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

从梨形四膜虫中分离出一种肌高蛋白样蛋白，由121个氨基酸残基组成。这比抹香鲸肌红蛋白小了32个残基，表明它与普通的珠蛋白有着不同的起源。因此，我们研究了这种独特的蛋白质的光谱和稳定性。结果发现，在0.1 M缓冲液中，在25℃下，在pH为4 - 12的宽范围内，四膜虫氧肌红蛋白的自氧化率与抹香鲸MbO2的自氧化率几乎相当。此外，两种ph谱都显示了远端组氨酸作为催化残基进行的质子辅助过程。这些动力学观察结果与纤毛原生动物肌红蛋白中存在远端组氨酸的光谱检查完全一致。同时，我们从梨状四膜虫和嗜热T.中分离出了珠蛋白基因，发现它们的基因组结构中没有内含子。这与以往文献中关于尾草履虫和真配子衣单胞菌相同类型的收缩或截断的珠蛋白基因形成鲜明对比。更确切地说，四膜虫基因似乎与蓝藻红蛋白基因密切相关。事实上，这些古老珠蛋白的比较表明，不仅在基因组DNA模式上，而且在蛋白质结构上都有显著的多样性。

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.