Principles of Protein Structure - H. Schulz 1982

Mechanisms of polypeptide chain folding and association
Aggregates of globular proteins
Symmetry of aggregates

Asymmetric aggregates perform complex Functions. All aggregates can be subdivided into asymmetric and symmetric ones. A typical asymmetric aggregate (which is also a component of RNA) is the ribosome. The functions of Ribosomes in Protein Synthesis are so complex that their execution requires a large group of different Proteins, and non-identical proteins can only aggregate asymmetrically.

Aggregates may possess spatial, linear, or point group symmetry. Symmetric aggregates can be divided into those with spatial or linear symmetry, as well as point group symmetry. Spatial group symmetry has been found in Insulin crystals, which are formed in the Pancreas and provide a shape that can be maintained at a negligible osmotic pressure [259]. Symmetry of the same type is observed in striated Muscles of vertebrates and insects [215]. Linear groups have been found in microcapillaries [181], tobacco mosaic virus [180], and filamentous phages [220]. Point group symmetry is very common. The symmetry of Amino Acids precludes point groups containing inversion or reflection centers, so that only the groups n, n2, 23, 432, 532 are possible for n = 1, 2, 3 ... [252, 260]. Examples of all these groups, except for 23, are given in Table 5.4.

As already discussed in previous sections, symmetric formations are energetically favorable. Apparently, only in the case of hexokinase is there observed asymmetric aggregation of identical units, and even then only in the crystalline state [261]. In some cases, a "pseudosymmetric" arrangement may occur. Normal human Hemoglobin, for example, has ideal 2 symmetry and 222 pseudosymmetry. The pseudosymmetry would be exact if the ß- and α-subunits were identical. If all subunits are chemically identical but have two slightly differing Conformations—i.e., forming rather close contacts—then so-called quasisymmetry arises. It is frequently encountered in spherical Viruses. In the presence of quasisymmetry, a single type of protein can form an icosahedral shell [262] that is approximately spherical. In tomato bushy stunt virus, the subunit contains two Structural domains. The conformational difference is caused by the relative mobility of these domains [263].