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It is a common fact that we have iron in our blood. But what if we had something different in its place? Like instead of iron what if it was copper, or silver, or what if we had gold in our blood stream instead? Theoretically, what would that do?

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    $\begingroup$ If we would use copper to transport oxygen in the blood, we would be more closely related to horseshoe crabs. $\endgroup$ – Karsten Theis Jan 24 at 19:01
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    $\begingroup$ Iron in blood is a part of heme group in haemoglobin. Heme is a complex molecule with a porphyrin ring attached to a iron center. Several other such molecules with similar structures are also there, Chlorophyll a has a similar structure with Magnesium at the centre of the Porphyrin ring. Researching on chlorophyll a will give you an idea. $\endgroup$ – Soura Jan 24 at 19:03
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    $\begingroup$ Instead of one Fe(II) atom like in our heme, the carrier hemocyanin in horseshoe crabs has two bound copper ions to bind to a single dioxygen. $\endgroup$ – Karsten Theis Jan 24 at 19:07
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    $\begingroup$ The whole biological system of a human is finely tuned. You can't just willy-nilly substitute some other metal for iron in hemoglobin. $\endgroup$ – MaxW Jan 24 at 19:08
  • $\begingroup$ Assuming that you could construct a recombinant protein that was not rejected by the immune system, and used some metal other than iron in its oxygen-binding cofactor, it makes for an interesting if bizarre question. Afaik release of oxygen is passive (ie does not require another protein, even if it's complex) so if you tuned the protein's $CO_2$ and oxygen binding constants just right, why well, I don't know, it might just work. Question is, why don't we have a copper-loaded oxygen carrier? Just because? $\endgroup$ – Buck Thorn Jan 24 at 20:01
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To start, transition metal complexes with large porphyrin-like molecules are so electronically complex that standard chemical intuition doesn't apply and you have to either do the experiment or run a (nearly impossible) calculation to determine what effect a non-physiological metal would have on the protein activities. In the case of hemoglobin, your best bet would be to think about metals which have two accessible oxidation states because the interconversion of Fe(II) and Fe(III) is critical to the function of hemoglobin.

To give a broader answer, let's think about the general question of substituting non-native metals into a protein. This is, in fact, an very interesting and open question. On smaller metalloproteins there have been studies probing whether nature 'picked' the best metal for a task or just a convenient/abundant one [1,2]. The focus of those studies is on catalysis, and there are cases where many non-physiological metals can function in the protein and even catalyze the reaction as good or even better than the wild type.

[1] https://pubs.acs.org/doi/10.1021/ar500227u

[2] https://pubs.acs.org/doi/abs/10.1021/acs.jpcb.6b00997

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