Protozoan myoglobin from Paramecium caudatum. Its unusual amino acid sequence.

A protozoan myoglobin (or monomeric hemoglobin) was isolated from Paramecium caudatum, and its complete amino acid sequence determined. It consists of 116 amino acid residues with a molecular mass of 12,565 daltons, this being much smaller than sperm whale myoglobin by 37 residues and even smaller than a bacterial hemoglobin from Vitreoscilla by 30 residues in terms of the monomer unit. A computer search showed no notable sequence homology with other hemoproteins. It contains two histidine residues at positions 68 and 84, but other lines of evidence seem to be needed to complete their final alignment. This is the first protozoan myoglobin to be sequenced. It may provide us with a new molecular basis for a further understanding of myoglobin-hemoglobin chemistry and their evolution.