Amorpha-4,11-diene synthase: Mechanism and stereochemistry of the enzymatic cyclization of farnesyl diphosphate

Recombinant amorpha-4,11-diene synthase from Artemisia annua, expressed in Escherichia coli, was incubated with the deuterium-labeled farnesyl diphosphates, (1R)-[1-H-2]FPP, (1S)-[1-H-2]FPP, and [1,1-H-2(2)]FPP. GC-MS analysis of amorpha-4,11-diene formed from the deuterated FPPs shows that the deuterium atoms are retained in the product. Furthermore, analysis of the MS-spectra obtained with the differently labeled substrate indicates that the H-1si-proton of FPP is transferred during the cyclization reaction to carbon 10 of amorphadiene while the H-1re-proton of FPP is retained on C-6 of the product. Proton NMR and COSY experiments proved that the original H-1si-proton of FPP is located at C-10 of amorpha-4,11-diene as a result of a 1,3-hydride shift following initial 1,6-ring closure. The results obtained support the previously suggested mechanism for the cyclization of farnesyl diphosphate by amorph-4,11-diene synthase involving isomerization of FPP to (R)-nerolidyl diphosphate (NPP), ionization of NPP, and C-1,C-6-ring closure to generate a bisabolyl cation, followed by a 1,3-hydride shift, 1,10-ring closure to generate the amorphane skeleton, and deprotonation at either C-12 or C-13 to afford the final product (1S,6R,7R,10R)-amorpha-4,11-diene. (c) 2005 Elsevier Inc. All rights reserved

Immunolocalization of the saposin-like insert of plant aspartic proteinases exhibiting saposin C activity. Expression in young flower tissues and in barley seeds

The plant- specific insert ( PSI) of cypro11 gene- encoding cyprosin, an aspartic proteinase from Cynara cardunculus, has been cloned by polymerase chain reaction ( PCR) into a bacterial expression vector. A rearranged form of this PSI in which the N- and C- terminal sequences were permutated to make it more similar to the structural arrangement observed in saposins was also cloned and expressed in the same system. The biological activities of the two purified recombinant proteins were compared to those of human saposins B and C. The proteins showed similar activity to saposin C, i. e. capacity to activate human glucosylceramidase. At a concentration of 5 mu M, wild- type PSI, saposin C, and rearranged PSI activated human glucosylceramidase two-, three-, and five- fold, respectively. The K-m for 4- methylumbelliferyl beta-glucopyranoside was around 7 mM in the presence of any of the three activators ( 5 mM). The neurotropic activity using NS20Y cells and lipid- binding properties of the plant recombinant proteins were tested. The two plant proteins showed lipid- binding properties similar to those of saposins but did not have any effect on neurite outgrowth. Immunolocalization of PSI showed its expression in protective tissues in flower meristem - protodermis, in C. cardunculus and embryonic root cap and coleorhiza in mature barley grains - as well as husk, pericarp, and the aleurone layer. Possible biological functions suggested for the plant homologue to saposins besides the general activation of enzymes involved in lipid metabolism would be involvement in plant defence