The Protein Subunit
Each subunit in Figure 2 contains regions with a coiled shape; many of the amino acids that make up the polypeptide chain interact to form this particular structure, called an alpha helix. In an alpha helix (Figure 3), each amino acid is "hydrogen-bonded" to the amino acid that is four residues ahead of it in the chain. In hemoglobin, the hydrogen-bonding interaction occurs between the H of an -NH group and the O of a -CO group of the polypeptide backbone chain; the amino-acid side chains extend outward from the backbone of the helix. Approximately 75% of the amino-acid composition of hemoglobin adopts an alpha-helical structure. Another common structural motif is the beta-pleated sheet, in which amino acids line up in straight parallel rows.
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This page created by Matt Traverso, Washington University in St Louis.
© 2004, Washington University.
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