[Human hemoglobin structure and respiratory transport].
Hemoglobin carries oxygen from the lungs to the tissues and helps to transport carbon dioxide back to the lungs. It fulfills this dual role by clicking back and forth between two alternative structures, designated T for tense and R for relaxed, which are defined by the theory of allostery. Like all proteins, it is made up of small molecules called amino acids. A hemoglobin molecule is made up of four polypeptide chains, two alpha chains of 141 amino acid residues each and two beta chains of 146 amino acid residues each. In the complete molecule, four subunits are closely joined, as in a three-dimensional jigsaw puzzle, to form a tetramer. In the T structure, the subunits of the molecule are clamped by salt bridges and hydrogen bonds against the pressure of springs and their narrow pockets impede the entry of oxygen. In the R structure, all the clamps have sprung open and the heme pockets are open wide enough to admit oxygen easily. Uptake of oxygen by the T structure would strain the clamps until they all burst open in concert and allow the molecule to relax to the R structure. Loss of oxygen will narrow the heme pockets and allow the T structure to re-form.
Imamura
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Department of Human Genetics, National Institute of Genetics.
D000494:Allosteric Regulation; D002245:Carbon Dioxide / Q000097:blood; D006454:Hemoglobins / Q000502:physiology* / Q000648:ultrastructure*; D006801:Humans; D010100:Oxygen / Q000097:blood; D015657:Respiratory Transport / Q000502:physiology*
D006454:Hemoglobins; D002245:Carbon Dioxide; D010100:Oxygen