Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

8 Enzymatic Addition/Elimination
Reactions
8.1 Introduction
The addition and elimination of the elements of water is a common process
in biochemical pathways. Particularly common is the dehydration of b-
hydroxy-ketones or b-hydroxy-carboxylic acids, which can be constructed by
aldolase and Claisen enzymes. In this chapter we shall examine the mechanisms
employed by these hydratase/dehydratase enzymes, and the related ammonia
lyases.
In particular we shall discuss several enzymes on the shikimate pathway,
which is depicted in Figure 8.1. This pathway is responsible for the biosynthesis
of the aromatic amino acids l-phenylalanine, l-tyrosine and l-tryptophan in
plants and micro-organisms. Nature synthesises the aromatic amino acids
starting from d-glucose via a pathway involving a series of interesting elimination
reactions. This is an important pathway for plants, since it is also responsible
for the biosynthesis of the precursors to the structural polymer lignin
encountered in Section 7.10. Animals, which do not utilise this pathway, have
to consume the aromatic amino acids as part of their diet.
The elimination of the elements of water involves cleavage of a C2H bond
and cleavage of a C2O bond. The timing of C2H versus C2O cleavage
HO
CO
−
2 HO −
CO
3-dehydroquinate
2
2-
O
CH 2 OPO 3 DAHP synthase
OH
synthase
CHO
PEP
2-
O3 POCH 2
OH
O OH
OH
OH
OH
(1) (2) (3)
CO 2
−
OH
O
CO 2
−
chorismate
synthase
2-
O3 PO
CO 2
−
OH
O
EPSP
synthase
PEP
− 2-
CO O3 2
PO
CO 2
−
OH
OH
3-dehydroquinate
dehydratase
shikimate
kinase
ADP
O
(4)
shikimate
dehydrogenase
HO
ATP
CO 2
−
OH
CO 2
−
OH
OH
NADPH
NADP +
OH
(8)
(7)
(6)
(5)
Figure 8.1 The shikimate pathway. (1) erythrose-4-phosphate; (2) 3-deoxy-d-arabinoheptulosonic
acid 7-phosphate (DAHP); (3) 3-dehydroquinate; (4) 3-dehydroshikimate; (5) shikimate;
(6) shikimate-3-phosphate; (7) 5-enolpyruvyl-shikimate-3-phosphate (EPSP); (8) chorismate.
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