A REFINED 3-DIMENSIONAL SOLUTION STRUCTURE OF A CARBOXY-TERMINAL FRAGMENT OF APOLIPOPROTEIN-CII

The three-dimensional structure of a synthetic fragment of human apoli poprotein CII (apo-CII) in 35%, 1,1,1,3,3,3-hexafluoro-2-propanol (HFP ) has been determined on the basis of distance and intensity constrain ts derived from two-dimensional proton nuclear magnetic resonance meas urements. The NOE crosspeak build-up rates were converted to distance constraints which were used in the distance geometry program DI-ANA. A set of one hundred structures were generated and of these ten structu res were used in molecular dynamics simulations using the program XPLO R. This program enabled a direct minimization between the difference o f the two-dimensional NOE intensities and those calculated from the fu ll relaxation matrix. In this way spin diffusion is fully taken into a ccount, which can be seen from the considerable improvement of the R-f actor after the relaxation matrix refinement. These calculations show that this fragment, which corresponds to the carboxy terminal 30 amino acids of intact apo-CII and which retains its ability to activate lip oprotein lipase, is essentially flexible, but has three defined second ary structural elements. The most significant one is an alpha-helix be tween residues 67 and 74. The following three residues adopt a turn-li ke structure. Another turn of alpha-helix is seen between residues 56 and 59. The effect of the solvent system on the secondary structure wa s studied by circular dichroism spectroscopy. The results show that th e mixed aqueous 35% HFP solvent induces secondary structure of a very similar nature to the one induced by sodium dodecyl sulphate.