Transferases in Polymer Chemistry - Enzymatic Polymerisation - page 44

HMG-CoA Acetoacetyl-CoA Acetyl-CoA

Mevalonate

Natural rubber
( cis -1,4-polisoprene)

Mevalonate-PP

MPP-D

+ IPP

CPT

Sterols
Chlorophyll
Carotenoids
Gibberelins
Squalene

IPP

+ IPP

FPP

IPP TPT GPP

TPT

DMAPP

Essential oils
Monoterpenes

Abscisic acid
Phytoalexins

Fig. 11 Natural rubber is produced from a side branch of the ubiquitous isoprenoid pathway, with
3-hydroxy-methyl-glutaryl-CoA (HMG-CoA) as the key intermediate derived from acetyl-CoA by
the general mevalonic-acid pathway. Mevalonate diphosphate decarboxylase (MPP-D) produces
IPP, which is isomerized to DMAPP by IPP isomerase (IPI). IPP is then condensed in several steps
with DMAPP to produce GPP, FPP and GGPP by the action of a trans -prenyltransferase (TPT).
The cis -1,4-polymerization that yields natural rubber is catalyzed by cis -prenyltransferase (CPT),
which uses the non-allylic IPP as substrate. Reprinted from [ 248 ], with permission from Elsevier

The exact mechanism of polymer initiation is unknown. Initiation of rubber syn-
thesis has been studied in several plants and a common finding is that the end
groups found in low molecular weight rubber (such as rubber from goldenrod and
H. brasiliensis leaves) are not made up of cis -isoprene units, unlike the bulk of the
rubber [ 259 , 260 ]. Structural studies [ 261 , 262 ] have led to the suggestion that the
C15 FPP may be the most common initiator in vivo, at least in H. brasiliensis .
Although many different APPs are effective initiators of rubber biosynthesis,
only IPP can be used as the source of isopentenyl monomer for the cis -1,4-
polymerization of the rubber polymer.
All rubber transferases exhibit similar kinetic constants and pH optima, and are
able to accept a similar range of APPs as initiating substrate [ 263 , 264 ] . In vitro
studies have shown that several compounds (DMAPP, GPP, FPP, and GGPP) can
initiate rubber biosynthesis, with a faster rate of rubber biosynthesis the longer the
APP (up to C15 or C20) [ 254 , 265 ] . Non-natural APPs were also shown to be able
to function as a primer for the rubber biosynthesis [ 266 ] .
The cDNAs of the cis -prenyltransferase of H. brasiliensis was successfully iden-
tified and expressed in E. coli . The in vitro polymerization of IPP after initiation
with FPP using the expressed cis -prenyltransferase resulted in low degrees of poly-
merization [ 267 , 268 ]. After addition of rubber particles to this polymerization, the
molecular weight increased tremendously [ 269 ] . It can be concluded that the rub-
ber particles are essential for rubber biosynthesis. Katarina Cornish established a
detailed structural model of the in vivo synthesis of natural rubber in the rubber par-
ticle monolayer membrane and partially explained this behavior (see Fig. 12 ) [ 251 ] .