| Monoterpenes are lipophilic natural products that accumulate in modified epidermal hairs known as glandular trichomes. The function(s) of monoterpenes in plants is not well known; however, they appear to serve ecological roles as agents of chemical defense or communication. Monoterpenes arise from the acetate-mevalonate pathway via cyclization of the common intermediate geranyl pyrophosphate. Enzymes that catalyze this conversion are known as monoterpene cyclases. Soluble enzyme preparations from isolated glandular trichome cells of Mentha x piperita catalyze the cyclization of geranyl pyrophosphate to 4S-limonene.;4S-Limonene synthase was purified to apparent homogeneity by dye-ligand, anion-exchange and hydrophobic interaction chromatography, and the properties of this cyclase were determined. The enzyme has a native molecule weight of 55,000 and is monomeric, shows a pH optimum near 6.7, an isoelectric point of 4.35, a ;Several mechanistic aspects of 4S-limonene synthase catalysis were also studied. The cyclization is considered to proceed, by analogy to other monoterpene cyclases, via the initial ionization and isomerization of geranyl pyrophosphate to 3S-linalyl pyrophosphate and the subsequent cyclization of this enzyme-bound, tertiary, allylic intermediate to the monocyclic 4S-;Amino acid residues involved in 4S-limonene synthase catalysis were investigated by chemical modification using group specific reagents and dye-sensitized photooxidation. Cysteine, histidine, and perhaps methionine appear to be essential for enzyme activity. The results of protection studies indicated that both cysteine and histidine are located at or near the active site of the enzyme. The results of experiments conducted with several other monoterpene cyclases supported the key role of histidine in cyclase catalysis. |
