Kaj Österlund

Solution structure of human plasma fibronectin as a function of NaCl concentration determined by small-angle X-ray scattering

The structure of human plasma fibronectin in 50 mM Tris-HCl buffer, pH 7.4, containing varying concentrations of NaCl, has been investigated using the small-angle X-ray method.Below 0.3 M NaCl the overall structure of the molecule is disc-shaped; at 0 M NaCl the axial ratio of the disc is about 1:7 and between 0.1 M to 0.3 M it is slightly more asymmetric, with an axial ratio of 1:10.At about 0.3 M NaCl there is a reversible transition to a more open structure, and, from 0.3 M up to 1.1 M NaCl the small-angle X-ray data can be explained by models consisting of ensembles of flexible, non-overlapping, bead-chains generated by a Monte Carlo procedure. Within this concentration range there is a gradual increase in the stiffness of the chains, as well as a decrease in bead radius, which indicates that the molecule becomes more open when the NaCl concentration is increased.The transition to a more open structure is also demonstrated by the average radius of gyration which increases gradually from 8.26 nm at 0 M NaCl to 8.75 nm at physiological or near-physiological conditions, and up to 16.2 nm at 1.1 M NaCl.

Resonance Raman study on yeast cytochrome c peroxidase Effect of coordination and axial ligands

Resonance Raman spectra are reported for native ferric cytochrome c peroxidase, its cyanide and fluoride compounds, those of the ferrous enzyme and its cyanide and carbonyl compounds, and the spectrum of the hydrogen peroxide compound, compound I. Band frequencies of ferric horseradish peroxidase isoenzyme C2 and its derivatives are also given. Comparison of the frequencies of the bands around 1400, 1500, 1560-1580, and 1610-1640 cm-1 with those of other hemoproteins and heme model compounds showed that in ferric highspin compounds in particular the bands are not only spin and oxidation sensitive, as has previously been reported, but that they also reflect the coordination of the heme iron. It is suggested that ferric cytochrome c peroxidase and horseradish peroxidase are pentacoordinated. In the hexacoordinated fluoride, cyanide and carbon monoxide derivatives the bands reflect the spin state and the out-of-plane position of the heme iron. The spectrum of cytochrome c peroxidase compound I supports previous studies that suggest that it has a lowspin heme iron in the Fe(IV) oxidation state.

Structural and functional features of Pseudomonas cytochrome c peroxidase.

The secondary structure of Pseudomonas cytochrome c peroxidase (ferrocytochrome c: hydrogen-peroxide oxidoreductase, EC 1.11.1.5) has been predicted from the established amino acid sequence of the enzyme using a Chou-Fasman-type algorithm. The amount of alpha-helicity thus obtained is in agreement with previously obtained results based on circular dichroic measurements at far UV. The two heme c moieties of the enzyme have earlier been shown to have widely different characteristics, e.g., the redox potentials of the hemes differ with about 600 mV, and carry out different functions in the enzyme molecule. The structural comparisons made in this study enlighten the observed functional differences. The first heme in the polypeptide chain, heme 1, has in its environment a folding pattern generally encountered in cytochromes. In the region of the sixth ligand, however, profound differences are noted. The cytochromal methionine has been replaced by a lysine with a concomitant lowering of redox-potential thus making peroxidatic activity possible. Around heme 2, extra amino acid residues have been added to the peroxidase as compared with Rhodospirillum molischianum cytochrome c2 core structure in the 20s loop. After completion of the cytochromal fold around heme 2 an additional tail consisting of 25 residues is linked. This tail shows no stabilizing elements of secondary structure, but contains a strongly hydrophobic segment which suggests a possible membrane contact site of this extrinsic membrane protein. Heme 2 is concluded to have a cytochromal function in the molecule. To further elucidate the functional properties of the enzyme, a noncovalent two-fragment complex was produced by specific cleavage of the peroxidase by Pseudomonas elastase. The complex was studied with respect to its properties to the native enzyme. The two-fragment complex of Pseudomonas peroxidase retains the overall conformation of the native enzyme showing, however, no heme-heme interaction. Thus, a comparison of the properties of the native enzyme with those of the two-fragment complex permitted some conclusions to be drawn on the structure of the enzyme as well as the mechanism of heme-heme interaction. From the present results we conclude that the two distal heme surfaces in the peroxidase are oriented toward each other. This structural arrangement allows an inter-heme communication in the enzyme molecule and it also forms the structural basis for the enzyme mechanism. The structural comparisons also give insight into the evolution of an ancestral cytochrome c into an efficient peroxidase that has a versatile control mechanism in heme-heme interaction.

Solution structure of human plasma fibronectin using small-angle X-ray and neutron scattering at physiological pH and ionic strength

Human plasma fibronectin has been investigated at physiological pH and ionic strength, by using small-angle X-ray and neutron scattering techniques. The results indicate that the molecule is disc shaped with an axial ratio of about 1:10. In fact, an ellipsoid of revolution with semiaxes a = 1.44 nm and b = c = 13.8 nm is in agreement with the experimental scattering data, and can also fully explain the rather extreme hydrodynamic parameters reported for fibronectin. The X-ray data gave a radius of gyration of 8.9 nm and a molecular weight of 510,000, whereas the neutron data gave slightly larger values, 9.5 nm and 530,000, respectively. From the volume of the best fitting ellipsoid we obtain a degree of hydration of 0.61 g H2O/g protein (dry weight). Neutron data, recorded at different D2O concentrations in the solvent, gave a match point of 43% D2O, which indicates that approximately 80% of the hydrogens bound to oxygen and nitrogen are exchangeable.

Librational modes and the structure of ammonium nitrate in its high temperature phases

Abstract We have measured the temperature dependence of the low frequency Raman spectrum of ammonium nitrate in its solid phases IV, II and I and also in its melt. Librational modes are observed in phases IV and II but disappear in both phase I and the melt. Our results indicate that the NO 3 group is almost freely rotating in the solid phase I.

Effect of chemical modification of arginine and lysine residues of fibronectin on its antigenic and gelatin-binding activity.

The effect of chemical modification of arginine and lysine residues of fibronectin on its antigenic and gelatin-binding activity was studied by enzyme immunoassay techniques. Both modifications strongly reduced the gelatin-binding activity. Using conformation-specific antibodies it was shown that modification of lysines caused extensive conformational changes in the molecule. No such changes could be detected in arginine-modified fibronectin. The results suggest that arginine residues are directly involved in the binding of fibronectin to gelatin. Lysine residues seem to be important for maintaining a native conformation necessary for gelatin-binding.