carb5.htm:
First, two monosaccharides are brought together such that two hydroxyl groups are close to each other.
In an enzyme-catalyzed reaction, a water molecule is eliminated, leaving a bond between C1 of glucose and C4 of fructose.
A glycosidic bond is left between the two monosaccharides. enzyme7.htm:
A water molecule is held in the active site by a hydrogen bond to aspartate 125. The net charge in the active site is -1. When substrate binds, several things happen at once. One proton of the bound water molecule is removed by aspartate 25. The oxygen atom of the water molecule starts to form a bond to the carbonyl carbon, and the pi electrons move onto the carbonyl oxygen. The amine nitrogen removes a proton from aspartate 25.
The carbonyl carbon is now in a tetrahedral transition state. The overall charge of the enzyme-substrate complex is still minus one.
An electron pair on the negatively charged oxygen atom moves back into a pi bond with the carbonyl carbon as the peptide bond between that carbon and the amine nitrogen breaks. The positive charge on aspartate
25's carboxyl carbon is neutralized by an electron pair on the negatively charged oxygen.
The peptide bond has been broken. The fragment with a new N-terminus is free to diffuse away, as is the fragment with the new C-terminus if the hydrogen bond between it and aspartate 125 is broken.