Investigation of serine hydroxymethyltransferase in methanogens

The cofactor specificity of serine hydroxymethyltransferase (SHMT) activities was tested in extracts of several methanogens using tetrahydromethanopterin (H4MPt) from Methanobacterium thermoautotrophicum Marburg, tetrahydrosarcinapterin (H4SPt) from Methanosarcina barkeri, and tetrahydrofolate (H4folate) as the potential C1 carrier. In Methanosphaera stadtmanae and Methanococcus thermolithotrophicus, the activities were H4MPt dependent. In Methanospirillum hungatei GP1, Methanosaeta concilii, Methanolobus tindarius, and Methanosarcina barkeri Fusaro, the activities were strictly H4folate dependent. H4SPt was reactive with the SHMT of Methanosphaera stadtmanae but not with that of Methanosarcina barkeri. In both Methanosarcina barkeri and Methanospirillum hungatei, pyridoxal phosphate stimulated SHMT activity. The apparent Km values for H4folate and L-serine were 0.086 and 0.29 mM in Methanosarcina barkeri and 0.065 and 0.31 mM in Methanospirillum hungatei, respectively. Key words: tetrahydromethanopterin, tetrahydrofolate, Archaea, serine hydroxymethyltransferase, methanogen.

Biochemical and Genetic Characterization of an FK506-Sensitive Peptidyl Prolyl cis-trans Isomerase from a Thermophilic Archaeon, Methanococcus thermolithotrophicus

ABSTRACT A peptidyl prolyl cis-trans isomerase (PPIase) was purified from a thermophilic methanogen, Methanococcus thermolithotrophicus. The PPIase activity was inhibited by FK506 but not by cyclosporine. The molecular mass of the purified enzyme was estimated to be 16 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 42 kDa by gel filtration. The enzyme was thermostable, with the half-lives of its activity at 90 and 100°C being 90 and 30 min, respectively. The catalytic efficiencies (k cat/Km ) measured at 15°C for the peptidyl substrates,N-succinyl-Ala-Leu-Pro-Phe-p-nitroanilide andN-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide, were 0.35 and 0.20 μM−1 s−1, respectively, in chymotrypsin-coupled assays. The purified enzyme was sensitive to FK506 and therefore was called MTFK (M. thermolithotrophicusFK506-binding protein). The MTFK gene (462 bp) was cloned from anM. thermolithotrophicus genomic library. The comparison of the amino acid sequence of MTFK with those of other FK506-binding PPIases revealed that MTFK has a 13-amino-acid insertion in the N-terminal region that is unique to thermophilic archaea. The relationship between the thermostable nature of MTFK and its structure is discussed.