Exploring the nature’s discriminating factors behind the selection of molybdoenzymes and tungstoenzymes depending on the biological environment

Pterin based molybdenum cofactor found in the molybdenum dependent enzymes catalyzes the oxo-transferase and hydroxylase activity. For tungsten, the pterin based tungsten cofactor is known for the similar biological low potential redox activities in anaerobic conditions in thermophilic microorganisms. Nature's selection of tungsten and molybdenum for the different working conditions is rationalized in terms of their relative bioavailabilities, thermodynamic stabilities of their compounds, kinetic inertness and the difference in relativistic effect experienced by these two congeners. The relativistic effect is the most important factor to justify the tungsten vs. molybdenum selectivity in different enzymes. The non-innocent dithiolene based pterin ligand tunes the biological redox activity of the enzymes by stabilising +4, +5 and +6 oxidation states of molybdenum and tungsten. In fact, it acts as a ‘redox buffer’ in their catalytic mechanism. Mechanistic aspects of the enzymatic activity are more investigated for the Mo-dependent enzymes compared to those of W-dependent enzymes. Strong controversies regarding the mechanisms of activity of the enzymes like Mo/W-FDH (formate dehydrogenase), Mo-Cu-CODH (carbon monoxide dehydrogenase), W-AOR (aldehyde oxidoreductase), W-AH (acetylene hydratase), W-BCRs (benzoyl-CoA-reductases), etc., indicate that this field is still an active area of research.