Scope of the Essentiality Model

It appears straightforward to use electrochemical data (redox potentials) for proteins containing Fe or Cu vice versa to estimate the strengths of complex binding of 

Stability and multitude/diversity of metal-ligand bonds which can be formed by some metal ion in redox-neutral to oxidizing aqueous media and its biocatalytic feasibility and thus possible essentiality are linked to each other. 

The above multitude/diversity of metal-ligand bonds and of (bio-)catalytic transformations is given by terms (parameters) which are typical for the given metal ion (although c and x values change with oxidation states (Fe, Ce, V, etc.) and c x are linked by correlations for given oxidation states). 

For biological reproduction the construction of autocatalytic cycles is required which reduces the options to use broadly diverse metals (elements in general) in biological systems. 

Tissue diversity (histological complexity) acts likewise. 

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