Chemistry Reference
In-Depth Information
Figure 2.4 Stereoview of a model for the
conjugate ALPB-pyridoxamine, showing
pyridoxamine attached to a site in the protein
interior. (Reprinted with permission from
Ref. 32. Copyright 1996 American Chemical
Society.)
a
as 50 turnovers were observed for the transamination between phenylalanine and
-
ketoglutarate, producing
L-
glutamate with an enantiomeric purity of 93% ee. The rate
was enhanced by as much as 4200-fold with the protein-pyridoxamine conjugate when
compared with the unliganded pyridoxamine phosphate. Although these results are
still quite primitive compared with natural enzyme systems, they clearly show pro-
gress in the right direction.
2.2.5
Polymeric and Dendrimeric Vitamin B6 Mimics
We recently utilized synthetic polymers and dendrimers to construct macromolecular
transaminase mimics. Initially, we covalently attached pyridoxamine to commercial
polyethylenimine (PEI) [35]. The PEI polymer is highly branched, has a molecular
number-average (M
n
) weight of about 60 000, and contains about 1400 monomer re-
sidues. About 25% of the amino groups of PEI are primary, about 50% secondary, and
the remaining are tertiary. To imitate the hydrophobic environment provided by the
enzymes, we covalently attached some alkyl groups to the PEI-pyridoxamine reagent
(Figure 2.5).
The PEI-pyridoxamine polymer was treated with excess pyruvic acid in various buf-
fer solutions, without added metal ions and with excess added EDTA. Kinetic studies
revealed that the attached polymer increased the rate of pyridoxamine transamination
with pyruvic acid by a factor of 6700-8300 at pH 5.0. Under higher pH conditions, the
rate enhancement decreased: At pH 7.0 the rate enhancement by the polymer was still
2300 times, while at pH 8.0, the optimum for pyridoxamine itself, it was 1900. We also
found that transamination by simple pyridoxamine showed strong metal ion catalysis
- adding 1 equiv. of CuCl
2
per pyridoxamine unit to the pH 5.0 solution increased the
