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Entry M0254    5.4.99.17    squalene-hopene cyclase

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Step 05

The water molecule polarised by Glu45 abstracts the proton or attacks the E-ring cation, to terminate the reaction.

GIF of Reaction Step M0254.stg05


Comment: Finally, the enzyme reaction is terminated by either regiospecific elimination of the Z-methyl group to the major product hop-22(29)-ene (80%), alternatively, addition of water forms the minor product hopan-22-ol (20%). It has been proposed that the water molecules are polarized by residues in the hydrogen-bonding network around Glu45 at the bottom of the active-site cavity; a polarized water molecule abstracts the proton or attacks the E-ring cation, to terminate the reaction. In fact, the product ratio of hopene and hopanol has been reported to be significantly altered in E45A and E45D mutants of A. acidocaldarius SHC [1].



Mechanisms

Proton Transfer

Mechanism Components

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

Amino acids involved in the reaction step.

Amino Acid Location of Function Activity Function
Asp376 Side Chain spectator Hydrogen Bond Acceptor
His451 Side Chain spectator Hydrogen Bond Donor
Electrostatic Stabiliser
Tyr495 Side Chain spectator Hydrogen Bond Donor
Electrostatic Stabiliser
Phe365 Side Chain spectator van der Waals interaction
Tyr420 Side Chain spectator van der Waals interaction
Polar Non-polar interaction
Trp489 Side Chain spectator van der Waals interaction
Polar Non-polar interaction
Tyr609 Side Chain spectator van der Waals interaction
Polar Non-polar interaction
Phe601 Side Chain spectator van der Waals interaction
Phe605 Side Chain spectator van der Waals interaction
Trp169 Side Chain spectator van der Waals interaction
Polar Non-polar interaction
Glu45 Side Chain spectator Hydrogen Bond Acceptor
Increase Basicity

Reactive Centre

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

View similar reactions in MACiE.


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