The ineluctable requirement for the trans-iron elements molybdenum and/or tungsten in the origin of life

Abstract

An evolutionary tree of key enzymes from the Complex-Iron-Sulfur-Molybdoenzyme (CISM) superfamily distinguishes "ancient" members, i.e. enzymes present already in the last universal common ancestor (LUCA) of prokaryotes, from more recently evolved subfamilies. The majority of the presented subfamilies and, as a consequence, the Molybdo-enzyme superfamily as a whole, appear to have existed in LUCA. The results are discussed with respect to the nature of bioenergetic substrates available to early life and to problems arising from the low solubility of molybdenum under conditions of the primordial Earth.

Figure 1

(A) Structural superposition of the catalytic Molybdenum subunits from respiratory nitrate reductase (Nar, light and dark purple) and polysulfide reductase (Psr, blue and green). (B) Schematic sequence alignments between Nar and Psr. The second and third lines represent the alignment obtained fully automatically by ClustalX and T-Coffee whereas the bottom-line alignment is based on the structural information gathered from (A) and further processed as described above. In both (A) and (B), light purple stretches indicate parts of the Nar polypeptide chain for which no structural equivalents exist in Psr. The same applies for the light blue sequences stretches in Psr. The green stretches denote the part of the sequences for which Clustal- or T-Coffee- and structure-based alignments coincide.

Figure 2

(A) 3D structure-based NJ-phylogenetic tree (see Supplementary Information) of the CISM-enzymes nitrate reductase (Nar), DMSO/TMAO reductase (Dms/Dor/Tor), arsenate reductase (Arr), formate dehydrogenase (Fdh), arsenite oxidase (Aro) and polysulfide reductase (Psr).Violet and orange denote eury- and crenarchaeal branches, dark green, cyan and light green stand for Proteobacteria, Firmicutes and other Bacteria, respectively. Open and closed dots indicate bootstrap values for the deep branchings exceeding 70 and 90%, respectively. In 3D structures, the Mo-subunit is in violet. (B) Redox potential range of relevant substrates and corresponding enzymes.