title: Structural comparisons of the native and reactive-centre-cleaved forms of alpha 1-antitrypsin by neutron- and X-ray-scattering in solution.
abstract: alpha 1-Antitrypsin is the best-characterized member of the serpin (serine-proteinase inhibitor) superfamily. Its solution structure was studied by high-flux neutron-scattering and synchrotron X-ray-scattering. Neutron data show that its absorption coefficient A1% 280,1cm is 5.4. The neutron radius of gyration RG at infinite contrast for native alpha 1-antitrypsin is 2.61 nm, characteristic of a moderately elongated structure, and its cross-sectional RG is 1.34 nm. The internal inhomogeneity of scattering densities within alpha 1-antitrypsin is high at 29 x 10(-5). The X-ray RG is 2.91 nm, in good agreement with the neutron RG of 2.82 nm in 1H2O. This RG is unchanged in reactive-centre-cleaved alpha 1-antitrypsin. These parameters are also unchanged at pH 8 in sodium/potassium phosphate buffers up to 0.6 M. The neutron and X-ray curves for native alpha 1-antitrypsin were compared with Debye simulation based on the crystal structure of reactive-centre-cleaved (papain) alpha 1-antitrypsin. After allowance for residues not visible in the crystallographic electron-density map, and rejoining the proteolysed site between Met-358 and Ser-359 by means of a relatively minor conformational re-arrangement, good agreement to a structural resolution of 4 nm is obtained with the neutron data in two contrasts and with the X-ray data. The structures of the native and cleaved forms of alpha 1-antitrypsin are thus similar within the resolution of solution scattering. This places an upper limit on the magnitude of the presumed conformational changes that occur in alpha 1-antitrypsin on reactive-centre cleavage, as indicated in earlier spectroscopic investigations of the Met-358-Ser-359 peptide-bond cleavage. Methods for scattering-curve simulations from crystal structures are critically assessed. The RG data lead to dimensions of 7.8 nm x 4.9 nm x 2.2 nm for native alpha 1-antitrypsin. The high internal inhomogeneity and the asymmetric shorter semi-axes of 4.9 nm and 2.2 nm suggest that the three oligosaccharide chains of alpha 1-antitrypsin are essentially freely extended into solvent in physiological conditions. This conclusion is also supported by the Debye simulations, and by modelling based on hydrodynamic parameters.
authors.affiliation: Department of Biochemistry and Chemistry, Royal Free Hospital School of Medicine, London, U.K.
authors.forename: K F
authors.initials: KF
authors.lastname: Smith
authors.forename: R A
authors.initials: RA
authors.lastname: Harrison
authors.forename: S J
authors.initials: SJ
authors.lastname: Perkins
mesh_terms: D001665:Binding Sites; D002021:Buffers; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D003460:Crystallization; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D015394:Molecular Structure; D009502:Neutrons; D010315:Particle Accelerators; D010710:Phosphates / Q000494:pharmacology; D011487:Protein Conformation; D012542:Scattering, Radiation; D012996:Solutions; D014965:X-Rays; D000515:alpha 1-Antitrypsin
chemical_list: D002021:Buffers; D010710:Phosphates; D012996:Solutions; D000515:alpha 1-Antitrypsin