A novel geranylgeranyl reductase from the methanogenic archaeon Methanosarcina acetivorans displays unique regiospecificity.

Saturation of a prenyl group to various levels is a frequently observed modification of isoprenoids. The members of the geranylgeranyl reductase family, however, are the only known enzymes responsible for such reductive modifications in archaea. A methanogenic archaeon, Methanosarcina acetivorans, has proteins homologous to phytoene desaturase CrtI, which is the carotenogenic enzyme that catalyzes oxidation/isomerization of phytoene to lycopene, but their function in carotenogenesis is unlikely in a methanogen that does not produce carotenoids. In the present study, we identified one of the homologues, MA1492, as a new type of archaeal geranylgeranyl reductase that is not homologous to known geranylgeranyl reductases. The expression of MA1492 in Escherichia coli cells, which were genetically modified to produce unsaturated archaeal-type lipids, led to the production of partially saturated lipid derivatives. Furthermore, we analyzed the substrate specificity of recombinant MA1492 via in vitro assays. The LC-MS, or radio-TLC, analysis of the reaction products showed that the enzyme was definitely specific to compounds containing C20 geranylgeranyl groups and reduced only one of four double bonds in a geranylgeranyl chain. The GC-MS analysis of the product from geranylgeraniol confirmed that the reduction selectively occurred on the ω-terminal double bond. The available crystallographic structure of an orthologue enzyme may explain the reaction mechanism that achieves the substrate specificity and regiospecificity.