January 14, 2004
A: FROM MENTOR NHA LE IN TX
In answer to Sanna's question about the different colors of
blood. It is true that in many sea creatures, such as
horseshoe crabs their blood color is blue from the copper
metal that is a component of the protein that binds oxygen.
You can find a more complete answer at this cool site
which asks and answers the exact same question that you are
having:
http://www.jbradforddelong.net/movable_type/archives/000566.html
There are many colors of blood depending on the type of
metals that helps to bind the Oxygen. Other differences
include the efficiency of oxygen binding and other
characterisitcs, i dont think these are known in real
depth, but would be cool to find out. Good that you have
these great questions, keep asking.
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A: FROM MENTOR JOAN LUSK IN RI

There are actually two (at least two we know of) other kinds of proteins that carry molecular oxygen in blood. Mollusks and arthropods have hemocyanin, which as you said, contains copper instead of iron. The two copper(I) ions are each held in the protein by three imidazole (histidine) ligands. One oxygen molecule binds between the coppers, making them formally cooper(II). Each atom of the oxygen binds to each of the coppers, bridgine between them, and staying bonded to the other oxygen atom. Karen Magnus has published the structure at http://moray.ml.duke.edu/projects/Magnus/images/ Another difference between us and the mollusks is that hemocyanin is simply dissolved in their blood, not packaged in cell the way our hemoglobin is packaged in red cells. There is sure to be an interesting story (if you're looking for a research pruject) about what difference this makes for their kidneys - presumably the mollusks and arthropods have to filter out hemocyanin from their equivalent of urine - and for CO2 transport and pH response - but I don't know anything about these issues.

The other non-heme oxygen carrier is called hemerythrin and is found in some sea worms. Two Fe(II) ions are bound each to three histidine imidazole's, rather like the configuration of the Cu(I) in hemocyanin, but also to two carboxyl groups in the protein. But when oxygen binds to hemerythrin, it binds end-on to one of the Fe's. It oxidizes both Fe's to Fe(III) and binds in teh form of peroxide anion (HOO-) to one Fe(III).

I didn't find a drawing of the hemerythrin ligand structure on the web, but I did find a crystal structure of the whole protein. You can see how different the folding of the protein is from that of the subunit of hemoglobin or myoglobin, if you have a picture of one of those in one of your books.
http://www.biochem.ucl.ac.uk/bsm/pdbsum/1hmo/main.html

So nature has solved the problem of how to carry oxygen around in blood, how to increase its solubility in blood, in three structurally different but functionally analogous ways.

The name "hemocyanin" implies that it is blue-green, the color "cyan", and "hemerythrin" implies that it's red.

The textbook from which I got this information (not trusting my memory) is Inorganic Chemistry by Shriver and Atkins, page 650. The structures around the copper or iron are clearly shown. The authors point out that some organisms that use hemerythrin also have myoglobin - the heme protein we use to store oxygen in muscles and which is homologous to hemoglobin (its sequence and foldiing patterns are similar).

I've always thought that if some process or structure is not _impossible_, some organism somewhere employs it!

Joan Lusk (a hemoglobin-based form of life not closely related to mollusks)