title: Polar Versus Non-polar Local Ordering at Mobile Sites in Proteins: Slowly Relaxing Local Structure Analysis of (15)N Relaxation in the Third Immunoglobulin-Binding Domain of Streptococcal Protein G.
abstract: We developed recently the slowly relaxing local structure (SRLS) approach for studying restricted motions in proteins by NMR. The spatial restrictions have been described by potentials comprising the traditional L = 2, K = 0, 2 spherical harmonics. However, the latter are associated with non-polar ordering whereas protein-anchored probes experience polar ordering, described by odd-L spherical harmonics. Here we extend the SRLS potential to include the L = 1, K = 0, 1 spherical harmonics and analyze (15)N-(1)H relaxation from the third immunoglobulin-binding domain of streptococcal protein G (GB3) with the polar L = 1 potential (coefficients c0(1) and c1(1)) or the non-polar L = 2 potential (coefficients c0(2) and c2(2)). Strong potentials, with ⟨c0(1)⟩ ∼ 60 for L = 1 and ⟨c0(2)⟩ ∼ 20 for L = 2 (in units of kBT), are detected. In the α-helix of GB3 the coefficients of the rhombic terms are c1(1) ∼ c2(2) ∼ 0; in the preceding (following) chain segment they are ⟨c1(1)⟩ ∼ 6 for L = 1 and ⟨c2(2)⟩ ∼ 14 for L = 2 (⟨c1(1)⟩ ∼ 3 for L = 1 and ⟨c2(2)⟩ ∼ 7 for L = 2). The local diffusion rate, D2, lies in the 5 × 10(9)-1 × 10(11) s(-1) range; it is generally larger for L = 1. The main ordering axis deviates moderately from the N-H bond. Corresponding L = 1 and L = 2 potentials and probability density functions are illustrated for residues A26 of the α-helix, Y3 of the β1-strand, and L12 of the β1/β2 loop; they differ considerably. Polar/orientational ordering is shown to be associated with GB3 binding to its cognate Fab fragment. The polarity of the local ordering is clearly an important factor.
authors.affiliation: The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University , Ramat-Gan 52900 Israel.
authors.forename: Oren
authors.initials: O
authors.lastname: Tchaicheeyan
authors.affiliation: The Mina and Everard Goodman Faculty of Life Sciences, Bar-Ilan University , Ramat-Gan 52900 Israel.
authors.forename: Eva
authors.initials: E
authors.lastname: Meirovitch
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D001665:Binding Sites; D007136:Immunoglobulins / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*
chemical_list: D001426:Bacterial Proteins; C034284:IgG Fc-binding protein, Streptococcus; D007136:Immunoglobulins