Biosynthesis of Methanopterin in Methanobacterium thermoautotrophicum

Abstract

The early steps in the biosynthesis of methanopterin in Methanobacterium thennoautotrophicum were analyzed by in vivo incorporation experiments of 13C labeled precursors and by enzymatic studies. M thermoautotrophicum was grown in mineral medium supplemented with 13C-Iabeled acetate, pyruvate, or methionine. Methanopterin was isolated from the cells, and the 13.C-Iabeling pattern was determined by 1H-and 13C-NMR spectroscopy. The labeling patterns of amino acids and nucleosides were also determined for comparison. The results indicate that the pteridine moiety of methanopterin is derived from a purine nucleoside. Both methyl groups of methanopterin originate from methionine. The benzenoid ring of the ribitylaniline moiety is derived from the shikimate pathway, and the ribityl part from the pentose pool. Enzymatic activity of GTP cyclohydrolase I has been detected in cell-free supernatant of M. thermoautotrophicum. This indicates that the early biosynthetic steps of folic acid and methanopterin proceed via common intermediates. Both pathways start from GTP, and 6-hydroxymethyldihydropterin is the probable branching point. The ribitylaniline moiety of methanopterin is synthesized from a product of the shikimate biosynthetic pathway and a ribose derivative, most probably p-aminobenzoate and phosphoribosylpyrophosphate, by decarboxylation of the aminobenzoate.

Keywords

• methanopterin
• methanobacterium
• biosynthesis
• nmr spectroscopy