Octaprenyl pyrophosphate synthase (OPPs) catalyzes consecutive condensation reactions offarnesyl pyrophosphate (FPP) with five molecules of isopentenyl pyrophosphate (IPP) to generate C
40octaprenyl pyrophosphate (OPP) which constitutes the side chain of menaquinone. We have previouslyreported the X-ray structure of OPPs from
Thermotoga maritima, which is composed entirely of
-helicesjoined by connec
ting loops and is arranged with nine core helices around a large central cavity [Guo, R.T., Kuo, C. J.,
Ko, T. P., Chou, C. C., Shr, R. L., Liang, P. H., and Wang, A. H.-J. (2004)
J. Biol. Chem.279, 4903-4912]. A76 and S77 are located on top of the active site close to where FPP is bound. A76Yand A76Y/S77F OPPs mutants produce C
20, indica
ting that the substituted larger residues interfere withthe substrate chain elongation. Surprisingly, the A76Y/S77F mutant synthesizes a larger amount of C
20than the A76Y mutant. In the crystal structure of the A76Y/S77F mutant, F77 is pushed away by Y76,thereby crea
ting more space between those two large amino acids to accommodate the C
20 product. Alarge F132 residue at the bottom of the tunnel-shaped active site serves as the "floor" and determines thefinal product chain length. The substitution of F132 with a small Ala, thereby removing the blockade, ledto the synthesis of a C
50 product larger than that produced by the wild-type enzyme. On the basis of thestructure, we have sequentially mutated the large amino acids, including F132, L128, I123, and D62, toAla underneath the tunnel. The products of the F132A/L128A/I123A/D62A mutant reach C
95, beyondthe largest chain length generated by all known
trans-prenyltransferases. Further modifications of theenzyme reaction conditions, including new IPP derivatives, may allow the preparation of high-molecularweight polyprenyl products resembling the rubber molecule.