A hemoglobin-like protein in ciliated protozoa : Its structure, function and molecular evolution

从梨形四膜虫中分离到一个肌球蛋白样蛋白，由121个氨基酸残基组成。这比抹香鲸的肌红蛋白小32个残基，这表明它与常见的球蛋白有着截然不同的来源。因此，我们对这种独特的蛋白质的光谱和稳定性进行了研究。结果表明，在25℃的0.1M缓冲液中，四膜虫氧合肌球蛋白的自氧化速率几乎与抹香鲸MbO2的自氧化速率相当。此外，这两个pH谱都显示了远端组氨酸作为其催化残基进行的质子辅助过程。这些动力学观察与纤毛虫肌红蛋白中存在远端组氨酸的光谱检查完全一致。同时，我们还分离了梨形四膜虫和嗜热四膜虫的珠蛋白基因，发现它们的基因组结构中没有内含子。这与之前关于尾草履虫和衣藻相同类型的收缩或截短珠蛋白基因的文献形成了鲜明的对比。相反，四膜虫基因似乎与发菜的蓝藻珠蛋白基因关系密切。事实上，对这些古老的珠蛋白的比较表明，不仅在基因组DNA图谱上，而且在蛋白质结构上也存在显著的多样性。

A myogobin-like protein isolated from Tetrahymena pyriformis is composed of 121 amino acid residues. This is much smaller than sperm whale myoglobin by 32 residues, suggesting a distinct origin from the common globin. We have therefore examined this unique protein for its spectral and stability properties. As a result, the rate of autoxidation of Tetrahymena oxymyoglobin was found to be almost comparable to that of sperm whale MbO2 over a wide range of pH 4 - 12 in 0.1 M buffer at 25℃. Moreover, both pH-profiles exhibited the proton-assisted process performed by the distal histidine as its catalytic residue. These kinetic observations are in full accord with spectral examinations for the presence of a distal histidine in ciliated protozoa myoglobins. At the same time, we have isolated the globin genes from Tetrahymena pyriformis and T. thermophila, and found that there is no intron in their genomic structures. This is in a sharp contrast to previous literatures on the same types of the contracted or truncated globin genes from Paramecium caudatum and Chlamydomonas eugametos. Rather, the Tetrahymena genes seemed to be closely related to the cyanobacterial globin gene derived from Nostoc commune. In fact, the comparison of these ancient globins show a marked diversity not only in the genomic DNA pattern but also in the protein structure.