Structure of coenzyme F-420 dependent methylenetetrahydromethanopterin reductase from two methanogenic archaea

Coenzyme F-420-dependent methylenetetrahydromethanopterin reductase (Mer) i s an enzyme of the Cl metabolism in methanogenic and sulfate reducing archa ea. It is composed of identical 35-40 kDa subunits and lacks a prosthetic g roup. The crystal structure of Mer from Methanopyrus kandleri (kMer) reveal ed in one crystal form a dimeric and in another a tetrameric oligomerisatio n state and that from Methanobacterium thermoautotrophicum (tMer) a dimeric state. Each monomer is primarily composed of a TIM-barrel fold enlarged by three insertion regions. Insertion regions 1 and 2 contribute to intersubu nit interactions. Insertion regions 2 and 3 together with the C-terminal en d of the TIM-barrel core form a cleft where the binding sites of coenzyme F -420 and methylene-tetrahydromethanopterin are postulated. Close to the coe nzyme F-420-binding site lies a rarely observed non-prolyl cis-peptide bond . It is surprising that Mer is structurally most similar to a bacterial FMN -dependent luciferase which contains a non-prolyl cis-peptide bond at the e quivalent position. The structure of Mer is also related to that of NADP-de pendent FAD-harbouring methylenetetrahydrofolate reductase (MetF). However, Mer and MetF do not show sequence similarities although they bind related substrates and catalyze an analogous reaction. (C) 2000 Academic Press.