Transferases in Polymer Chemistry - Enzymatic Polymerisation - page 45

FPP and IPP in the cytosol

Integral membrane protein

Phospholipid head groups

1.1 nm Rubber transferase active

site with bound IPP and
FPP
Rubber transferase active
site with growing rubber
polymer

-

5

1.4 nm

7

Stearic acid with doxyl spin
probe positions

12

Mingling of probes and fatty
acids chains with rubber

0.8 nm

16

17

Fatty acid chains

Rubber core

Fig. 12 Scheme depicting, to scale, a section of the surface monolayer biomembrane surrounding
a P. argentatum rubber particle and the intermingling of the phospholipid fatty acid chains with the
pure rubber core. Natural rubber is synthesized from IPP with an APP, most probably FPP, as an
essential initiator. Both substrates are hydrophilic and are synthesized in the cytosol. The rubber
transferase active site containing IPP ( yellow ) and allylic-PP ( green ) binding sites, and a non-
specific hydrophobic binding region ( pink ) is drawn twice. The site on left of the schema contains
bound IPP and FPP (the magnesium cofactor is not shown). Catalysis then would occur, leading to
condensation of the two molecules and the release of pyrophosphate. The active site drawn in the
middle of the schema is shown containing a short molecule of natural rubber ( cis -1,4-polyisoprene)
originally initiated by FPP. The trans,trans tail of the rubber molecule derived from FPP is visible.
The active sites are positioned so that the hydrophilic pyrophosphate of FPP is aligned to the polar
headgroup region of the membrane. Kinetic studies suggest that FPP traverses the active site as
shown. Both models show the same physical position for the membrane-rubber interface [ 251 ]-
Reproduced by permission of The Royal Society of Chemistry

Rubber appears to be a metabolic dead-end because there have been no findings
of enzymes capable of breaking down the rubber in latex. The exact termination
reaction of the rubber polymerization is not known. Different end-groups have
been detected by NMR in rubber purified from a range of species, indicating that
molecule dephosphorylation and release may involve esterification, cyclization, or
hydrolysis [ 262 ] .
In Hevea, the MW of the polymer has a bimodal distribution with some polymers
in the range of 106 Da and others 105 Da. Both the Hevea polymers and the C55-
C120 oligomeric isoprenes appear to have well-controlled molecular weights [ 270 ] .
In vitro studies reveal the size of the polymer is related to the relative concentrations
of the putative primer (farnesyl pyrophosphate or geranylgeranyl pyrophosphate)