3.1 VIRUS PARTICLES ARE CONSTRUCTED FROM SUBUNITS

While proteins may have regular secondary structure elements in the form of α helix and β structure, the tertiary structure of the protein is not symmetrical. This is a consequence of hydrogen bonding, disulfide bridges, and the intrusion of proline in the secondary structure. Although it may be naïvely thought that the nucleic acid could be covered by a single, large protein molecule, this cannot be the case since proteins are irregular in shape, whereas most virus particles have a regular morphology (Fig. 3.1). However, that viruses must contain more than a single protein can also be deduced solely from considerations of the coding potential of nucleic acid molecules. A coding triplet has an Mr of approximately 1000 but specifies a single amino acid with an average Mr of about 100. Thus a nucleic acid can at best only specify one-tenth of its mass of protein. Since viruses frequently contain more than 50% protein by mass, it is apparent that more than one protein must be present. Obviously, less genetic material is required if the single protein molecule specified is to be used as a repeated subunit. However, it is not essential that the coat be constructed from identical subunits, provided the combined molecular weights of the different subunits is sufficiently small in relation to the nucleic acid molecule which they protect. There is a further advantage in constructing a virus from subunits, since any misfolding of protein (occurring commonly at a frequency of 1 in 1000 and for which there is no repair mechanism) affects only a small part of a structural unit. Thus, provided that faulty subunits are not included in the virus particle during assembly, an error-free structure can be constructed with the minimum of wastage. The necessary physical condition for the stability of any structure is that it be in a state of minimum free energy, so it can be assumed that the maximum number of interactions is formed between the subunits of a virus particle. Since the subunits themselves are nonsymmetrical, the maximum number of interactions can be formed only if they are arranged symmetrically, and there are a limited number of ways this can be done. Shortly after their seminal work on the structure of DNA, Watson and Crick predicted on theoretical grounds that the only two ways in which asymmetrical subunits could be assembled to form virus particles would generate structures with either cubic or helical symmetry. (However an important rider to the energy status of virus particles is that at least some are suspected of being metastable, and that their true minimum energy state is reached only after they have undergone uncoating during the process of infecting a cell


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