Two distinct subunits of hemerythrin from the brachiopod Lingula reevii: an apparent requirement for cooperativity in O2 binding

Reported are results on the subunit composition of octameric hemerythrin (Hr) from the brachiopod Lingula reevii. Unlike most other Hrs, L. reevii Hr shows cooperativity in O2 binding. Purified L. reevii Hr was found to consist of two different subunits in approximately equimolar proportions. These two subunits differ in molecular weight by approximately 1000. Amino acid sequence data for the first 24 residues of the two subunits, labeled alpha and beta, show 70% identity with each other. Comparisons to amino acid sequences of other Hrs show approximately 50% identity in the first 24 residues and that both the alpha and beta subunits of L. reevii Hr have one residue deleted at their amino termini. Very recently, one other Hr, that from the brachiopod Lingula unguis, was also shown to contain equimolar proportions of two different subunits Satake, K., Yugi, M., Kamo, M., Kihara, H., & Tsugita, A. (1990) Protein Seq. Data Anal. 3, 1-5], and this Hr also shows cooperativity in O2 binding. An alpha 4 beta 4 octamer is, therefore, proposed to be a common feature of those Hrs that show such cooperativity. Likely arrangements of alpha and beta subunits within an alpha 4 beta 4 octamer having the same configuration of subunits as that in other octameric Hrs are proposed. The most probable arrangements can be readily derived from physically reasonable restrictions on the types of intersubunit interactions and on transmission of allosteric effects.