Why enzymes are needed Flashcards
What is the sugar before the glycosidic bond called?
Glycone
What is the sugar after the glycosidic bond called?
Aglycone
Where is the glycosidic bond cleaved?
Between the anomeric carbon of the glycone and the glycosidic oxygen
Describe the process of the glycosidic bond cleaving?
The bond is attacked by a proton and the glycosidic oxygen on the departing aglycone is protonated
The glycone is distorted into an oxocarbenium ion transition state which is attacked by a hydroxyl group at the anomeric carbon resulting in the liberation of the glycone sugar
What is the PKa of sugar hydroxyls?
12
What does the PKa tell us?
The pH at which 50% of the molecule is charged
Are the hydroxyls on sugars charged at pH 7?
No
What is the geometry of the positively charged oxocarbenium ion?
Changes from chair to half chair where the C1 carbon and the ring oxygen are in the same plane to facilitate sharing and stabilising of the positive charge
What is needed to hydrolyse the glycosidic bond in solution?
A low pH to generate the H+ and OH- needed
A high temperature to provide the energy for the generation of the oxocarbenium transition state
Why do we need enzymes to cleave the glycosidic bond?
The aforementioned conditions are unlikely in the biological cell
What is the rate enhancement that GHs cleave the glycosidic bond at in room temperature and a neutral pH?
10^17
Is the transition state a high energy molecule when in the active site of the enzyme?
No
How do GHs provide the H+ and OH- needed?
Through acid-base assisted catalysis
