SOLUTION STRUCTURE, ROTATIONAL DIFFUSION ANISOTROPY AND LOCAL BACKBONE DYNAMICS OF RHODOBACTER-CAPSULATUS CYTOCHROME C2

The solution structure, backbone dynamics and rotational diffusion of the Rhodobacter capsulatus cytochrome c(2) have been determined using heteronuclear NMR spectroscopy. In all, 1204 NOE-derived distances wer e used in the structure calculation to give a final ensemble with 0.59 (+/-0.08) Angstrom rms deviation for the backbone atoms (C, C-alpha an d N) with respect to the mean coordinates. There is no major differenc e between the solution structure and the previously solved X-ray cryst al structure (1.07(+/-0.07) Angstrom rms difference for the backbone a toms), although certain significant local structural differences have been identified. This protein contains five helical regions and a hist idine-heme binding domain, connected by a series of structured loops. The orientation of the helices provides an excellent sampling of angul ar space and thus allows a precise characterization of the anisotropic diffusion tensor. Analysis of the hydrodynamics of the protein has be en performed by interpretation of the N-15 relaxation data using isotr opic, axially asymmetric and fully anisotropic diffusion tensors. The protein can be shown to exhibit significant anisotropic reorientation with a diffusion tensor with principal axes values of 1.405(+/-0.031) x 10(7) s(-1), 1.566(+/-0.051) x 10(7) s(-1) and 1.829(+/-0.054) x 10( 7) s(-1). Hydrodynamic calculations performed on the solution structur e predict values of 1.399 x 10(7) s(-1), 1.500 x 10(7) s(-1) and 1.863 x 10(7) s(-1) when a solvent shell of 3.5 Angstrom is included in the calculation. The optimal orientation of the diffusion tensor has been incorporated into a hybrid Lipari-Szabo type local motion-anisotropic rotational diffusion model to characterize the local mobility in the molecule. The mobility parameters thus extracted show a quantitative i mprovement with respect to the model-free analysis assuming isotropic reorientation; helical regions exhibit similar dynamic properties and fewer residues require more complex models of internal motion. While t he molecule is essentially rigid, a tripeptide loop region (residues 1 01 to 103) exhibits flexibility in the range of 20 to 30 ps, which app ears to be correlated with the order in the NMR solution structure. (C ) 1998 Academic Press.