Medical Pharmacology Chapter 36:  Antiviral Drugs

• A virus is defined by a nucleic acid which is surrounded by at least one protein.¹ 

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    Viral capsid formed by multiple copies of two proteins

    Antares42: "Schematic diagram of the hexon of a virus capsid made from two protein molecules."3

  • An outer-membrane envelope may also be present and viruses are classified as a parasite since their replication is dependent on host cells.

  • Viral nucleic acids are insufficient encoding for the numerous enzyme proteins required for lipid, carbohydrate, or protein metabolism.

    • Viral nucleic acids are principally concerned with replicating and packaging viral nucleic acids while residing inside host cells.¹ 

  • With respect to the range of infectious agents, viruses are smallest, with diameters extending from about 20 nm to 300 nm in diameter.

    • If one defines the infectious unit as including the nucleic acid, its protein shell and lipid containing membrane, if present, the infectious unit defines the virion.⁴

      • Terms similar to viruses include:   virusoids, viroids, and prions.¹

        • Virusoids are described as nucleic acids which depend on helper viruses for nucleic acid packaging into virus-like particles.¹

          • Dependency on helper viruses classifies virusoids as "satellites."

          • Also, virusoids may be described as sub-viral particles.

        • Viroids ("virus-like") are "small, circular plant pathogens."^1,5,6  

        • Lastly, prions are infectious, abnormal protein molecules which reproduce by altering the structure of their normal cellular protein counterparts.¹ 

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            Multimeric Prion Replication Mechanism⁸

            "A diagram of prion propagation" PrPc is the normal protein. PrPSc is the infectious protein from of PrP.  PrPSc  can induce a change in normal PrPc structure to the infectious form.8,9

          •  

            Fibril Model of Prion Replication¹⁰

            Fibril Model of Prion Replication9, 10

          • Prions have been suggested as possibly causative in certain neurodegenerative diseases including Creutzfeldt-Jakob disease and "mad cow disease", also known as human bovine spongiform encephalopathy.¹

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              Bovine Spongiform Encephalopathy⁷

              "This micrograph of brain tissue reveals the cytoarchitectural histopathologic changes found in bovine spongiform encephalopathy (BSE). "The presence of vacuoles, i.e. microscopic ' holes' in the gray matter, gives the brain of BSE--affected cows a sponge-like appearance when tissue sections are examined in the lab."7

           

• Viral Structure

  • The viral genome may consist of several nucleic acid structural forms.¹

    • Some viruses are based on single-strand or double-strand DNA (1); whereas, others depend on single-strand sense RNA or positive-sense RNA (2).

      • Other possibilities include a single-strand are segmented anti-sense (negative strand) RNA (3) or a double-strand segmented RNA (4).¹

        • A sense strand allows comparison of the RNA base sequence to that of mRNA.^1,11 

          • With positive-sense RNA, the RNA base sequence is the same as mRNA and may serve directly as a protein synthesis template.

          • In negative-sense the viral RNA exhibits a base sequence complementary to that of mRNA.

            • A negative strand RNA virus is also referred to as an anti-sense-strand RNA virus.¹

  • Viruses encode nucleoproteins which are found at the viral particle core and these nucleoproteins associated with viral nucleic acids.

    • The combination of nucleoproteins and viral nucleic acids is enclosed in a protein capsid.

      • Capsids consist of multimeric identical capsomeres which themselves are composed of one or just a few proteins.

        •  

          TMV viral capsid structure^13,14

          "Diagram of the structure of tobacco mosaic virus (TMV)"13	Colm G "Diagram of the structure of tobacco mosaid virus"14

      • Capsid structures are described as having icosahedral or helical symmetry.

        • Icosahedral structures, although resembling spheres, exhibit 2-fold, 3-and 5-fold symmetry axes.

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            Icosahedral Viral Structure¹²  

            "Structure of the icosahedral Cowpea mosaic virus (CPMV) based on PDB ID 2BFU."12

          •  

            Icosahedral structure¹⁴ 

            "Two adenoviruses with a cartoon to show their icosahedral structure."14

  • Viral structural characteristics are important both for classification and describing structural and functional properties of viral proteins.⁴ 

  • Specific functional features that described virus families are themselves specified by virion function such as "morphogenesis and release from infected cells", transmission two new viral hosts and then attachment, penetration, and uncoating in those newly infected cells.⁴

    • More about viral particle symmetry⁴:

      • Several techniques have been used to obtain basic information concerning viral structure at small dimensions.

      • These techniques include x-ray diffraction methods, cryoelectron microscopy and electron microscopy.

        • In electron microscopy surface structure is typically enhanced through the use of heavy metal stains.

          • Sample preparation typical for electron microscopy may result in alterations of particle morphology.

        • Rapidly frozen viruses in combination with cryoelectron microscopy enhances fine structural details.⁴ 

          •  

            Mammalian Reovirus as analyzed by Cryoelectron Microscopy and Surface Reconstruction^16,17

            Left image:  Surface rendering Right image:  Orthogonal section through the viral structure.17

             

        • A higher level of resolution is provided by x-ray crystallographic techniques; however, the application of this method is somewhat limited, since the specimen to be analyzed must be crystalline thus limiting consideration to small, nonenveloped viruses..⁴  

          An example of a common cold virus has nonetheless had its structure resolved by x-ray crystallographic methods.¹⁶ 

          The common cold virus, rhinovirus 14 serves as an example.

