Trans-cis isomerization of geraniol and geranyl phosphate by cell-free enzymes from higher plants Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ff3657675

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  • Neryl pyrophosphate, rather than geranyl pyrophosphate, is believed to be the immediate biological precursor of the cyclohexanoid monoterpenes because the cis-2, 3-double bond readily permits cyclization. Biosynthesis of neryl pyrophosphate could occur by either of two principal mechanisms: direct cis condensation of dimethylallyl pyrophosphate with isopentenyl pyrophosphate or trans-cis isomerization of geranyl pyrophosphate, Flavin-dependent enzymes that catalyze the isomerization of geraniol and geranyl phosphate to nerol and neryl phosphate, respectively, have now been isolated from peppermint and pea leaves, and carrot tops. Isomerization also occurs non-enzymatically but is greatly accelerated in the presence of the enzyme extracts. Various factors are required in order to demonstrate enzymatic isomerization. These factors are: (1) a flavin (FAD or FMN); (2) a thiol or sulfide (glutathione, dithiothreitol, (i-rnercaptoethanol, or Na₂S); and (3) light (400-500 nm wavelength). If the flavin is partially reduced chemically, light is not required. Non-enzymatic isomerization also occurs if sulfite is the sulfur compound added. When a cell-free peppermint extract is utilized, enzymatic isomerization in the light is a function of: enzyme, substrate, FAD, and dithiothreitol concentrations; temperature (at 35° the rate is twice the rate at 25° ); and pH (7. 5 optimum). Isomerization is also increased by anaerobic conditions. Complete reduction of the flavin inhibits isomerization. Inhibition is also observed if N-ethylmaleimide is present during the reaction, but pretreatment of the enzyme with NEM or 2-hydroxymercuribenzoate, which is removed prior to incubation with substrate, causes no inhibition. The various substrates are isomerized at different rates, Nerol appears to be isomerized twice as fast as geraniol, and the apparent equilibrium for the reaction is 67% geraniol and 33% nerol. Geranyl phosphate is isomerized at about one-half the rate of geraniol. The results are discussed in relation to postulated in vivo isomerization of geranyl pyrophosphate to neryl pyrophosphate.
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