title: Modelling of the serine-proteinase fold by X-ray and neutron scattering and sedimentation analyses: occurrence of the fold in factor D of the complement system.
abstract: Solution scattering is a powerful means of determining the overall arrangement of domains in the multidomain proteins of complement. the serine-proteinase domain is central to all proteolytic events during complement activation. As models of this domain, bovine beta-trypsin, trypsinogen, alpha-chymotrypsin and chymotrypsinogen A were studied by neutron and X-ray synchrotron solution scattering. At pH 7, all the X-ray and neutron M(r) values corresponded to monomeric proteins. The X-ray radii of gyration, RG, of beta-trypsin, trypsinogen, alpha-chymotrypsin and chymotrypsinogen A (measured in positive solute-solvent contrasts) were 1.59 nm, 1.78 nm, 1.71 nm and 1.76 nm (+/- 0.05-0.11 nm) in that order. Neutron contrast variation showed that the RG at infinite contrast, RC, for these four proteins were 1.57 nm, 1.70 nm, 1.67 nm and 1.78 nm (+/- 0.03 nm) in that same order. The radial inhomogeneity of neutron-scattering density, alpha, was positive at (5-13) x 10(-5), and corresponds to the preponderance of hydrophilic residues near the protein surface. On trypsinogen activation, a small reduction in the RG value of 0.13 +/- 0.07 nm was just detectable, while the RG of chymotrypsinogen A was unchanged after activation. The RC and alpha values of the four proteins can be calculated by using crystallographic co-ordinates. The reduced RG of beta-trypsin relative to trypsinogen was explained in terms of the removal of the extended N-terminal hexapeptide of trypsinogen. The full X-ray and neutron-scattering curves in positive and negative contrasts agreed well with scattering curves calculated from crystallographic coordinates to a nominal structural resolution of 4.5 nm, provided that the internal structure was considered in neutron modelling, and that the hydration was considered in X-ray modelling. Sedimentation-coefficient data also provide information on the disposition of domains in multidomain proteins. It was found that the hydrated X-ray sphere model could be directly utilized to calculate sedimentation coefficients. X-ray scattering on factor D showed from its RG of 1.78 nm that this is monomeric and very similar in structure to beta-trypsin. The X-ray-scattering curve of factor D was readily modelled using the beta-trypsin crystal structure after allowance for sequence changes. The success of these modellings provides a basis for the constrained modelling of solution scattering data for the multidomain proteins of complement.
authors.affiliation: Department of Biochemistry and Chemistry, Royal Free Hospital School of Medicine, London, U.K.
authors.forename: S J
authors.initials: SJ
authors.lastname: Perkins
authors.forename: K F
authors.initials: KF
authors.lastname: Smith
authors.forename: J M
authors.initials: JM
authors.lastname: Kilpatrick
authors.forename: J E
authors.initials: JE
authors.lastname: Volanakis
authors.forename: R B
authors.initials: RB
authors.lastname: Sim
mesh_terms: D002918:Chymotrypsin / Q000737:chemistry; D002919:Chymotrypsinogen / Q000737:chemistry; D011416:Complement Factor D / Q000737:chemistry*; D003198:Computer Simulation; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D009502:Neutrons; D011487:Protein Conformation; D012542:Scattering, Radiation; D012697:Serine Endopeptidases / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D014357:Trypsin / Q000737:chemistry; D014362:Trypsinogen / Q000737:chemistry; D014461:Ultracentrifugation; D014965:X-Rays
chemical_list: D012996:Solutions; D014362:Trypsinogen; D002919:Chymotrypsinogen; D012697:Serine Endopeptidases; D002918:Chymotrypsin; D014357:Trypsin; D011416:Complement Factor D