STRUCTURAL AND ELECTROSTATIC PROPERTIES OF GLOBULAR-PROTEINS AT A POLYSTYRENE WATER INTERFACE

The structural and electrostatic properties of two similar-sized globu lar proteins, hen egg-white lysozyme and bovine milk alpha-lactalbumin , adsorbed to microspheres of a negatively charged polystyrene latex, are determined from adsorption isotherms, plateau adsorption values, a nd isothermal titration microcalorimetry data. Particular attention is given to the application of proton-titration techniques for which a n ew robust protocol is presented which accurately predicts the contribu tions of the aqueous solution (i.e., the blank) to the overall titrati on behavior of the protein-containing sample. Plateau values for the a dsorbed proteins show a complex dependence on pH with a maximum occurr ing at the isoelectric point of the protein/sorbent complex. Adsorbed- state titration curves differ markedly from the corresponding dissolve d-state curves; the data suggest that alpha-lactalbumin undergoes subs tantial changes in secondary and tertiary structure upon adsorption. I n the adsorbed state, alpha-lactalbumin does not exhibit the low-pH na tive-to-apo-state transition which promotes release of the Ca2+ ion fr om the ion-binding pocket. Average pK(a) data for the two proteins sug gest that a fraction of the charged carboxyl groups are in close proxi mity to the sorbent surface and the adsorbed protein is largely denatu red. The data also indicate that protein adsorption is driven by a syn ergistic combination of sorbent dehydration, protein structural change s, and charge redistribution. (c) 1994 Academic Press, Inc.