Proteins, self-assembly quaternary protein structure

Quaternary structure is akin to the mesostructure of lipid or surfactant self-assemblies, such as the aggregates characteristic of mesh-structures in bacterial protein coats (described in Chapter 4), or the cholesteric liquid crystals found in... 

In this context, some comments on protein crystallisation can be made. The process of crystallisation can be viewed as one of self-assembly of the quaternary structure, although the constituent units now have a well-defined arrangement in space, in contrast to their less rigid shape in liquid crystalline mesophases. Indeed, twisted structures are very commonly found in globular protein crystals, which are reminiscent of the hyperbolic forms of micro- and mesoporous zeolites, described in Chapter 2. 

Important differences between DNA and RNA appear in their secondary and tertiary structures, and so we shall describe these structural features separately for DNA and for RNA. Even though nothing in nucleic acid structure is directly analogous to the quaternary structure of proteins, the interaction of nucleic acids with other classes of macromolecules (for example, proteins) to form complexes is similar to the interactions of the subunits in an oligomeric protein. One well-known example is the association of RNA and proteins in ribosomes (the polypeptide-generating machinery of the cell) another is the self-assembly of tobacco mosaic virus, in which the nucleic acid strand winds through a cylinder of coat-protein subunits. 

One of the most remarkable properties of self-assembly is its ability to generate exceedingly coinplica ted supramolecular structures from fairly simple components. Perhaps the most elegant embodiment of this phenomenon is protein stmcture. Proteins exhibit at least four hierarchies of stmcture primary, secondary, tertiary, and quaternary stmctures. Primary stmcture describes the covalent connections making up the sequence of amino acids in each strand. Secondary stmcture involves local architectural elements created when portions of a strand... 

Self-Association of Proteins The idea that many proteins are non-covalent complexes of two or more identical subunits can be traced back to the early work of Svedberg with the analytical ultracentrifuge (ref. 4). Such protein assembly is referred to as quaternary structure, a topic reviewed by Klotz et al. in 1970 (ref. 5). 

In contrast to supermolecular materials, a supramolecular system is a self-assembled, noncovalently bonded entity, in which molecular units are assembled through noncovalent forces to create a complex structureFor large entities, such supramolecular systems are similar in constitution to those of quaternary structures found in proteins. Thus, for a supramolecular system, it is possible to have variations in the constitution depending on how many molecular units are present and the nature of the bonding interactions. The subunits in the case of supramolecular systems need not be mesomorphic as long as the assembled entity is. 

Quaternary structure (Section 23.4) The structure of a self-assembled aggregate of two or more protein units. 

The subject of this section is the self-assembly of different peptoid (macro-) molecules, something which could be also termed the quaternary structure. We are aware that for structural biologists, quaternary structure is a more or less exactly defined supramolecular assembly of multiple folded proteins. We shall not be so strict, if only to leave room for creativity in what might be accomplished with peptoids in the future. In this section, we will discuss briefly supramolecular structures that have been reported using peptoids. Naturally, this is a field much closer to the heart of the polymer chemist. 

In a related work, a design of enzyme-inspired star block-copolymers with branched topologies and protein-Hke tertiary or quaternary structures was performed. These polymers incorporate hydrophihc, superhydrophobic, and polydentate metal-binding sites and self-assemble in water, their mode of assembly being controlled by the composition of the polymer. An important feature of the star block-copolymers is that they incorporate perfluorocarbons and, due to that, their emulsions in water can attract and preconcentrate O2 in the vicinity of the active metal site. Addition of Cu(II) and TEMPO leads to an effective catalytic system for oxidation of alcohols to aldehydes in water. " ... 

Hydrophobic forces are responsible for self-organization in many molecular systems [53-55]. These forces are recognized as important determinants of tertiary and quaternary structure in proteins, and are responsible for the assembly of biological membranes and other supramolecular systems. In pre-... 

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