Vitamin B6 Enzyme Models - Artificial Enzymes - page 52

Figure 2.6 Structure of the PAMAM-pyridoxamine systems.
(Reprinted from Ref. 38. Copyright 2003 American Chemical Society.)

mine) (PAMAM) dendrimers in which we located one pyridoxamine unit at the core
(Figure 2.6) [38]. Unlike PEIs, whose structures are poorly defined, PAMAM dendri-
mers are good mimics of globular proteins. We tuned the structure and size of the
dendrimeric enzyme models, which greatly affected the transamination rates. We es-
tablished that a single pyridoxamine in the core of a PAMAMdendrimer is comparable
to a pyridoxamine unit linked to M n = 60 000 PEI in transaminating pyruvic and phe-
nylpyruvic acids to alanine and phenylalanine, but it is less effective than the laurylated
PEI-pyridoxamine.

2.3
Racemization

Amino acid racemases are important for bacteria because they need D- alanine in the
biosynthesis of cell walls. These enzymes require pyridoxal as the active cofactor. A
racemization reaction starts with the aldimine complex between pyridoxal and an a -

amino acid (Scheme 2.4). Deprotonation occurs at the -carbon of amino acid, due to

the electron-sink effect of pyridoxal. Reprotonation of the quinonoid intermediate at
the opposite side provides the desired product (pathway a in Scheme 2.4). However,
reprotonation may also take place at the C4 0 of pyridoxal (pathway b in Scheme 2.4).
This kills the catalyst because one of its product, pyridoxamine, can no longer racemize
an amino acid.
Thus, to attain the racemase reactivity it is important to learn how to block the trans-
amination process. Our idea was to place a catalytic group that can protonate only the
a

a

-carbon of the amino acid unit but not the C4 0 of the pyridoxal moiety in the quino-
noid intermediate (Scheme 2.5). Thus we synthesized catalyst 38 , which carries a rigid
pyridine side chain [39]. Catalyst 39 , which lacks the double bond, was also synthesized
as a less rigid control. Both catalysts catalyzed loss of optical activity from the aldimine
equally well - about twice as fast as simple pyridoxal. However, 39 could catalyze the
transamination reaction 2.5 times faster than 38 . Therefore, 38 showed a small pre-
ference for racemization over the transamination reaction as compared with 39 by a