Model peptide studies of sequence repeats derived from the intracrystalline biomineralization protein, SM50. I. GVGGR and GMGGQ repeats

We report solution-state pulsed field gradient nmr studies of a native sequence-derived 23-residue peptidomimetic, N(α)-acetyl- QPGVGGRQPGMGGQPGVGGRQPG-C(α)-amide, that incorporates the prevalent GVGGR and GMGGQ repeats found in the sea urchin embryo intracrystalline spicule matrix protein, SM50 (Strongylocentrotus purpuratus). These repeats are sequence homologues of elastin protein repeats (VPGVG, VGGVG, and APGVGV) and spider dragline silk protein repeats (GPGG, GQGG, and QPGYG). Using rotating frame nuclear Overhauser effect (ROE) connectivities, CH(α) proton conformational shifts, ³J(NH-CHα) coupling constants, amide temperature shift coefficients, and pulsed field gradient ROE spectroscopy solvent exchange measurements, we find that the 23-mer peptidomimetic possesses a multiple β-turn structure in aqueous solution, in equilibria with an extended or coil structure (60% β-turn: 40% random coil). The GVGGR sequence adopts a double β-turn conformation that is stabilized by two hydrogen bonds (R7→V4, R20→V17; G6→G3, G19→G16). The GMGGQ region adopts a single β- turn conformation that is stabilized by a hydrogen bond involving residues Q14 and M11. Repeating β-turn structures, or β-spirals, may play an important role with regard to matrix assembly, protein stability, molecular elasticity, and/or protein-crystal recognition within the spicule mineralized matrix.