Organic Chemistry

Section 25.6 Pyridoxal Phosphate: Vitamin B 6 1055

In each of these transformations, one of the bonds to the of the amino
acid substrate is broken in the first step of the reaction. Decarboxylation breaks the
bond joining the carboxyl group to the transamination, racemization, and
break the bond joining the hydrogen to the and
bond cleavage breaks the bond joining the R group to the

PLP becomes attached to its enzyme by forming an imine with the amino group of a
lysine side chain. The first step in all PLP-requiring enzymes is a transimination
reaction, a reaction in which one imine is converted into another imine. In the transim-
ination reaction, the amino acid substrate reacts with the imine formed by PLP and the
enzyme, forming a tetrahedral intermediate. The lysine group of the enzyme is expelled,
forming a new imine between PLP and the amino acid.

COO−

NH 2 CH

bond broken

in decarboxylation

bond broken in transamination,

racemization, and , -elimination

bond broken in

C C bond cleavage

R

a-carbon.

a,b-elimination a-carbon; Ca¬Cb

a-carbon;

a-carbon

O O

O

XCH 2 CHCO−

E

PLP

, -elimination

+NH 3

CH 3 CCO− + X− + NH 4

+

O O

HOCHCHCO−

R

E

PLP

C C bond cleavage

+

NH 3

O

R

CH + CH 2 CO−

+

NH 3

+

O

R

NH 3

H

E

PLP

L-amino acid

racemization

+NH

3

+NH 3

C O−

O

R

H

L-amino acid

C O−

O

R

H

D-amino acid

+ C O−

O

RCHCO− +

E

PLP

-ketoglutarate

transamination

O

RCCO−

glutamate

O O

O O

−OCCH

2 CH 2 CCO

− +

O O

−OCCH

2 CH 2 CHCO

−

+NH

3

+NH

3