BP07 Myoglobin, a globular protein

Globular proteins carry out most chemical reactions that take place in the cell. In such proteins the polypeptide chain is folded compactly, unlike that of the fibrous proteins (see BP 10 and 11).

A typical globular protein is myoglobin: about 70 % of its chain exists as an a-helix (coloured red). The eight a-helix domains (sometimes indicated by the letters A to H), bonded by chain-parts without a specific secondary structure (yellow colour), fold themselves up into a ball.
A groove in the 3D-structure contains a prosthetic group, the heme (a porphyrin ring with a Fe (II) atom, the structural formula is shown on the right).

In globular proteins the hydrophobic residues are found in the middle of the protein whilst the hydrophilic residues are found on the surface, in contact with the aqueous surroundings.

The polypeptide chain of myoglobin is only represented schematically in the figure: the a-helix domains are shown as red spirals, the loops as a yellow ribbon. Side chains are not shown. The prosthetic group is shown in more detail as a ball and stick model. The bonded oxygen molecule is clearly shown. The volume that is occupied by this group is also represented. The figure is based on the crystal structure determination of oxymyoglobin done by Phillips (Reference: SEV Phillips, Structure and Refinement of oximyoglobine at 1.6Å resolution, J. Mol. Biol. 142, 531 (1980), Protein Databank code: 1mbo).