A primitive myoglobin from Tetrahymena pyriformis: its heme environment, autoxidizability, and genomic DNA structure

A myoglobin-like protein isolated from Tetrahymena pyriformis is composed o f 121 amino acid residues. This is much smaller than sperm whale myoglobin by 32 residues, suggesting a distinct origin from the common globin gene. W e have therefore examined this unique protein for its structural, spectral and stability properties. As a result, the rate of autoxidation of Tetrahym ena oxymyoglobin (MbO(2)) was found to be almost comparable to that of sper m whale MbO(2) over a wide range of pH 4-12 in 0.1 M buffer at 25 degreesC. Moreover, both pH profiles exhibited the remarkable proton-assisted proces s, which can be performed in sperm whale myoglobin by the distal (E7) histi dine as its catalytic residue. These kinetic observations are also in full accord with spectral examinations for the presence of a distal histidine in ciliated protozoa myoglobin. At the same time, we have isolated the globin genes both from T. pyriformis and Tetrahymena thermophila, and found that there is no intron in their genomic structures. This is in sharp contrast t o previous reports on the homologous globin genes from Paramecium caudatum and Chlamydomonas eugametos. Rather, the Tetrahymena genes seemed to be rel ated to the cyanobacterial globin gene from Nostoc commune. These contracte d or truncated globins thus have a marked diversity in the cDNA, protein, a nd genomic structures. (C) 2000 Elsevier Science B.V. All rights reserved.