Isologous association

Cyclic (Rotation) symmetry It is the simplest arrangement possible, involving isologous association of subunits. The disposition of the subunits is such that there is an n-fold rotational axis where n is the number of subunits. A rotation of 360°/n transposes the structure into itself. This type of symmetry is designated as Cn. 

Fig. 2.37. Modes of association of protomers in oligomeric proteins (I) isologous associations (II) heterologous associations (III) heterologous tetramer (IV) isologous tetramer (pseudotetrahedral) and (V) exact and pseudoisologous associations in a tetramer I, II, III from Monod et ai, 1965) (IV), V from Matthews and Bernhard, 1973).

FIGURE 6.42 Isologous and heterologous associations between protein subunits. 

Now consider the association of A and B to form oligomers in which the intrinsic binding constants KAX and Kbx have the same values as in the monomers. Since more enzymes apparently exist as isologous dimers than as any other oligomeric form,116 it is appropriate to consider the behavior of such dimers in some detail. Monod et al. emphasized that both... 

In the isologous tetrahedron (Fig. 7-5) the third set of paired interactions cl must be taken into account. (However, the third interaction constant will not be an association constant of the type represented by Xfl AA and KhkAA but a dimensionless number.) On the other hand, the heterologous square has only a single interaction constant. 

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