ABSTRACT
Essentially proteins are amino acids linked together by polypeptide linkages (–CO-NH). Th ese linkages are formed between the carboxyl group (–COOH) of one amino acid to the amino (–NH2) group of another. Several peptides together make a polypeptide. A linear polypeptide has two terminals, N and C. N terminals usually occur at the left hand side whereas –C terminals occur at the right hand side. Th ese terminals regulate the acidity and basicity of the amino acids. Th e properties of the polypeptides vary with the diff erences in the sequences of the acidic and basic amino acids of the polypeptides. Natural, biologically active, protein molecules are not always simple linear polypeptides as they may be spirally or helically coiled to form secondary structures which may be further bended or folded to give rise to tertiary structures; interactions between them may give rise to quaternary structures. Formation of these complex structures from the native linear conformation contributes to their varied biological properties. Such complex protein molecules can be denatured and renatured by changing the environment in which they operate. In principle viral coat proteins do not differ from those found in the cells of the organisms except in their amino acid sequences. Th e amino acid composition and amino acid sequences diff er from virus to virus and are an important criterion for diff erentiating one virus from another. Th e size of plant viral proteins normally ranges from 150 to 600 or more amino acid residues. Normally, in simple virus particles, only one type of amino acid sequence is found, whereas in the particles of complex viruses, more than one polypeptide occurs and diff erent types of amino acid sequences are observed depending upon the number of polypeptide species. Coat proteins of most of the viruses have acetylated N terminals. As for structures, mostly secondary and tertiary forms are found; amongst the secondary forms the α-helix arrangement is common in helical particles. Th e size of the protein, its structural conformation and amino acid sequence determine the properties of the coat protein and this contributes to the stability, antigenicity, transmissibility by vectors and several other properties of the virus particles.
