Mutational and structural studies of (ßα)8-barrel fold methylene-tetrahydropterin reductases utilizing a common catalytic mechanism.

Methylene-tetrahydropterin reductases catalyze the reduction of a methylene to a methyl group bound to a reduced pterin as C1 carrier in various one-carbon (C1) metabolisms. F420-dependent methylene-tetrahydromethanopterin (methylene-H4MPT) reductase (Mer) and the flavin-independent methylene-tetrahydrofolate (methylene-H4F) reductase (Mfr) use a ternary complex mechanism for the direct transfer of a hydride from F420H2 and NAD(P)H to the respective methylene group, whereas FAD-dependent methylene-H4F reductase (MTHFR) uses FAD as prosthetic group and a ping-pong mechanism to catalyze the reduction of methylene-H4F. A ternary complex structure and a thereof derived catalytic mechanism of MTHFR is available, while no ternary complex structures of Mfr or Mer are reported. Here, Mer from Methanocaldococcus jannaschii (jMer) was heterologously produced and the crystal structures of the enzyme with and without F420 were determined. A ternary complex of jMer was modeled on the basis of the jMer-F420 structure and the ternary complex structure of MTHFR by superimposing the polypeptide after fixing hydride-transferring atoms of the flavins on each other, and by the subsequent transfer of the methyl-tetrahydropterin from MTHFR to jMer. Mutational analysis of four functional amino acids, which are similarly positioned in the three reductase structures, indicated despite the insignificant sequence identity, a common catalytic mechanism with a 5-iminium cation of methylene-tetrahydropterin as intermediate protonated by a shared glutamate. According to structural, mutational and phylogenetic analysis, the evolution of the three reductases most likely proceeds via a convergent development although a divergent scenario requiring drastic structural changes of the common ancestor cannot be completely ruled out.

[Fe]-Hydrogenase, Cofactor Biosynthesis and Engineering.

[Fe]-hydrogenase catalyzes the heterolytic cleavage of H2 and reversible hydride transfer to methenyl-tetrahydromethanopterin. The iron-guanylylpyridinol (FeGP) cofactor is the prosthetic group of this enzyme, in which mononuclear Fe(II) is ligated with a pyridinol and two CO ligands. The pyridinol ligand fixes the iron by an acyl carbon and a pyridinol nitrogen. Biosynthetic proteins for this cofactor are encoded in the hmd co-occurring (hcg) genes. The function of HcgB, HcgC, HcgD, HcgE, and HcgF was studied by using structure-to-function analysis, which is based on the crystal structure of the proteins and subsequent enzyme assays. Recently, we reported the catalytic properties of HcgA and HcgG, novel radical S-adenosyl methionine enzymes, by using an in vitro biosynthesis assay. Here, we review the properties of [Fe]-hydrogenase and the FeGP cofactor, and the biosynthesis of the FeGP cofactor. Finally, we discuss the expected engineering of [Fe]-hydrogenase and the FeGP cofactor.

Crystal structure of FAD-independent methylene-tetrahydrofolate reductase from Mycobacterium hassiacum.

FAD-independent methylene-tetrahydrofolate (methylene-H4 F) reductase (Mfr), recently identified in mycobacteria, catalyzes the reduction of methylene-H4 F to methyl-H4 F with NADH as hydride donor by a ternary complex mechanism. This biochemical reaction corresponds to that of the ubiquitous FAD-dependent methylene-H4 F reductase (MTHFR), although the latter uses a ping-pong mechanism with the prosthetic group as intermediate hydride carrier. Comparative genomics and genetic analyses indicated that Mfr is indispensable for the growth of Mycobacterium tuberculosis, which lacks the MTHFR encoding gene. Therefore, Mfr appears to be an excellent target for the design of antimycobacterial drugs. Here, we report the heterologous production, enzymological characterization, and the crystal structure of Mfr from the thermophilic mycobacterium Mycobacterium hassiacum (hMfr), which shows 78% sequence identity to Mfr from M. tuberculosis. Although hMfr and MTHFR have minor sequence identity and different catalytic mechanisms, their structures are highly similar, thus suggesting a divergent evolution of Mfr and MTHFR from a common ancestor. Most of the important active site residues of MTHFR are conserved and equivalently positioned in the tertiary structure of hMfr. The Glu9Gln variant of hMfr exhibits a drastic reduction of the catalytic activity, which supports the predicted function of the glutamate residue as proton donor in both hMfr and MTHFR. Thus, highly similar binding modes for the C1 -carriers and the reducing agents in hMfr and MTHFR are assumed.

Development and application of a highly efficient CRISPR-Cas9 system for genome engineering in Bacillus megaterium.

Bacillus megaterium has become increasingly important for the biotechnological production of valuable compounds of industrial and pharmaceutical importance. Despite recent advances in rational strain design of B. megaterium, these studies have been largely impaired by the lack of molecular tools that are not state-of-the-art for comprehensive genome engineering approaches. In the current work, we describe the adaptation of the CRISPR-Cas9 vector pJOE8999 to enable efficient genome editing in B. megaterium. Crucial modifications comprise the exchange of promoter elements and associated ribosomal binding sites as well as the implementation of a 5-fluorouracil based counterselection system to facilitate proper plasmid curing. In addition, the functionality and performance of the new CRISPR-Cas9 vector pMOE was successfully evaluated by chromosomal disruption studies of the endogenous ß-galactosidase gene (BMD_2126) and demonstrated an outstanding efficiency of 100 % based on combinatorial pheno- and genotype analyses. Furthermore, pMOE was applied for the genomic deletion of a steroid esterase gene (BMD_2256) that was identified among several other candidates as the gene encoding the esterase, which prevented accumulation of pharmaceutically important glucocorticoid esters. Recombinant expression of the bacterial chloramphenicol acetyltransferase 1 gene (cat1) in the resulting esterase deficient B. megaterium strain ultimately yielded C21-acetylated as well as novel C21-esterified derivates of cortisone.