E. Terzi et al., ALZHEIMER BETA-AMYLOID PEPTIDE-25-35 - ELECTROSTATIC INTERACTIONS WITH PHOSPHOLIPID-MEMBRANES, Biochemistry, 33(23), 1994, pp. 7434-7441
The role of lipids in the aggregation of three Alzheimer model peptide
s was investigated with circular dichroism spectroscopy and high-sensi
tivity titration calorimetry under conditions of low ionic strength. I
n solution, the peptides beta AP(25-35)OH and beta AP(25-35Nle)NH2 exh
ibit a reversible random-coil reversible arrow beta-sheet (or beta-str
uctured aggregate) transition. Addition of lipid vesicles containing n
egatively charged lipids shifts the random-coil reversible arrow beta-
sheet equilibrium almost completely toward beta-sheet structure, which
can be explained by the specific conditions created at the membrane s
urface: the cationic peptides are attracted to the negatively charged
membrane, and the increase in peptide concentration together with the
partial alignment of the peptide molecules then facilitates beta-sheet
formation. The third peptide, beta AP-(25-35)NH2, also binds to the l
ipid membrane but was found to adopt an essentially random-coil struct
ure, both with and without lipids. A quantitative characterization of
the binding equilibrium was possible with high-sensitivity titration c
alorimetry. All three peptides exhibited exothermic binding enthalpies
which varied between Delta H approximate to-2 kcal/mol for beta AP(25
-35)OH and -8 kcal/mol for beta AP(25-35)NH2. The apparent binding con
stants, calculated with bulk concentrations, were large and varied bet
ween 500 and 5 x 10(4) M(-1), depending on the experimental conditions
. However, after correction for electrostatic charge effects using the
Gouy-Chapman theory, the intrinsic binding constants were found to be
constant and much smaller with K similar to 2-10 M(-1). The low intri
nsic binding constants exclude significant hydrophobic interactions be
tween the lipid membrane and the three Alzheimer model peptides even t
hough residues 29-35 are considered to be part of a membrane-spanning
domain in the full-length precursor protein. The predominance of elect
rostatic forces was demonstrated by addition of 0.1 M NaCl, which abol
ished the peptide-lipid interaction.