Chemistry and Biology of Proteins - F. Haurowitz 1953

Protein Synthesis
Viral Particles

Viral particles were first purified and isolated in crystalline form by Stanley, who successfully recrystallized tobacco mosaic virus using ammonium sulfate fractionation. In some respects, the virus behaves like a pure protein. For instance, it can be dissolved and subsequently recrystallized by precipitation with ammonium sulfate. However, the resulting viral crystals appear to be "living," as they retain The ability to multiply in plants inoculated with trace amounts of the virus. In this process, The amount of virus can increase approximately 1,000,000-fold compared to the initial inoculum [112].

As might be expected, the report that crystalline preparations are capable of reproduction aroused immense interest among biologists and chemists alike. The ability to crystallize has traditionally been regarded as a property of non-living matter, such as minerals, and was associated with a rigid Structure incapable of change. Since life implies a continuous alteration of Composition and Structure, the capacity to crystallize seemed entirely incompatible with all prevailing concepts regarding The Nature of living matter.

It should not be forgotten, however, that a crystalline form signifies, essentially, nothing more than the regular repetition of structural elements. If a vast number of small Structural elements of identical size and shape aggregate to form a regular three-dimensional spatial lattice of identical particles, this lattice will appear to our eyes as a crystal, even if a continuous redistribution of atoms and molecules takes place within each particle. This applies to inorganic crystals as well. Using Infrared Spectroscopy and other Physicochemical Methods, it has been demonstrated that many molecular groups within these crystals undergo continuous vibrational motion.

The Study of Viruses is of paramount importance for solving The problem of METABOLISM/35.html">Protein Biosynthesis. This is indicated by the fact that all viruses, even the simplest ones, contain PROTEIN AND NUCLEIC acids [113]. Lower viruses contain only ribonucleic acid, whereas higher viruses include both deoxyribonucleic acid and ribonucleic acid. The high nucleic acid content of viruses suggests that a significant portion of their Proteins consists of acidic Nucleoproteins. Within the pH range lying between the isoelectric points of proteins and Nucleic Acids, they can combine with serum albumin and other proteins to form mesomorphic fibers that are insoluble at low Ionic strength [114].

The shape and Structure of viral particles are determined primarily by their constituent proteins rather than nucleic acids, since the latter can be cleaved off with appropriate Enzymes without disrupting either the particle shape or structure [115]. Nevertheless, Treatment with Nucleases results in the loss of viral activity and their ability to reproduce.

The ability of a virus to crystallize is not impaired by the loss of its nucleic acid component. Turnip yellow mosaic virus can be obtained in crystalline form both before and after the removal of nucleic acid [116].

Investigation of viral particles in polarized ultraviolet light has shown that the planes of all purine rings of the nucleic acid are parallel to one another and positioned perpendicularly to the central axis of the elongated particles [117].

The Molecular Weight of tobacco mosaic virus particles is close to 50,000,000 [118]. The validity of this high figure, determined by ultracentrifugation, was confirmed by studies of Flow Birefringence and Electron Cell/15.html">Microscopy. Similar values have been found for Other types of viruses. The molecular weight of the rabbit papilloma virus is approximately 20,000,000 [119], while the molecular weight of a bacteriophage isolated from staphylococci is 300,000,000 [120].

Electron micrography has established that tobacco mosaic virus particles have the shape of elongated rods 15 mμ in thickness and 300 mμ in length [121]. The diameter of Influenza virus particles is approximately 100–210 mμ [122]. In concentrated urea solutions, tobacco mosaic virus dissociates into particles with a molecular weight of 100,000 [123]. Disintegration of the viral protein can also be induced by treatment with sodium dodecyl sulfate [124].

The size and diameter of viral particles vary within wide limits; the smallest viruses, such as FOOT-and-Mouth disease viruses, have a diameter of about 10 mμ, whereas the diameter of the largest viral particles, such as those of the psittacosis virus, reaches an average of 275 mμ [118].

