A model for vicilin solubility at mild acidic pH, based on homology modelling and electrostatics calculations

The crystallographic structures of jack bean canavalin and Frenchbean phaseolin have been used to construct a homology modelof the storage vicilin of cocoa. Reported molecular weightsfor cocoa storage protein subunits correlate with proteolysisat the site of a large hydrophilic insert in the mature protein.Burial of the hydrophobic amino acids on trimer formation isa strongly conserved feature in the vicilin family. Histidineresidues also sit at the monomer-monomer interfaces of the trimerand are likely to contribute to the decreased solubility ofcocoa vicilin at mild acidic pH, which is generally consideredto be caused solely by aggregation near to the isoelectric point.Electrostatic calculations suggest that such an arrangementof histidine residues in the absence of specific counterionbinding will not favour the particular geometry of trimer formationbelow neutral pH. Higher order aggregates that do not excludehistidine charge from the solvent may be favoured, aiding theprecipitation of cocoa vicilin at mild acidic pH. This suggestionis considered for the vicilin family. The hypothesis could contributeto an understanding of the pH and ionic strength dependenceof vicilin solubility in vitro, and possibly of the behaviourof vicilins in the seed storage environment