A hemoglobin-like protein from Protozoan ciliates : Its structure and function

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A myoglobin-like protein from Tetrahymena pyriformis is composed of 121 amino acid residues with a molecular mass of 14,343 daltons including the heme moiety. This is much smaller than sperm whale myoglobin by 32 residues and shows no notable degree of sequence similarity with other myoglobins and hemoglobins. In order to examine this unique protein for its spectral and stability properties, we have isolated a large amount of its native oxy-form directly from the packed cells by using gel filtration on Sephadex G-50 followed by CM-cellulose chromatography. As a result, Tetrahymena oxymyoglobin was found to be quite resistant against the autoxidation reaction over a wide range of pH4-12 in 0.1M buffer at 25ﾟC,its rate-constant being almost the same as that of sperm whale MbO_2. We therefore conclude that Tetrahymena myoglobin can bind molecular oxygen as stably as mammalian myoglobins can do, in spite of its very contracted structure probably due to a large number of amino acid deletions.