Anyone know what people refer to when they say "free" Iron or "free" copper in cells? They always use "free" but don't say what "free" is. Is it just that it depends on the environment? In a similar environment is copper and iron going to bind similar ligands or not? In a cellular environment what are those ligands that constitute the "free" form of metals?

 

This is not a homework question I'm just wondering because I'm sifting through papers and no one actually says what is "free".

"Free" Iron in a system is that proportion of the total Iron content which is accessible for use in chemical reactions.

 

In biological systems, iron which is bound to functioning proteins (eg haemoglobin) is not available for other reactions and is not regarded as Free Iron

Technically there is no free iron in cells as they are all associated one way or another with biomolecules. It is correct, however that the term refers to only loosely associated as opposed to tightly bound iron. Generally only specific binding elements are therefore considered non-free. However, even in cases of specific binding, the strength of the bond can be so weak that it will be easily released in standardized assays to determine free iron.

The unspecific binding is similar for most heavy metals, however there are different proteins and other molecules that specifically bind iron or copper in certain configurations. This could e.g. be in the form of iron-sulfur clusters in specific proteins, the binding regions of certain transporters, etc.