Glutathione synthetase homologs encode α-L-glutamate ligases for methanogenic coenzyme F₄₂₀ and tetrahydrosarcinapterin biosyntheses

Proteins in the ATP-grasp superfamily of amide bond-forming ligases have evolved to function in a number of unrelated biosynthetic pathways. Previously identified homologs encoding glutathione synthetase, D-alanine:D-alanine ligase and the bacterial ribosomal protein S6:glutamate ligase have been vertically inherited within certain organismal lineages. Although members of this specificity-diverse superfamily share a common reaction mechanism, the nonoverlapping set of amino acid and peptide substrates recognized by each family provided few clues as to their evolutionary history. Two members of this family have been identified in the hyperthermophilic marine archaeon Methanococcus jannaschii and shown to catalyze the final reactions in two coenzyme biosynthetic pathways. The MJ0620 (mptN) locus encodes a tetrahydromethanopterin:α-L-glutamate ligase that forms tetrahydrosarcinapterin, a single carbon-carrying coenzyme. The MJ1001 (cofF) locus encodes a γ-F₄₂₀-2:α-L-glutamate ligase, which caps the γ-glutamyl tail of the hydride carrier coenzyme F₄₂₀. These two genes share a common ancestor with the ribosomal protein S6:glutamate ligase and a putative α-aminoadipate ligase, defining the first group of ATP-grasp enzymes with a shared amino acid substrate specificity. As in glutathione biosynthesis, two unrelated amino acid ligases catalyze sequential reactions in coenzyme F₄₂₀ polyglutamate formation: a γ-glutamyl ligase adds 1-3 L-glutamate residues and the ATP-grasp-type ligase described here caps the chain with a single α-linked L-glutamate residue. The analogous pathways for glutathione, F₄₂₀, folate, and murein peptide biosyntheses illustrate convergent evolution of nonribosomal peptide biosynthesis through the recruitment of single-step amino acid ligases.