Chemical Structure of Viruses
- Virus particles are composed of different kinds of polymeric molecules.
- Viral proteins are divided into structural and non-structural.
- Supercapsid and capsid proteins are referred to structural proteins.
- Supercapsid proteins are subdivided into attachment and fusion proteins.
- Attachment proteins initiate the recognition of susceptible cell by virus and provide specific interaction of virion with cell surface receptors.
- Attachment glycoproteins are presented in viral spikes that bind to the target cells (e.g. hemagglutinin of influenza virus).
- Fusion proteins accelerate viral fusion with the cell membrane.
- Capsid proteins form nucleocapsid protein units (capsomers).
- Non-structural proteins comprise the early precursors of viral proteins; viral enzymes (DNA- and RNA-polymerases, proteases, neuraminidase, etc.); genomic and regulatory proteins, which control transcription and translation.
- Most of non-structural viral proteins are expressed inside the cells in the course of viral infection.
- Viral proteins possess antigenic properties, reacting with specific antibodies and immune cell receptors.
- Besides its own proteins, during maturation the virus may capture the proteins of the host cells. For instance, HIV harbors cellular protein cyclophilin up to 30% of its weight; without this protein HIV particles are non-infectious.
- Viral lipids are obtained from the infected cells, coming into the structure of viral envelope. They provide viral infectivity and stimulate hydrophobic interaction of virus particles with cell membranes facilitating viral entry. Overall, lipid structures comprise up to 30% of enveloped virion mass.
- Carbohydrate residues are usually bound to proteins in glycoprotein complexes of spikes and other viral receptors. They cover about 10-15% of virion contents