PENTAMERIC AND DECAMERIC STRUCTURES IN SOLUTION OF SERUM AMYLOID-P COMPONENT BY X-RAY AND NEUTRON-SCATTERING AND MOLECULAR MODELING ANALYSES

Human serum amyloid P component (SAP) is a normal plasma glycoprotein and the precursor of amyloid P component which is a universal constitu ent of the abnormal tissue deposits in amyloidosis. X-ray and neutron scattering data showed that pentameric or decameric ring structures fo r SAP in solution are readily distinguished. Further neutron data coll ection showed that SAP pentamers were reproducibly obtained in the pre sence of Ca2+ at pH 5.5 or in the presence of methyl 6-O-(1-carboxyeth ylidene)-beta-D-galactopyranoside (MO beta DG) and Ca2+ at pH 6.0 to 8 .0, while SAP decamers were obtained in the presence of EDTA between p H 5.5 and 8.0. SAP pentamers have a mean X-ray R-G Of 3.99(+/-0.11) nm and a mean neutron R-G of 3.69(+/-0.12) nm in 100% (H2O)-H-2. SAP dec amers have a mean X-ray R-G of 4.23(+/-0.12) nm and a mean neutron R-G Of 4.09(+/-0.14) nm in 100% (H2O)-H-2. The absorption coefficients of SAT pentamers and decamers differ by 10%. If we infer that the two al pha-helical A-faces are In contact with each other in the SAP decamer, the lack of structural change of the decamer with pH may be explained by the absence of His residues from the A-face of the SAP pentamer, a nd the change in absorption coefficients is compatible with the presen ce of Trp residues at this A-face. The rigid ring structure of pentame ric SAP provided a test of scattering curves calculated from crystal s tructures. The only structural unknown is the orientation of the five chemically homogeneous oligosaccharide chains relative to the protein, but extended oligosaccharide structures were found to account for its scattering curve. X-ray scattering curves were best calculated using a hydrated structure, while neutron scattering curves were best calcul ated using an unhydrated structure. The outcome of these analyses was used to model the structure of decameric SAP. The evaluation of 640 st ructures for two SAP pentamers brought face-to-face to form SAP decame rs gave better curve fits for structures in which the two A-faces were in contact with each other, in which it is likely that the two pentam ers were out of alignment by a rotation of 36 degrees and the oligosac charide chains were extended. (C) 1997 Academic Press Limited.