Vk. Mishra et al., EFFECT OF THE ARRANGEMENT OF TANDEM REPEATING UNITS OF CLASS-A AMPHIPATHIC ALPHA-HELIXES ON LIPID INTERACTION, The Journal of biological chemistry, 270(4), 1995, pp. 1602-1611
Exchangeable apolipoproteins possess tandem repeating units of class A
amphipathic helical segments and many of them are linked together by
proline residues. To understand the optimal arrangement of the amphipa
thic helixes for lipid association, we have studied the interactions o
f three model class A amphipathic helical peptides with lipids. The th
ree peptides are: 37pA, a dimer of 18A (DWLKAFYDKVAEKLKEAF) linked tog
ether by a Pro (18A-Pro-18A); 37aA, a dimer of 18A linked together by
an Ala (18A-Ala-18A); and 36A, a dimer of 18A without any linker resid
ue (18A-18A). Circular dichroism (CD) spectra showed that the peptides
are predominantly alpha-helical in aqueous and lipid environments. Te
mperature dependent CD studies indicated that in buffer helix stabilit
y decreases in the order 36A > 37aA > 37pA; however, in the presence o
f dimyristoyl phosphatidylcholine (DMPC), the above order is reversed.
The retention times of the peptides on a C-18 reversed-phase high per
formance liquid chromatography column decreased in the order 36A > 37a
A > 37pA, consistent with the lengths of the nonpolar faces of the alp
ha-helixes being in the same order; the retention time of the parent 1
8A was shorter than 37pA. While 37pA adsorbed to egg phosphatidylcholi
ne monolayers most strongly, the degree and rate of association of 36A
were significantly lower. Differential scanning calorimetry indicated
that, while 37pA was most effective in reducing the enthalpy of the g
el to liquid-crystalline phase transition of DMPC multilamellar vesicl
es, 36A was least effective; 36A was even less effective than 18A. Flu
orescence quenching experiments with iodide and acrylamide indicated t
hat, in the presence of DMPC, Trp residues in 36A are most exposed to
the quenchers while in 37pA they are least exposed. In the presence of
DMPC, shielding of Trp in 18A from the quenchers was more than that o
bserved with Trp residues in 36A. The results of this study suggest th
at the arrangement of tandem repeating amphipathic helical units which
results in the formation of a class A amphipathic helix with a nonpol
ar face longer than five or six turns reduces the ability of the helix
to associate with phospholipid.