Hydrophobic Surface Patch Research Articles

Effect of lysine ionization on the structure and electrochemical behaviour of the Met44-->Lys mutant of the blue-copper protein azurin from Pseudomonas aeruginosa.

The structural and spectrochemical effects of the replacement of Met44 in the hydrophobic surface patch of azurin from Pseudomonas aeruginosa by a lysine residue were studied as a function of the ionization state of the lysine. In the pH range 5-8, the optical absorption, resonance Raman, EPR and electron spin-echo envelope modulation spectroscopic properties of wild-type and Met44-->Lys (M44K) azurin are very similar, indicating that the Cu-site geometry has been maintained. At higher pH, the deprotonation of Lys44 in M44K azurin (pKa 9-10) is accompanied by changes in the optical-absorption maxima (614 nm and 450 nm instead of 625 nm and 470 nm) and in the EPR gII value (2.298 instead of 2.241), indicative of a change in the bonding interactions of Cu at high pH. The strong pH dependence of the electron self-exchange rate of M44K azurin supports the assignment of Lys44 as the ionizable group and demonstrates the importance of the hydrophobic patch for electron transfer. The pH dependence of the midpoint potentials of wild-type and M44K azurin can be accounted for by the ionizations of His35 and His83 and by the additional electrostatic effect of the mutation.

Molecular modeling of the domain structure of C9 of human complement by neutron and x-ray solution scattering

C9 is the most abundant component of the membrane attack complex of the complement system of immune defense. This is a typical mosaic protein with thrombospondin (TSR) and low density lipoprotein receptor (LDLr) domains at its N-terminus and an epidermal growth factor-like (EGF) domain at its C-terminus. Between these lies a perforin-like sequence. In order to define the arrangement in solution of these four moieties in C9, high-flux neutron and synchrotron X-ray solution scattering studies were carried out. The neutron radius of gyration RG at infinite contrast is 3.33 nm, and its cross-sectional RG (RXS) is 1.66 nm. Similar values were obtained by synchrotron X-ray scattering after allowance for radiation effects. Stuhrmann analyses showed that the neutron radial inhomogeneity of scattering density alpha is 35 X 10(-5) from the RG data and 16 X 10(-5) from the RXS data. These values are typical for soluble glycoproteins and show no evidence for the existence of any large hydrophobic surface patches on free C9 that might form contacts with lipids. Indirect transformation of the neutron and X-ray scattering curves into real space showed that C9 had a maximum dimension estimated at 12 +/- 2 nm, and this suggests that the lengths of 7-8 nm deduced from previous electron microscopy studies in vacuo are underestimated. Molecular modeling of the C9 scattering curves utilized small spheres in the Debye equation, in which the analyses were constrained by the known volumes of the four moieties of C9 and the known sizes of the TSR and EGF-like domains. The most likely models for C9 suggest that these four regions of C9 are arranged in a V-shaped structure, with an angle of 10 degrees between the two arms, each of length 11.1 nm. This structure has a more hydrophobic character between the two arms. The scattering model is fully consistent with hydrodynamic sedimentation data on C9. Similar V-shaped hydrodynamic models could be developed for C6, C7, C8, and C9 of complement. Such a compact structure is atypical of other multidomain complement proteins so far studied by solution scattering and is fully compatible with mechanisms in which C9 is postulated, on activation, to undergo a drastic unfolding of its domain structure and to expose a more hydrophobic surface which can be embedded into lipid bilayers.

Solution behaviour of Chromobacter viscosum and Pseudomonas sp. lipases. No evidence of self-association

1. The size of two bacterial lipases was studied by SDS/PAGE, sedimentation velocity and sedimentation equilibrium to test for possible self-association behaviour. 2. Mr values of selected lipases were obtained from SDS/PAGE and sedimentation-velocity measurements, together with an absolute determination by sedimentation equilibrium 3. The Mr values obtained in a variety of aqueous solvents indicate that lipases do not self-associate in solution, suggesting the absence of surface hydrophobic patches.

