A set of wild-type and mutant human, woodchuck, and duck
hepatitis viral core proteins have been prepared and used to study the free
thiol groups and the
disulfide bonding pattern present within the core particle. Human (
HBcAg) and woodchuck (WHcAg) core
proteins contain 4
cysteine residues, whereas duck (DHcAg) core
protein contains a single
cysteine residue. Each of the cysteines of
HBcAg has been eliminated, either singly or in combinations, by a two-step mutagenesis procedure. All of the
proteins were shown to have very similar physical and immunochemical properties. All assemble into essentially identical core particle structures. Therefore
disulfide bonds are not essential for core particle formation. No intra-chain
disulfide bonds occur. Cys107 is a free
thiol buried within the particle structure, whereas Cys48 is present partly as a free sulfhydryl which is exposed at the surface of the particle. Cys61 is always and Cys48 is partly involved in interchain
disulfide bonds with the identical residues of another monomer, whereas Cys183 is always involved in a
disulfide bond with the Cys183 of a different monomer. WHcAg has the same pattern of bonding, whereas DHcAg lacks any
disulfide bonds, and the single free sulfhydryl, Cys153 which is equivalent to Cys107 of
HBcAg, is buried.