Large viruses may exceed small Bacteria in size. Only plant viruses form rod-shaped crystals. Animal viruses have not yet been obtained in crystalline form.

The chemical composition of viruses with small particles is characterized by simplicity; they contain proteins and nucleic acid but are almost devoid of free Introduction/36.html">CARBOHYDRATES and Lipids. Larger viral particles, on the other hand, have a significantly more complex composition and include not only lipids but also various enzymes [125]. Analysis of the T-2 bacteriophage of Escherichia coli revealed that it contains 51% protein, 5–6% lipids, and 40% nucleic acid; the nucleic acid fraction comprises 1/6 ribonucleic acid and ⁵/₆ deoxyribonucleic acid [126].

Analysis of the Amino Acid Composition of T-4 bacteriophage E. coli proteins established the extremely interesting fact that The amino acid composition of this viral protein is nearly identical to the amino acid COMPOSITION OF THE intestinal bacteria themselves [127]. This remarkable similarity in amino acid makeup suggests that the viral protein is formed by the rearrangement of host Amino Acids and that The conversion of host protein into viral protein may not be accompanied by deamination or other deep-seated degradation processes of amino acids. On the other hand, however, it has been demonstrated that N¹⁵H₃ added to the medium in which the virus multiplies penetrates into the virus, whereas bacterial nitrogen is not utilized by the virus [128]. This would seem to suggest that the bacteriophage viral protein is formed at the expense of substances present in the nutrient medium rather than from bacterial protein. Nevertheless, experiments with radioactive phosphorus showed that about 75% of the phosphorus entering into this virus consists of radioactive phosphorus from the medium, while bacterial phosphorus is utilized for building the remaining 25%. It is very difficult to interpret these contradictory data. The possibility cannot be excluded that the host bacterium provides the enzymes necessary for the synthesis of viral proteins [128].

Their small size, crystalline form, and ability to reproduce make viruses one of the most fascinating objects of biological research. If viruses are considered truly living entities, it must be acknowledged that they are the simplest of all living creatures. Yet, are they genuinely alive? At present, this paramount question cannot yet be answered satisfactorily, despite the participation of leading specialists in the Discussion of the problem. The most important difference between viruses and other living organisms, such as bacteria, is that viruses reproduce only within living Cells.

The presence of living host cells is an essential prerequisite for virus growth. Dead tissue represents a barrier through which multiplying viruses cannot penetrate [131]. Unfortunately, we still do not know the cause of this phenomenon. It is possible that it is related, to some extent, to the surface CHARACTERISTICS OF THE host cell structural elements. Apparently, viruses cannot live in homogeneous solutions, and their existence requires them to be in end-to-end contact with each other or in contact with the structural elements of host cells [129].

Since viruses can be recrystallized in the same manner as typical proteins, many researchers view viral particles as individual molecules, referring to the particle weight determined by ultracentrifugation or electron microscopy as the molecular weight. However, isolated viral particles are not entirely uniform in size, and the molecular weight found represents an average value. In any case, we are hardly justified in calling viral particles molecules at all [130].

Viral reproduction bears a certain resemblance to the autocatalytic conversion of a proenzyme into an enzyme. Minute amounts of Trypsin are capable of converting a significant quantity of trypsinogen into trypsin; similarly, The addition of traces of Pepsin is sufficient to transform a large quantity of pepsinogen into pepsin [156]. An analogy can be drawn between virus growth and enzyme activation by stating that the increase in the amount of trypsin observed upon introducing traces of trypsin into a trypsinogen solution is the result of trypsin multiplication. On the other hand, the animal host protein can be termed a provirus and virus reproduction regarded as The process of converting provirus into virus [131, 156].

Obviously, with equal justification, we can call a virus both "living" and "non-living" [129]. We must admit that at present there is no criterion for an unambiguous solution to this critical problem.