1H nuclear magnetic resonance study of the protonation behaviour of the histidine residues and the electron self-exchange reaction of azurin from Alcaligenes denitrificans

The proton nuclear magnetic resonance spectrum of azurin from Alcaligenes denitrificans at pH 6.0 and 309 K is reported. Proton signals from all methionine and histidine residues (among them the copper ligands) have been assigned. The data have been used to study the pH behaviour of His35 and to establish the electron self-exchange rate of the protein. His35 appears to be protonated at pH <4.5, possibly after rupture of a salt bridge. No effects of this protonation on the tertiary structure around the copper site are observed, however, contrary to the case of Pseudomonas aeruginosa azurin. The electron self-exchange rate amounts to 4 × 10 5 M −1 s −1 at pH 6.7 and 297 K. The data support the conclusion that the electron self-exchange takes place by way of the hydrophobic surface patch around His 117, and that His35 is not involved in this reaction. Oxidation of azurin increases the acidity of the freely titrating His32 and His83 by 0.07 and 0.25 p K a units, respectively. The data can be used to test the theory of electrostatic interactions in proteins. The optical extinction coefficient at 625 nm was experimentally determined and amounts to 4.8(±0.1) × 10 3 M −1 cm −1.

Neurotoxic Proteins in Scorpion Venom

Scorpion venoms are complex mixtures of mucous, low molecular weight components (salts and organic compounds) and many basic, neurotoxic proteins (MR < 8000). These neurotoxins alter the properties of Na channels in excitable cells in three ways. One group of toxins delays inactivation of the Na channel, thereby greatly prolonging the action potential. A second effect, produced by other scorpion toxins, is a transient shift in the voltage dependence of activation, which is demonstrated upon application of a depolarizing pulse. This effect, shown thus far only by toxins from New World species of scorpions, results in an increased tendency of the cell to fire spontaneously and repetitively. The third effect produced by some of the toxins is a reduction of ionic currents (both Na and K currents) with no changes in the kinetics of activation and inactivation. Many observations from pathophysiological studies with scorpion venoms and isolated toxins can be explained on the basis of their effects on Na channels.The three-dimensional structure of one scorpion toxin (Toxin Var3) is known. Because of homology in the amino acid sequences of many different scorpion toxins, it is believed that the structure proposed for Toxin Var3 from Centruroides sculpturatus (Fig. 2 and 3) is typical of the scorpion toxins in general. Featured in this structure are a dense core of protein secondary structure and a surface hydrophobic patch. Many of the conserved amino acids in the scorpion toxins are associated with this patch thus suggesting that this part of the structure is associated with the biological action of the toxins.

Structure of azurin from Alcaligenes denitrificans at 2·5 Å resolution

The structure of the blue copper protein, azurin, from Alcaligenes denitrificans has been determined from an electron density map at a nominal resolution of 3.0 A. Four isomorphous heavy-atom derivatives, prepared with KAu(CN)2, uranyl acetate, Hg(NH3)2Cl2 and KAu(CN)2 + uranyl acetate (a double derivative) were used to calculate phases by the method of isomorphous replacement. The overall figure of merit was 0.61. The two molecules in the asymmetric unit are related by an approximate 2-fold axis. Independent interpretations of the density were made for the two molecules, and the structures have since been partially refined. After 12 refinement cycles, using the Hendrickson-Konnert restrained least-squares program, the R factor is 0.318 for data to 2.5 A resolution and there are no major conformational differences between the two molecules. Refinement is continuing. Eight extended strands of the polypeptide chain form a beta-barrel structure whose topology is the same as that of plastocyanin and the alternative folding proposed for Pseudomonas aeruginosa azurin. As in the latter two proteins, the copper atom forms three short bonds, with His-46 N delta 1, His117 N delta 1 and Cys112 S gamma, and one longer bond, with Met121 S delta, these four ligands forming a very distorted tetrahedron. A possible additional interaction, between copper and the carbonyl oxygen of Gly45, cannot be discounted at the present stage of the analysis. A surface hydrophobic patch, around the edge of the imidazole ring of His117 appears the most likely electron transfer locus. The sequences of azurin and plastocyanin have been aligned and the homology between the two proteins is discussed.