Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

160 Chapter 7
Figure 7.4 Structure of rabbit muscle fructose-1,6-bisphosphate aldolase (PDB Wle 1ADO), a class
I aldolase. Catalytic residues Lys-229, Glu-187, Lys-146 and Asp-33 shown in red. Bound substrate
analogue shown in black. Picture prepared using RASMOL.
three histidine residues, as shown in Figure 7.6. The proS hydrogen is removed
by an active site base, which appears to be Glu-182, to give a dienolate
intermediate, as shown in Figure 7.7. Reaction of the si-face of the enolate
with the si-face of G3P forms the C2C bond, with Asp-109 protonating
the aldehyde carbonyl of G3P.
Fructose-1,6-bisphosphate aldolase has been used for enantioselective
carbon–carbon bond formation in organic synthesis. The class I enzyme is
highly selective for the DHAP substrate, but can react with a wide range
of aldehyde substrates. Reaction with racemic 2-hydroxy-3-azido-propionaldehyde
on a 1–10 mmol scale yields a single enantiomer of the aldol product
containing three chiral centres. Biotransformation of the same substrates
with aldolase enzymes of diVerent stereospeciWcity yields diastereomeric
products as shown in Figure 7.8. Dephosphorylation of the products followed
by reduction of the azido group generates the corresponding amines, which
undergo stereospeciWc reductive amination reactions with the free keto
group. The cyclic amine products can be readily converted into a range
of aza-sugar analogues which are of considerable interest as glycosidase
inhibitors.