Solubility Classification of Proteins

Proteins can be classified into two main groups on the basis of their water solubility. These are the insoluble fibrous proteins above, and the soluble globular proteins. The latter contain polypeptide chains which are folded into a compact structure of globular shape. This class of proteins is considerably more complex than the fibrous variety. They tend to expose a maximum number of their polar (amino acid) groups to the external aqueous environment, and at the same time orient a maximum number of their non-polar groups internally. Fibrous proteins are rich in the latter and these are oriented externally. 

Whereas the fibrous proteins have a more permanent existence and are used for structural purposes within the body, the globular proteins are essentially mobile and serve a variety of functions in the maintenance and regulation of life processes. Many proteins are enzymes, and almost all enzymes are globular proteins (Chapter 11.4) (Table 10.11) [9-11]. 

Atoms in the folded myoglobin or hemoglobin protein complexes are normally attached above and below the plane of the Fe-porphyrin (haeme) groups. When the proteins act as oxygen carriers at least one of these linkages is broken to allow the O2 molecule to attach itself to the Fe atom as 

FIGURE 10.18 Globular protein structures—twisted tube representation. Wider parts of tube contain a-helices and narrower parts correspond approx to random coiled chains. Location of Fe-containing haeme groups in these very similar oxygen storage proteins is denoted by H. 

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