Entry M0236    3-dehydro-L-gulonate-6-phosphate decarboxylase

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Step 02

The carbonyl of the intermediate reforms and the C1 position is protonated. This can occur at the si-face (involving the si-water and His136, shown here) or at the re-face (involving the re-water and Arg139).

GIF of Reaction Step M0236.stg02

Comment: The enediolate can be protonated from either the re- or si-face. The si-face water is protonated by His136 to allow it to act as a proton shuttle. However, if the protonation occurs at the re-face, Arg139A has been proposed to be the proton source. Since protonation at the si-face is favoured in a roughly 2:1 ratio, this is the mechanism that has been shown.


Assisted Keto-Enol Tautomerisation
Proton Transfer

Mechanism Components

Bond Cleavage
Bond Formation
Bond Order Change
Intermediate Terminated
Overall Product Formed
Proton Relay

Amino acids involved in the reaction step.

Amino Acid Location of Function Activity Function
Asp67B Side Chain spectator Hydrogen Bond Acceptor
Electrostatic Stabiliser
Glu112A Side Chain spectator Hydrogen Bond Acceptor
Increase Acidity
Electrostatic Stabiliser
His136A Side Chain reactant Hydrogen Bond Donor
Proton Donor
Lys64A Side Chain spectator Hydrogen Bond Donor
Electrostatic Stabiliser

Metal Cofactors involved in Step 02

Metal Type Metal Identity Chain Activity Function
magnesium MG 5300 _ spectator Substrate Binding
Electrostatic Stabiliser

Reactive Centre

Bonds Formed Bonds Cleaved Bonds Changed in Order Atom Types Involved
The C-O bond changes from a single to double bond
The C-C bond changes from a double to single bond

View similar reactions in MACiE.