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Mammals and many other groups of animals usually transport oxygen using haemoglobin and other complex proteins, the core of which is based on an iron coordinated to a porphyrin. There are plenty of subtle differences in the protein structures but are there any notable alternatives to that central iron-porphyrin structure?

Obviously there are other chemicals that can do a similar job. Polyfluorinated hydrocarbons were once considered as a viable emergency alternative to donor-blood as they are very good at transporting oxygen. But those are not something that living creatures are likely to develop.

So the question is are there either possible or known alternative chemistries for transporting oxygen in living creatures?

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Yes! A variety of metalloproteins are used for oxygen transport. These include proteins that bind to porphyrins with different central metal atoms, as well as proteins that bind the metal atoms directly, without a porphyrin ligand. Fun fact- hemocyanin, which uses two copper atoms as prosthetic groups to bind oxygen, is responsible for the blue color of horseshoe crab blood!

Here are some relevant links:

https://en.wikipedia.org/wiki/Respiratory_pigment

https://en.wikipedia.org/wiki/Hemocyanin

https://en.wikipedia.org/wiki/Hemoglobin (scroll to "other oxygen-binding proteins" near the bottom)

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