Variable Subunit Contact and Cooperativity of Hemoglobins |
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Authors: | Masafumi Shionyu Ken-ichi Takahashi Mitiko Gō |
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Institution: | (1) Division of Biological Science, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan, JP |
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Abstract: | Tertiary structures of proteins are conserved better than their primary structures during evolution. Quaternary structures
or subunit organizations, however, are not always conserved. A typical case is found in hemoglobin family. Although human,
Scapharca, and Urechis have tetrameric hemoglobins, their subunit contacts are completely different from each other. We report here that only one
or two amino acid replacements are enough to create a new contact between subunits. Such a small number of chance replacements
is expected during the evolution of hemoglobins. This result explains why different modes of subunit interaction evolved in
animal hemoglobins. In contrast, certain interactions between subunits are necessary for cooperative oxygen binding. Cooperative
oxygen binding is observed often in dimeric and tetrameric hemoglobins. Conformational change of a subunit induced by the
first oxygen binding to the heme group is transmitted through the subunit contacts and increases the affinity of the second
oxygen. The tetrameric hemoglobins from humans and Scapharca have cooperativity in spite of their different modes of subunit contact, but the one from Urechis does not. The relationship between cooperativity and the mode of subunit contacts is not clear. We compared the atomic interactions
at the subunit contact surface of cooperative and non-cooperative tetrameric hemoglobins. We show that heme-contact modules
M3–M6 play a key role in the subunit contacts responsible for cooperativity. A module was defined as a contiguous peptide
segment having compact conformation and its average length is about 15 amino acid residues. We show that the cooperative hemoglobins
have interactins involving at least two pairs of modules among the four heme-contact modules at subunit contact.
Received: 12 January 2001 / Accepted: 3 April 2001 |
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Keywords: | : Cooperativity — Heme-contact module — Subunit contact — Molecular evolution — Quaternary structure — Hemoglobin — Neutral evolution — Three-dimensional structure |
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