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762 Chapter 23
Copyright © 2017 Pearson Education, Inc.
14.
The
a
-keto group that accepts the amino group from pyridoxamine is converted into an amino group.
a.
O
O
O
_
O
O
NH
3
_
pyruvate
alanine
+
b.
O
O
O
O O
_
O
_
_
O
NH
3
O
oxaloacetate
aspartate
+
O
_
15.
The first step (after transimination) in all amino acid transformations catalyzed by PLP is removal of a
substituent from the
a
-carbon of the amino acid. The electrons left behind when the substituent is removed are
delocalized onto the positively charged nitrogen of the pyridine ring. If the ring nitrogen were not protonated,
it would be less attractive to the electrons. In other words, it would be a less effective electron sink.
O
H HC
C
H
X R
B
−
+
N
N
H
+
16.
The hydrogen of the OH substituent forms a hydrogen bond with the nitrogen of the imine linkage. (See
Problem 15.) This puts a partial positive charge on the nitrogen, which makes it easier for the amino acid
to add to the imine carbon in the transimination reaction that attaches the amino acid to the coenzyme. It
also makes it easier to remove a substituent from the
a
-carbon of the amino acid. If the OH substituent is
replaced by an
OCH
3
substituent, a proton is no longer available to form the hydrogen bond.