Question the first:

A long time ago I was struck by the similarity of hemoglobin and chlorophyll, and I have always wondered if there was some evolutionary connection.

So is there a connection? Some similarity in the DNA that codes for them?

Question the second:

When I learned of molluscs, arthropods, and assorted deep sea invertibrates having blue blood, I thought: replace the iron with copper, no suprise.

Now in an answer to another question on this site I discover that blue blooded creatures use hemocyanins. I look up hemocyanins and discover that they are polymers of histidine peptides, not similar at all.

So I research hemoglobin and chlorophyll and discover: there are whole families of them, and that they are porphyrins.

Now porphyrins with iron are hemes, and there are may of them, porphyrins with magnesium are chlorins, lots of those too, but apparently there are porphyrins with copper, cobalt, nickel, and zinc.

So what biological functions are performed by the porphyrins with copper, cobalt, nickel, and zinc?

Apologies for the late reply, but this is a difficult question.

After hunting around for some time, I've learned that biosynthesis of porphyrins involve the same enzymatic pathways - the metabolic pathways that produce chlorophyll and heme are the same. Porphyrin synthesis is highly conserved across biology and it may have been acquired very early on in the evolution of organisms. Some sulfate-reducing bacteria have a primitive form of porphyrin biosynthesis. Vitamin B12, which has a very similar structure to porphyrins, is also produced by bacteria. Evidently, porphyrin synthesis in both plants and animals have the same roots - way back when life was at the stage of prokaryotes (cells without nuclei).

As for the second part of your question, I'm not entirely sure about the biological functions of the different porphyrins. However, I did manage to track down other compounds which are related to porphyrins, a group called the tetrapyrroles. Tetrapyrroles are known with cobalt (vitamin B12), nickel (cofactor F430), and copper (pigments). Vitamin B12 is a coenzyme that participates in enzyme-catalysed reactions. Cofactor F430 is another coenzyme that is important in methane production. Pigments provide colourings in plants and animals.

Last edited by Manabu Sakamoto (20th Mar 2007 18:24:21)