Complexation of Proteins with Polyelectrolytes in a Salt-Free System and Biochemical Characteristics of the Resulting Complexes

The present chapter deals with the complexation of proteins with polyelectrolytes in a salt-free system with regard to the following two aspects. The first is how an irregular distribution of ionizable groups attached along the length of inflexible protein molecules affects the stoichiometry of salt-linkage formation with oppositely charged flexible polyelectrolytes. And the second is to what extent biochemical ability is maintained in the resulting complexes. Three proteins, human serum albumin (HSA), human or bovine hemoglobin (Hb), and bovine trypsin (BT) were chosen as samples in view of the information available on their amino acid sequences, three-dimensional conformations, structures of biochemically active sites, etc. Poly(diallyldimethylammonium chloride) (PDDA) and potassium poly(vinyl alcohol sulfate) (KPVS), the polyion charges of which are independent of pH within the range 2 to 13, were employed as the polyelectrolytes. The stoichiometry of complexation was studied as a function of pH using a colloid titration method. The biochemical properties of the resultant stoichiometric complexes were examined in terms of the binding affinity of cyanide ions with the heme (ferri-protoporphyrin IX) in the ternary KPVS/Hb/PDDA complex, and also of the protease activity of the BT/KPVS complex. A model for stoichiometric complexes of proteins with polyelectrolytes is proposed on the basis of the results obtained. Also presented are the functional capabilities of the ternary KPVS/Hb/PDDA complex as a cyanide ion exchanger, and of the BT/KPVS complex as an immobilized enzyme.