Viruses hexagonal

Viruses essentially consist of genetic material (nucleic acids, green strands in (A) and a capsular envelope made up of proteins (blue hexagons), often with a coat (gray ring) of a phospholipid (PL) bilayer with embedded proteins (small blue bars). They lack a metabolic system but depend on the infected cell for their growth and replication. Targeted therapeutic suppression of viral replication requires selective inhibition of those metabolic processes that specifically serve viral replication in infected cells. 

Fig-1 Transmission electron micrographs of thin sections of infected cells of Phaeocystis pouchetii. (a) The virus-like particles (indicated by arrow) are found in the cytoplasm of the cells, (b) Detail of virus-like particles showing the hexagonal outline of the viruses... 

Multiple interactions in the same plane can lead to the formation of sheets where, for example, each monomer can interact with six neighbors in a hexagonal close-packing arrangement (Fig. 5-8). Sheets can, with a slight readjustment, be converted into cylindrical tubes (Fig. 5-8) or even into spheres. These closed structures can provide even greater stability since they maximize the number of interactions that can be made. The protein coats of certain viruses are excellent examples of this. Microtubules, which consist of the protein tubulin, can be converted readily between sheet and tubular forms, at least in the purified form. 

As an example of large unit cell size, data have been recorded from crystals of cowpea mosaic virus to 4.3 A resolution at LURE [234]. The hexagonal unit cell has dimensions a=451 A, c = I038 A. The oscillation range was 0.4°. 

Figure 10.18 (a) A 0.4° oscillation photograph of the hexagonal crystal form of cow pea mosaic virus obtained at LURE. At the start of the exposure the crystal had been rotated 6.4°... 

Fig. 16.2. A section of the alignment of sequences of aspartic proteinases achieved by comparing the three-dimensional structures using COMPARER [14]. HIV human immunodeficiency virus RSV Rous sarcoma virus APE endothiapepsin APP penicillopepsin APR rhizopuspepsin PEP hexagonal porcine pepsin CHY calf chymosin. The last letter refers to the amino (N) or car-boxy (C) terminal domains of the pepsins. The coordinates of the three-dimensional structures were obtained from the PDB databank [24]. The amino acid code is the standard one-letter code (see Appendix C) formatted using the following conventions [7] ...

For evaluation of 6 we have assumed that the virus would be hexagonally closest-packed in a two-dimensional array on the adsorbent surface where 6 equals unity. The quantity x was estimated to be 50 from space filling models where approximately 3% of a 27-nm diameter icosahedral virion face is estimated, on the basis of globular protein structure (12), to approach the surface of the solid close enough to displace interfacial water molecules. If our estimate is changed to 10 or 250, instead of a most probable value of about 50, Equation 2 predicts that the AGads will be shifted by only zb 4 kj mol, which is comparable to the uncertainty given by experimental data scatter. 

Dilute, highly purified poliovirus, which would occupy less than 0.03 cm if hexagonally closest packed in a two dimensional array, was emulsified with equal volumes (10 mL) of 0.02 Z, pH 7 buffer and C2CI3F3 in borosilicate extraction tubes at 25°C. The emulsion was allowed to separate spontaneously, which took about 5 min, and the aqueous phase was sampled for residual virus. In Table IX we see only a small decrease in infectivity and radioactivity in the C2Cl3F3-extracted samples, compared to controls run in parallel but not containing C2CI3F3. This experiment was repeated and essentially identical results were obtained. 

The overall shape of a virus varies. The classic viral shape most often seen in the literature has a hexagonal capsid with a rod sticking out of it that attaches to the host cell and acts like a syringe to inject the nucleic acid. Figure 14.2 shows the T2 bacteriophage of E. colt, a classic example of a virus of this shape. Tobacco mosaic virus (TMV), on the other hand, has a rod shape, as shown in Figure 14.3. 

FIGURE 14.2 An electron micrograph of a hexagonal virus. The bacteriophage T2 virus was gently disrupted, releasing the DNA, which can be seen as many loops outside the virus. 

What is the structure of a virus At the center of a virus is its nucleic acid. This is surrounded by a protein coat called a capsid. The combination of the nucleic acid and the capsid is called the nucleocapsid. Many viruses also have a membrane envelope surrounding the nucleocapsid. Some also have protein spikes that help the virus attach to a host cell. Viruses have various shapes. Some are rod shaped, like the tobacco mosaic virus. Others have a hexagonal shape, like the bacteriophage T2 virus. 

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