8 MACROMOLECULAR COMPLEXES 



action are so sensitive to distance, a small imperfection in fit pro- 

 duces a large change in interaction energy. 



A product of the short-range interaction between two molecules 

 might well be a complex having new properties. A clear case of this 

 tvpe is the dissociation or fragmentation of the protein ribonuclease 

 into two enzymatically inactive portions ( Richards and Vithayathil, 

 1959 ) . Although the fragments have been produced by hydrolyzing 

 a covalent bond, the fragments, on mixing, combine to give a com- 

 plex which has full enzymatic activity. The ruptured covalent bond 

 is not re-foriped on mixing. A variety of exciting possibilities is 

 brought to our attention by this discovery. For example, the inter- 

 action of a steroid hormone molecule with an inactive protein might 

 induce enzymatic activity in the latter, this combination controlling 

 a host of subsequent reactions. 



Specific interactions are undoubtedly also involved in establish- 

 ing condensed regions, which usually appear as filaments, lamellae, 

 or helices, at levels of size within an order of magnitude of 100 A. 

 Currently, the lamellar structure containing lipid and protein ap- 

 pears to be a conspicuous component of cells, the assumption being 

 made that the lipid within the lamella will be present as a double 

 layer or layers. Certainly such an organization of lipid would be ex- 

 pected from the properties of soaps and lipid emulsions (Luzzati 

 et ai, 1958; Lawrence, 1958; Palmer and Schmitt, 1941) and the 

 structure of the myelin sheath (Schmitt, 1959). The double-layer 

 arrangement permits the bulky non-polar portions of the lipid mole- 

 cules to associate, leaving surface layers of polar groups to interact 

 with each other and with water, protein, or other polar substances. 



In many instances the development of a lamella may not involve 

 any high degree of fit (specificity) of the polar and non-polar por- 

 tions of the molecules involved. We would expect a low degree of 

 specificity mainly when the interaction leads to the development of 

 a structure which carries out some non-critical phvsical function, 

 a safe example being that of fat storage— although another might be 

 the essentially two-dimensional expansion of non-functional por- 

 tions of a plasma or other membrane. Certain functional aspects, 

 however, may require a specific arrangement of non-polar and polar 

 portions which will provide an energy barrier to penetration (per- 

 meability). Although the physical conditions establishing the basis 

 for interaction in monomolecular films are probably not duplicated 

 often in biological systems, the effects of short-range interaction in 



