Macromolecular assembly


The term macromolecular assembly refers to massive chemical structures such as viruses and non-biologic nanoparticles, cellular organelles and membranes and ribosomes, etc. that are complex mixtures of polypeptide, polynucleotide, polysaccharide or other polymeric macromolecules. They are generally of more than one of these types, and the mixtures are defined spatially, and with regard to their underlying chemical composition and structure. Macromolecules are found in living and nonliving things, and are composed of many hundreds or thousands of atoms held together by covalent bonds; they are often characterized by repeating units. Assemblies of these can likewise be biologic or non-biologic, though the MA term is more commonly applied in biology, and the term supramolecular assembly is more often applied in non-biologic contexts. MAs of macromolecules are held in their defined forms by non-covalent intermolecular interactions, and can be in either non-repeating structures, or in repeating linear, circular, spiral, or other patterns. The process by which MAs are formed has been termed molecular self-assembly, a term especially applied in non-biologic contexts. A wide variety of physical/biophysical, chemical/biochemical, and computational methods exist for the study of MA; given the scale of MAs, efforts to elaborate their composition and structure and discern mechanisms underlying their functions are at the forefront of modern structure science.
, which catalytically translate the information content contained in mRNA molecules into proteins. The animation presents the elongation and membrane targeting stages of eukaryotic translation, showing the mRNA as a black arc, the ribosome subunits in green and yellow, tRNAs in dark blue, proteins such as elongation and other factors involved in light blue, the growing polypeptide chain as a black thread growing vertically from the curve of the mRNA. At end of the animation, the polypeptide produced is extruded through a light blue SecY pore into the gray interior of the ER.

Biomolecular complex

A biomolecular complex, also called a biomacromolecular complex, is any biological complex made of more than one biopolymer or large non-polymeric biomolecules. The interactions between these biomolecules are non-covalent.
Examples:
The biomacromolecular complexes are studied structurally by X-ray crystallography, NMR spectroscopy of proteins, cryo-electron microscopy and successive single particle analysis, and electron tomography.
The atomic structure models obtained by X-ray crystallography and biomolecular NMR spectroscopy can be docked into the much larger structures of biomolecular complexes obtained by lower resolution techniques like electron microscopy, electron tomography, and small-angle X-ray scattering.
Complexes of macromolecules occur ubiquitously in nature, where they are involved in the construction of viruses and all living cells. In addition, they play fundamental roles in all basic life processes. In each of these roles, complex mixtures of become organized in specific structural and spatial ways. While the individual macromolecules are held together by a combination of covalent bonds and intramolecular non-covalent forces, by definition MAs themselves are held together solely via the noncovalent forces, except now exerted between molecules.

MA scales and examples

The images above give an indication of the compositions and scale associated with MAs, though these just begin to touch on the complexity of the structures; in principle, each living cell is composed of MAs, but is itself an MA as well. In the examples and other such complexes and assemblies, MAs are each often millions of daltons in molecular weight, though still having measurable component ratios at some level of precision. As alluded to in the image legends, when properly prepared, MAs or component subcomplexes of MAs can often be crystallized for study by protein crystallography and related methods, or studied by other physical methods.
MAs. Yellow-orange indicates hydrophobic lipid tails; black and white spheres represent PL polar regions. Bilayer/liposome dimensions : hydrophobic and polar regions, each ~30 Å "thick"—the polar from ~15 Å on each side.
, with 30 copies of each of its coat proteins, the small coat protein and the large coat protein, which, along with 2 molecules of positive-sense RNA constitute the virion. The assembly is highly symmetric, and is ~280 Å across at its widest point.
Virus structures were among the first studied MAs; other biologic examples include ribosomes, proteasomes, and translation complexes, procaryotic and eukaryotic transcription complexes, and nuclear and other biological s that allow material passage between cells and cellular compartments. Biomembranes are also generally considered MAs, though the requirement for structural and spatial definition is modified to accommodate the inherent molecular dynamics of membrane lipids, and of proteins within lipid bilayers.

Research into MAs

The study of MA structure and function is challenging, in particular because of their megadalton size, but also because of their complex compositions and varying dynamic natures. Most have had standard chemical and biochemical methods applied. In addition, their methods of study include modern proteomic approaches, computational and atomic-resolution structural methods, small-angle X-ray scattering and small-angle neutron scattering, force spectroscopy, and transmission electron microscopy and cryo-electron microscopy. Aaron Klug was recognized with the 1982 Nobel Prize in Chemistry for his work on structural elucidation using electron microscopy, in particular for protein-nucleic acid MAs including the tobacco mosaic virus. The crystallization and structure solution for the ribosome, MW ~ 2.5 MDa, an example of part of the protein synthetic 'machinery' of living cells, was object of the 2009 Nobel Prize in Chemistry awarded to Venkatraman Ramakrishnan, Thomas A. Steitz, and Ada E. Yonath.

Non-biologic counterparts

Finally, biology is not the sole domain of MAs. The fields of supramolecular chemistry and nanotechnology each have areas that have developed to elaborate and extend the principles first demonstrated in biologic MAs. Of particular interest in these areas has been elaborating the fundamental processes of molecular machines, and extending known machine designs to new types and processes.

General reviews