          • X-Ray Crystallographic-Based Rhinovirus 14 Structure^16,18

            Panel A, above left, depicts atoms shown as spheres including van der Walls radii16,19 Panel B, above right, shows each α-carbon depicted as a 4 Å sphere.16, 20

          •  

            X-Ray Crystallographic-Based Rhinovirus 14 Structure^16,21

            Panel C, above, shows the virus rendered as a molecular surface with the coloring proportional to the distance from the viral center. In the above panels, viral protein 1 (VP1) is in blue, viral protein 2 (VP2) is in green, and viral protein 3 (VP3) is rendered in red with viral protein 4, an interior protein is not visible.16

    • Viral Structural Symmetry

      • As noted above, one type of viral symmetry is cubic symmetry and with animal viruses the pattern is icosahedral.

        • The icosahedron has 20 faces, each being an equilateral triangle and exhibits twofold axes of rotational symmetry.

        • The icosahedron is shown in the figure below.

          •  

            Icosahedron²²

             

          • The surface of an icosahedron is described by 60 identical subunits.⁴  

            • Accordingly, 60 structural units are required to create the viral shell that encapsulates its genome.

            • Despite the virus expressing icosahedral symmetry, viral shape in terms of physical appearance is spherical.⁴

              •  

                Rotavirus

                The rotavirus which belongs to the Reoviridae virus family is an example of a virus exhibiting iscosahedral capsid symmetry.23 Rotavirus infection is the most common cause of severe diarrhea in young children and infants. 24, 25

            • Parvoviruses represent another example of virions with icosahedral symmetry.²⁸ 

              • This virus contain about 5 kb of single-stranded DNA.

              • The parvovirus capsid protein is described by a "jelly-roll β barrel."²⁸

                • A β-barrel is described as a large β-sheet which twists and coils forming a close structure in which the first strand binds to the last.

                  •  

                    "Jelly-Roll Barrels"²⁷

                    Representation of jelly-roll barrel structure, using "Greek-key" motifs to connect strands across barrels.27

              • Poliovirus is another example of a virus exhibiting icosahedral symmetry.

                • The viral genome codes for three structural proteins.

                • The three poliovirus capsid proteins exhibit the jelly-roll β-barrel structure described earlier.²⁸

                  •  

                    Polio Virus (electron microscopic image)²⁹

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              Poliomyelitis:  Clinical Considerations³⁰

              The polio virus is responsible both for paralytic poliomyelitis and aseptic meningitis.30 Poliomyelitis now considered eradicated in the Western Hemisphere and elsewhere. The virus contains a single-stranded, positive-polarity RNA, acting as an mRNA. The message is translated to a polypeptide that is subsequently cleaved by viral protease into viral proteins. Replication occurs both in the pharynx and in the gastrointestinal tract with the virus spreading to regional lymph nodes and through the bloodstream to the central nervous system. Although most infections are very mild and are asymptomatic, viral infection infrequently may lead to paralysis secondary to anterior horn spinal cord motor neuron death. Although antiviral therapy is unavailable, the disease is presented by either inoculation with inactivated vaccine (Salk vaccine) or attenuated vaccine (Sabin vaccine). These vaccines induce humoral antibodies that neutralize bloodstream virus. The oral vaccine results in intestinal IgA induction limiting transmission by preventing G.I. tract infection. Advantages of the oral administered vaccine include its ability to prevent gastrointestinal tract infections and induce longer immunity. However, the documentation of a few vaccine-induced cases of paralytic polio caused by poliovirus, which had transitioned to a virulent state in the oral vaccine,  led to a recommendation, in some countries, favoring administration of the killed virus.30

          • An isometric (icosahedral) virus particle contains a condensed viral nucleic acid structure.

            • The organization required for the condensed viral nucleic acid structure is facilitated by viral-encoded core proteins or for some viruses cellular histones.

              • Viral cubic structural symmetry can be found in some examples of both DNA and RNA viruses.⁴

        • Helical Symmetry

          • Another type of symmetry is helical symmetry in which protein subunits associate with viral nucleic acid in a helical manner.

            • This process leads to a filamentous nucleic acid-protein assembly (its nucleocapsid) which is then localized in the envelope.

            • Animal viruses exhibiting helical symmetry are typically RNA viruses (rhabdoviruses represent an exception) and form "ball shape" nucleocapsid structures.⁴

              •  

                Example of a virus showing helical symmetry^26,27

                Splettstoesser T ""Diagram of the structure of tobacco mosaid virus (TMV)"26	Colm G TMV Structure"14

          • Rhabdoviruses represent an example of an animal virus exhibiting helical symmetry.

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              Vesicular Stomatitis Viral Capsid³¹

            • Vesicular stomatitis is caused by a Rhabdovirus, specifically vesicular virus from the Rhabdoviridae family.

              Vesicular Stomatitis:  Clinical Considerations³²

              This virus causes vesiculation and ulceration of oral tissues, feet, and teats and may present in animals in a manner clinically indistinguishable from foot-and-mouth disease, which is more dangerous. Human transmission occurs as a result of direct contact with cattle most typically and occasionally following laboratory exposure. In human disease, clinical manifestations include an early conjunctivitis followed by an acute influenza-like presentation with fever, chills, nausea, headache, vomiting, myalgias, retrobulbar pain, substernal pain, pharyngitis, malaise, and lymphadenitis. Vesicular lesions may be noted on buccal mucosa or on fingers. Encephalitis rarely occurs. The illness lasts about 3-6 days resulting typically in complete recovery. Serologic diagnosis is based on an increase in complement-fixing or neutralizing antibody titer elevation.