NMR studies of model peptides of PHGGGWGQ repeats within the N-terminus ofprion proteins: A loop conformation with histidine and tryptophan in closeproximity

The N-terminal region of the prion protein from human and mouse contains fi ve tandem repeats with the consensus sequence of PHGGGWGQ. NMR. studies wer e performed in water for two cyclic peptides, cyclo-[C(1)R(2)Q(3)P(4)H(5)G( 6)G(7)S(8)W(9)G(10)Q(11)R(12)D(13)C(14)] (C1) and cyclo[C-1 R(2)D(3)P(4)H(5 )G(6)G(7)G(8)W(9)G(10)Q(11)P(12)H(13)G(14)G(15)G(16)W(17)G(18)Q(19)R(20)D(2 1)C(22)] (C2), which are cyclized by a disulfide bridge between the Cys res idues at the N- and C-termini, and for their corresponding linear peptides (L1 and L2) which are formed by reduction. The patterns of the CalphaH chem ical shift difference of these four peptide mimetics were very similar to t hose observed for the tandem repeats of human prion protein reported by oth er researchers, The medium-range NOE connectivities were found between the CbetaH of the H5 and the proton of the W9 side chain for L1, The correspond ing NOEs were also observed in H5-W9 and H13-W17 of L2 with ambiguity. Thes e observations indicate that histidine (i) is in close proximity to tryptop han (i+4). d(alpha N) (i,i+2) NOE connectivities were observed between W9 a nd Q11 of L1 and L2, and d(NN) (i,i+1) NOE connectivities were also observe d for G10-Q11 of L1 and L2 and for G18-Q19 of L2. Significantly lower tempe rature coefficients of amide proton chemical shifts were obtained for Q11 a nd Q19 of L2 and C2, Structure calculations for L1 showed that HGG(G/S)W an d (G/S)WGQ adopt a loop conformation and a beta-turn, respectively, These r esults strongly suggest that the tandem repeats within prion protein adopt a non-random structure.