HYDROGEN DEUTERIUM EXCHANGE KINETICS OF APOLIPOPHORIN-III IN LIPID-FREE AND PHOSPHOLIPID-BOUND STATES - AN ANALYSIS BY FOURIER-TRANSFORM INFRARED-SPECTROSCOPY/

Attenuated total reflection Fourier transform infrared spectroscopy wa s used to probe the kinetics of hydrogen/deuterium exchange in Manduca sexta apolipophorin-III (apoLp-III), ApoLp-III is an exchangeable apo lipoprotein that is made up of five elongated amphipathic alpha-helice s in a helical bundle conformation in the monomeric lipid-free form, U pon interaction with phospholipids, it is postulated to undergo a larg e con formational change whereby the hydrophobic interior is exposed, facilitating binding to the lipid surfaces, We have used the lipid-fre e and dimyristoylphosphatidylcholine-bound apoLp-III to study the dyna mically variable domains in the two forms, Three populations of amide protons varying in their hydrogen/deuterium exchange rates were found to exist: slow, intermediate, and fast exchanging, which could corresp ond to completely buried, partially buried, and solvent-exposed domain s on the protein in both the states. In lipid free apoLp-III, 36, 12, and 52% of the total residues contributed to the slow, intermediate, a nd fast exchanging populations, respectively, In the dimyristoylphosph atidylcholine-bound form, the corresponding distribution was 20, 16, a nd 64%, representing a 12% increase in the number of exposed residues. The results are discussed in terms of increased solvent accessibility due to gross tertiary structural reorganization.