Chemistry Reference
In-Depth Information
−
OPP
OPP
OPP
CHCH
2
-OPP
HH
C
10
(DMAPP) C
5
(IPP) C
5
C
15
C
20
H
−
CH
2
CHCH
2
−
OPP
n
C
5n
O
−
O
−
O
−
OPP
O
P
O
P
OO
Fig. 10
Reactions catalyzed by prenyltransferases
The prenyl chain elongation catalyzed by prenyltransferases is quite unique be-
cause the reaction proceeds consecutively and terminates precisely at discrete chain
lengths according to the specificities of the enzymes. The chain length of products
varies so widely that it ranges from geraniol (C10) to natural rubber (C
5000).
The identification and characterization of the genes and enzymes involved in
rubber biosynthesis have been slow compared with those involved in the synthesis
of other biopolymers. In fact, most of the studies thus far reported begin with rubber
particles.
Enzymatically active, partially purified (washed) rubber particles can be isolated
such that, when provided with an appropriate APP primer, magnesium ion cofactor,
arated by centrifugation into three phases. The bottom fraction (20% of the latex)
contains membrane-bound organelles. The middle fraction is called the C-serum.
The top fraction phase contains the rubber particles. Biochemical studies have es-
tablished that latex in this fractionated form is unstable. These studies also suggest
that the bottom fraction is required for initiation of polymer synthesis.
The essential precursor (APP) and monomer (IPP) are synthesized in vivo via the
isoprenoid pathway, with cytosolic acetyl-CoA being the primary building block for
the synthesis of rubber. Acetyl-CoA is converted to IPP through a pathway involv-
into dimethylallyl diphosphate (DMAPP) by isomerization. DMAPP primes the se-
quential head-to-tail condensations of IPP molecules by
trans
-prenyltransferases to
form geranyl diphosphate (GPP, C10, monoterpenoids), farnesyl diphosphate (FPP,
C15, sesquiterpenoids) and geranylgeranyl diphosphate (GGPP, C20, diterpenoids)
Rubber molecules are synthesized from one APP molecule, which initiates the
reaction, and the rubber polymer (
cis
-1,4-polyisoprene) is then polymerized by se-
quential condensations of the non-allylic IPP (magnesium cations are a required
cofactor) with release of a diphosphate at each condensation. After initiation and
elongation, a termination event occurs in which the rubber molecule is released
from the enzyme. Despite the similar process, remarkable differences exist between
plant species with respect to enzymatic reaction mechanisms and product molecular
weight.
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