title: Substrate and metal specificity in the enzymic synthesis of cyclic monoterpenes from geranyl and neryl pyrophosphate.
abstract: A partially purified enzyme (carbocyclase) from the flavedo of Citrus limonum formed alpha-pinene, beta-pinene, limonene, and gamma-terpinene from geranyl pyrophosphate (GPP) and neryl pyrophosphate. The maximum specific activities obtained were 7.0 and 3.6 nmol/min/mg, respectively. Cross-inhibition by the two substrates were observed and the ability to utilize neryl pyrophosphate was almost completely lost with aging. Citronellyl pyrophosphate and dimethylallyl pyrophosphate were the most effective inhibitors of carbocyclase. Isopentenyl pyrophosphate, the monophosphate esters of nerol and geraniol, as well as inorganic pyrophosphate were much less effective inhibitors. The enzyme had an absolute requirement for Mn2+. It could be replaced with about 2% effectiveness by Mg2+ and Co2+. Kinetic studies showed that the observed reaction rate correlates with the calculated concentration of the GPP (Mn2+)2 species. Previous evidence with nonenzymatic reactions and the results presented support the view that the mechanism of carbocyclase may be the intramolecular analog of prenyltransferase.
mesh_terms: D019883:Alkyl and Aryl Transferases; D004789:Enzyme Activation / Q000187:drug effects; D008670:Metals / Q000494:pharmacology*; D010944:Plants; D011106:Polyisoprenyl Phosphates / Q000378:metabolism*; D013379:Substrate Specificity; D013729:Terpenes / Q000096:biosynthesis*; D014166:Transferases / Q000037:antagonists & inhibitors / Q000378:metabolism*
chemical_list: D008670:Metals; D011106:Polyisoprenyl Phosphates; D013729:Terpenes; C015234:geranyl pyrophosphate; D014166:Transferases; D019883:Alkyl and Aryl Transferases; C037934:terpene carbocyclase