Amphipathic helices support function of blood coagulation factor IXa

Blood coagulation factor IXa gains proteolytic efficiency upon binding to a phospholipid membrane. We have found that an amphipathic, membrane-binding peptide from the C2 domain of factor VIII, fVIII(2303-23), enhances proteo lytic efficiency of factor IXa in the absence of phospholipid membranes. Th is enhancement is the result of a reduction in the K-M for the substrate, f actor X, with little effect on the k(cat). Enhanced function requires inter action of the gamma-carbortyglutamic acid (Gla) domains of factor IXa and f actor X since (i) a synthetic peptide comprising the Gla domain of factor I Xa and antibodies directed to the Gla domain of factor IXa inhibit this acc eleration, (ii) the acceleration is Ca(II) dependent, and (iii) conversion of Gla-domainless factor X is not affected by the presence of fVIII(2303-23 ) The effect of fVIII(2303-23) on factor IXa parallels the enhanced functio n produced by phosphatidylserine-containing bilayers, and fVIII(2303-23) do es not further enhance function of factor IXa when phospholipid vesicles ar e present. The critical feature of fVIII(2303-23) is apparently its amphipa thic helix-forming structure [Gilbert, G. E., and Baleja, J. D. (1995) Bioc hemistry 34, 3022-3031] because other alpha-helical peptides such as a homo logous peptide from the C2 domain of factor V and melittin have similar eff ects. Diastereomeric analogues of fVIII(2303-23) and melittin, which have r educed helical content, do not support factor IXa activity. A truncated pep tide of fVIII(2303-23) with three C-terminal residues deleted retains alpha -helical content but loses capacity to enhance factor X cleavage, suggestin g that a minimum length of alpha-helix is required. Although these results probably do not illuminate the physiologic function of the factor VIII pept ide corresponding to fVIII(2303-23), they demonstrate a novel, membrane-mim etic role of amphipathic helical peptides in supporting function of factor IXa.