it is called phosphorolysis)

Pathway Involvement

Glycolysis AND gluconeogenesis
This reaction easily goes both directions depending on the concentrations of the reactants. (see discussion of ΔG below)

Cofactors/Cosubstrates

Glycolysis cosubstrates: NAD+ and PO4= are both required. NADH is a coproduct
Gluconeogenesis cosubstrate: NADH; NAD+and PO4= are coproducts