Vectorially oriented monolayers of detergent-solubilized Ca(2+) -ATPase from sarcoplasmic reticulum

Hydration state of single cytochrome c monolayers on soft interfaces via neutron interferometry

Yeast cytochrome c (YCC) can be covalently tethered to, and thereby vectorially oriented on, the soft surface of a mixed endgroup (e.g., -CH3/-SH = 6:1, or -OH/-SH = 6:1) organic self-assembled monolayer (SAM) chemisorbed on the surface of a silicon substrate utilizing a disulfide linkage between its unique surface cysteine residue and a thiol endgroup. Neutron reflectivities from such monolayers of YCC on Fe/Si or Fe/Au/Si multilayer substrates with H2O versus D2O hydrating the protein monolayer at 88% relative humidity for the nonpolar SAM (-CH3/-SH = 6:1 mixed endgroups) surface and 81% for the uncharged-polar SAM (-OH/-SH = 6:1mixed endgroups) surface were collected on the NG1 reflectometer at NIST. These data were analyzed using a new interferometric phasing method employing the neutron scattering contrast between the Si and Fe layers in a single reference multilayer structure and a constrained refinement approach utilizing the finite extent of the gradient of the profile structures for the systems. This provided the water distribution profiles for the two tethered protein monolayers consistent with their electron density profile determined previously via x-ray interferometry (Chupa et al., 1994).

Vectorially oriented monolayers of the cytochrome c/cytochrome oxidase bimolecular complex

Vectorially oriented monolayers of yeast cytochrome c and its bimolecular complex with bovine heart cytochrome c oxidase have been formed by self-assembly from solution. Both quartz and Ge/Si multilayer substrates were chemical vapor deposited with an amine-terminated alkylsiloxane monolayer that was then reacted with a hetero-bifunctional cross-linking reagent, and the resulting maleimide endgroup surface then provided for covalent interactions with the naturally occurring single surface cysteine 102 of the yeast cytochrome c. The bimolecular complex was formed by further incubating these cytochrome c monolayers in detergent-solubilized cytochrome oxidase. The sequential formation of such monolayers and the vectorially oriented nature of the cytochrome oxidase was studied via meridional x-ray diffraction, which directly provided electron density profiles of the protein(s) along the axis normal to the substrate plane. The nature of these profiles is consistent with previous work performed on vectorially oriented monolayers of either cytochrome c or cytochrome oxidase alone. Furthermore, optical spectroscopy has indicated that the rate of binding of cytochrome oxidase to the cytochrome c monolayer is an order of magnitude faster than the binding of cytochrome oxidase to an amine-terminated surface that was meant to mimic the ring of lysine residues around the heme edge of cytochrome c, which are known to be involved in the binding of this protein to cytochrome oxidase.

Molecular orientation distributions in protein films: III. Yeast cytochrome c immobilized on pyridyl disulfide-capped phospholipid bilayers

Molecular orientation in a hydrated monolayer film of yeast cytochrome c, immobilized via disulfide bonding between Cys-102 and a pyridyl disulfide-capped phospholipid bilayer deposited from an air-water interface onto glass substrates, was investigated. The orientation distribution of the heme groups in the protein film was determined using a combination of absorption linear dichroism, measured in a planarintegrated optical waveguide-attenuated total reflection geometry- and fluorescence anisotropy, measured in a total internal reflection geometry. A gaussian model for the orientation distribution was used to recover the mean heme tilt angle and angular distribution about the mean, which were 40 and 11 degrees, respectively. Additional experiments showed that a large fraction of the cytochrome c was disulfide bonded to the bilayer, which correlates with the high degree of macroscopic order in the protein film. However, a subpopulation of yeast cytochrome c molecules in the film (approximately 30% of the total) appeared to be nonspecifically adsorbed. The orientation distribution of this subpopulation was found to be much broader than the specifically bound fraction.

Molecular dynamics simulations of a protein on hydrophobic and hydrophilic surfaces

Molecular dynamics simulations have been used to investigate the behavior of the peripheral membrane protein, cytochrome c, covalently tethered to hydrophobic (methyl-terminated) and hydrophilic (thiol-terminated) self-assembled monolayers (SAMs). The simulations predict that the protein will undergo minor structural changes when it is tethered to either surface, and the structures differ qualitatively on the two surfaces: the protein is less spherical on the hydrophilic SAM where the polar surface residues reach out to interact with the SAM surface. The protein is completely excluded from the hydrophobic SAM but partially dissolves in the hydrophilic SAM. Consequently, the surface of the thiol-terminated SAM is considerably less ordered than that of the methyl-terminated SAM, although a comparable, high degree of order is maintained in the bulk of both SAMs: the chains exhibit collective tilts in the nearest-neighbor direction at angles of 20 degrees and 17 degrees with respect to the surface normal in the hydrophobic and the hydrophilic SAMs, respectively. On the hydrophobic SAM the protein is oriented so that the heme plane is more nearly parallel to the surface, whereas on the hydrophilic surface it is more nearly perpendicular. The secondary structure of the protein, dominated by alpha helices, is not significantly affected, but the structure of the loops as well as the helix packing is slightly modified by the surfaces.