I am not sure what you mean by "pH of protein" ?? Do you mean the proteins isoelectric point pI ?? That is the pH at which the proteins total charge is zero. This is the balance of the charge states of all the -COOH -NH2, -OH charged side chains on the protein as well as the terminal COOH and terminal NH2 ? If this is the case I am not sure it would be 'calculable' as the presence of counter ions and other factors especially the tertiary structure of the protein may affect those charge states ?