Structure determination of a lone alpha-helical antifreeze protein from winter flounder.

The X-ray crystal structure of a lone alpha-helical antifreeze protein from winter flounder has been determined to 1.5 A using a combination of molecular-replacement and isomorphous-replacement techniques. Molecular replacement involved a multiparameter search using X-PLOR with two 37-mers of alanine in idealized alpha-helical conformations as the search models. Identified were a large number of potential solutions from which the correct solution was not distinguishable. Commitment of the top 1620 solutions to cycles of rigid-body, positional and simulated-annealing refinement identified the correct solution by a small margin in R factor. Low-resolution electron-density maps generated with phasing information from TbNO(3) and LaNO(3) derivatives were consistent with the top molecular-replacement solution. These derivatives also provided a means to filter and compare the large number of other molecular-replacement solutions with reasonable R factor statistics. The structure-solution strategy described herein may prove useful for the determination of other relatively simple alpha-helical X-ray structures.