Self-assembly of glutamic dehydrogenase into ordered superstructures: Multichain tubes formed by association of single molecules

Abstract

Linear polymers of bovine liver glutamic dehydrogenase formed by association of single enzyme molecules further associate to generate tubular structures consisting of four helical polymer chains. The helical parameters and the number of chains comprising the tubes were derived from an analysis of optical diffraction patterns and Fourier transforms of their electron microscopic images. Each of the four polymer chains is inclined to the tube axis at an angle of 28.5 ° ± 1.5 ° and describes a helical path having a pitch of approximately 800 Å containing between 9 and 9 1 3 molecules per turn of the helix. The distance between molecules along the chain direction was found to be 100 Å. Examination of several thousand particles revealed the existence of morphologically related species of tubes which differ from one another in the relative disposition of the polymer chains. In the simplest of these, the four polymer chains comprising the tube are related by a 4-fold rotation axis coincident with the tube axis (line group sr, N = 4 in the nomenclature of Klug, Crick & Wyckoff (1958)). Due to the rotation axis this species of tube displays a characteristic annular appearance reminiscent of the stacked disc form of tobacco mosaic virus protein. The spacing between adjacent annuli is 80 Å. In a second form of the tube, the polymer chains are staggered one-quarter of a molecular length relative to one another and thus the polymer chains are no longer related by the 4-fold rotation axis. The positions of the molecules in both structures may be described by a lattice (helix net) in which the environment of each molecule is equivalent to that of every other molecule. The structure of the third form of tube examined in detail, displays an additional level of complexity. In this species each polymer chain undergoes a sinusoidal displacement of one-quarter of a molecular length relative to its neighbor. The effect of the sinusoidal displacement is to place each of the four polymer chains in a different environment. Thus in this form of tube, the enzyme molecules occupy four symmetrically distinct but quasiequivalent positions. The organization of the different species of tubes is analyzed in terms of two different sets of bonding vectors. One set of bonds gives rise to the polymerization of individual enzyme molecules to form the linear chains and the second set defines the mode of association of the chains to form the tubes. The morphologically distinct forms of the tubes appear to arise as a result of a multiplicity of approximately equally probable configurations of this latter set of bonds. This view is supported by calculations which suggest that linear polymers of glutamic dehydrogenase possess a higher degree of flexibility than has been suspected.