CONFORMATIONAL STUDIES OF THE N-TERMINAL LIPID-ASSOCIATING DOMAIN OF HUMAN APOLIPOPROTEIN C-I BY CD AND H-1-NMR SPECTROSCOPY

A peptide comprising the N-terminal 38 residues of human apolipoprotei n C-I (apoC-I(1-38)) was synthesized using solid-phase methods and its solution conformation studied by CD and H-1 NMR spectroscopy. The CD data indicate that apoC-I(1-38) has a similar helical content (55%) in the presence of saturating amounts of SDS or egg yolk lysophosphatidy lcholine. A structural ensemble of SDS-bound apoC-I(1-38) was calculat ed from 464 NOE-based distance restraints using distance geometry meth ods. ApoC-I(1-38) adopts a helical structure between residues V4 and K 30 and an extended C-terminus from Q31 when associated with SDS. The r egion K12-G15 undergoes slow conformational exchange as indicated by a bove-average amide resonance linewidths, large temperature coefficient s, and fast exchange (<2 h) of backbone amide protons with deuterium. The mobility of K12-G15 is reflected in the poorly defined dihedral an gles of K12 and E13 in the calculated ensemble of structures. The aver age structure of apoC-I(1-38) is curved toward its hydrophobic face wi th bends of 125 degrees, centered at K12/E13, and 150 degrees, centere d at K21. This curvature appears to be driven by the interaction of tw o hydrophobic clusters, one formed by residues L8, L11, F14, and L18, and the other by L25, I26, and I29, with the amphiphile SDS. Based on our present structural definition of apoC-I(1-38) and the previously o btained structure of the fragment apoC-I(35-53), we propose the second ary structure of intact apolipoprotein C-I.