The unique structures of protozoan myoglobin and yeast hemoglobin: an evolutionary diversity.

A hemoglobin-like protein is found in some of the single-celled organisms, but its structure is quite different from that of mammalian myoglobin or hemoglobin. For instance, a protozoan myoglobin isolated from Paramecium caudatum consists of 116 amino acid residues, so that this contracted form is nearly two thirds of sperm whale myoglobin. Yeast hemoglobin from Candida norvegensis, on the other hand, is composed of a single polypeptide chain with 387 amino acid residues, but of two distinct domains carrying different functions; that is the N-terminal, heme-containing region and the C-terminal, FAD-containing reductase domain. The very unique structures of these ancient hemoproteins tell us their own strategies to overcome many difficulties in the reversible and stable binding of molecular oxygen, a very strong oxidizing agent, to the heme iron(II) in aqueous solutions.