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

从梨形四膜虫中分离的肌球蛋白样蛋白由121个氨基酸残基组成。这比抹香鲸肌红蛋白小得多，只有32个残基，表明它与普通球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。结果表明，在25℃、0.1M缓冲液中，四膜虫氧化肌红蛋白在pH 4 - 12范围内的自氧化速率与抹香鲸氧化肌红蛋白的自氧化速率基本相当。此外，这两个pH值的档案表现出质子辅助的过程中进行的远端组氨酸作为其催化残基。这些动力学观察是在完全雅阁检查的远端组氨酸在纤毛虫原生动物肌红蛋白的存在。同时，我们还分离了梨形四膜虫（Tetrahymena pyriformis）和T.嗜热菌，并发现在其基因组结构中没有内含子。这与先前文献报道的草履虫和真配子体衣原体中相同类型的收缩或截短的珠蛋白基因形成鲜明对比。相反，四膜虫的基因似乎是密切相关的蓝藻珠蛋白基因来自念珠藻。事实上，这些古老的球蛋白的比较不仅在基因组DNA模式上，而且在蛋白质结构上都显示出明显的多样性。

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.