Monoterpene biosynthesis: Demonstration of a geranyl pyrophosphate: Sabinene hydrate cyclase in soluble enzyme preparations from sweet marjoram ( Majorana hortensis)

Abstract

A soluble enzyme preparation from the leaves of sweet marjoram ( Majorana hortensis Moench) catalyzes the divalent cation-dependent cyclization of [1- 3H]geranyl pyrophosphate to the bicyclic monoterpene alcohols (+)-[6- 3H] cis- and (+)-[6- 3H]- transsabinene hydrate, providing labeling patterns consistent with current mechanistic considerations. No free intermediates were detectable in the conversion of geranyl pyrophosphate to the sabinene hydrates as determined by isotopic dilution experiments. Label from H 2 18O water was quantitatively incorporated into the products, indicating that the hydroxyl oxygen atoms of both cis- and trans-sabinene hydrate are derived from water and not from the pyrophosphate ester moiety of the substrate. The two enzymatic activities were inseparable by several chromatographic procedures, and differential inactivation studies suggested that the two activities reside with the same enzyme. The sabinene hydrate cyclase (synthase) has an apparent molecular weight of 56,000, shows a pH optimum near 7.0, and requires a divalent metal ion (either Mn 2+ or Mg 2+) for activity. The enzyme preparation is also capable of cyclizing neryl pyrophosphate, the cis-isomer of geranyl pyrophosphate, and analysis of mixed substrate incubations indicated that the two precursors are mutually competitive. Kinetic analysis and comparison of V rel K m values revealed that geranyl pyrophosphate is the more efficient substrate. This is the first report on an enzyme preparation capable of cyclizing geranyl pyrophosphate and neryl pyrophosphate to the isomeric sabinene hydrates.