The neuromuscular junction nicotinic acetylcholine receptor (AChR), a pentameric membrane glycoprotein, is the autoantigen involved in the autoimmune disease myasthenia gravis (MG). In animals immunized with intact AChR and in human MG, the anti-AChR antibody response is polyclonal. However, a small extracellular region of the AChR alpha-subunit, the main immunogenic region (MIR), seems to be a major target for anti-AChR antibodies. A major loop containing overlapping epitopes for several anti-MIR monoclonal antibodies (mAbs) lies within residues alpha 67-76 at the extreme synaptic end of each alpha-subunit: however, anti-MIR mAbs are functionally and structurally quite heterogeneous. Anti-MIR mAbs do not affect channel gating, but are very effective in the passive transfer of MG to animals; in contrast, their Fab or Fv fragments protect the AChR from the pathogenic effects of the intact antibodies. Antibodies against the cytoplasmic region of the AChR can be elicited by immunization with denatured AChR and the precise epitopes of many such mAbs have been identified; however, it is unlikely that such antibodies are present in significant amounts in human MG. Antibodies to other extracellular epitopes on all AChR subunits are present in both experimental and human MG; these include antibodies to the acetylcholine-binding site which affect AChR function in various ways and also induce acute experimental MG. Finally, anti-AChR antibodies cross-reactive with non-AChR antigens exist, suggesting that MG may result from molecular mimicry. Despite extensive studies, many gaps remain in our understanding of the antigenic structure of the AChR; especially in relation to human MG. A thorough understanding of the antigenic structure of the AChR is required for an in-depth understanding, and for possible specific immunotherapy, of MG.

PILB has been described as being involved in the virulence of bacteria of Neisseria genus. The PILB protein is composed of three subdomains. In the present study, the central subdomain (PILB-MsrA), the C terminus subdomain (PILB-MsrB), and the fused subdomain (PILB-MsrA/ MsrB) of N. meningitidis were produced as folded entities. The central subdomain shows a methionine sulfoxide reductase A (MsrA) activity, whereas PILB-MsrB displays a methionine sulfoxide reductase B (MsrB) activity. The catalytic mechanism of PILB-MsrB can be divided into two steps: 1) an attack of the Cys-494 on the sulfur atom of the sulfoxide substrate, leading to formation of a sulfenic acid intermediate and release of 1 mol of methionine/mol of enzyme and 2) a regeneration of Cys-494 via formation of an intradisulfide bond with Cys-439 followed by reduction with thioredoxin. The study also shows that 1) MsrA and MsrB display opposite stereoselectivities toward the sulfoxide function; 2) the active sites of both Msrs, particularly MsrB, are rather adapted for binding proteinbound MetSO more efficiently than free MetSO; 3) the carbon C␣ is not a determining factor for efficient binding to both Msrs; and 4) the presence of the sulfoxide function is a prerequisite for binding to Msrs. The fact that the two Msrs exhibit opposite stereoselectivities argues for a structure of the active site of MsrBs different from that of MsrAs. This is further supported by the absence of sequence homology between the two Msrs in particular around the cysteine that is involved in formation of the sulfenic acid derivative. The fact that the catalytic mechanism takes place through formation of a sulfenic acid intermediate for both Msrs supports the idea that sulfenic acid chemistry is a general feature in the reduction of sulfoxides by thiols.Peptide methionine sulfoxide reductase (MsrA) 1 activity is described as being involved in the virulence of the pathogens Escherichia coli, Streptococcus pneumoniae, Erwinia chrysanthemi, Mycoplasma genitalium, and Neisseria gonorrhoeae (1-4). Inspection of the alignment of the corresponding protein sequences shows that all possess in common a sequence that displays an MsrA activity. This MsrA activity has now been well characterized at the structural level (5, 6) and the enzymatic level (7). In particular, a sulfenic acid intermediate has been shown to be formed on Cys-51 of E. coli MsrA during the reduction of the sulfoxide function of methionine sulfoxide (MetSO). The active site can be represented as an open basin in which Cys-51, located at the N terminus of an ␣-helix, is accessible. Compared with the E. coli MsrA, the MsrAs from S. pneumoniae and from N. meningitidis or N. gonorrhoeae (called PILB) contain, in addition, an extension at the C terminus and at the C and N termini, respectively. This raised the question of the role of these extensions, in particular of the C-terminal extension. Sequence comparisons of the C-extension of PILB show amino acid identities with open reading frames of which no function has been...

N,N'-linked oligoureas with proteinogenic side chains are peptide backbone mimetics belonging to the gamma-peptide lineage. In pyridine, heptamer 4 adopts a stable helical fold reminiscent of the 2.6(14) helical structure proposed for gamma-peptide foldamers. In the present study, we have used a combination of CD and NMR spectroscopies to correlate far-UV chiroptical properties and conformational preferences of oligoureas as a function of chain length from tetramer to nonamer. Both the intensity of the CD spectra and NMR chemical shift differences between alphaCH2 diastereotopic protons experienced a marked increase for oligomers between four and seven residues. No major change in CD spectra occurred between seven and nine residues, thus suggesting that seven residues could be the minimum length required for stabilizing a dominant conformation. Unexpectedly, in-depth NMR conformational investigation of heptamer 4 in CD3OH revealed that the 2.5 helix probably coexists with partially (un)folded conformations and that Z-E urea isomerization occurs, to some degree, along the backbone. Removing unfavorable electrostatic interactions at the amino terminal end of 4 and adding one H-bond acceptor by acylation with alkyl isocyanate (4 --> 7) was found to reinforce the 2.5 helical population. The stability of the 2.5 helical fold in MeOH is further discussed in light of unrestrained molecular dynamics (MD) simulation. Taken together, these new data provide additional insight into the folding propensity of oligoureas in protic solvent and should be of practical value for the design of helical bioactive oligoureas.

necessary to produce reliable results.The rotamer populations of the -CHIOH side chain were calculated according to equations 4.1 1-4.13 of D a v i e~.~~ The results are as follows: 45% gauche', 36% trans, and 19% gauche-. As usual, the gauche' conformation predominates, but its contribution is not as large as in purine nucleosides. These results can be attributed to electrostatic repulsion between N(2) and O(5'). The distance between these two atoms would be very short (<3.0 A) if the furanose ring was in the C(2') endo/C(3') exo and the side chain in the gauche' conformation. Consequently, the gauche' rotamer can occur only when the ring pucker is C(3') endo/C(4') exo.et Figures 1 and 4 were drawn with the ORTEP program of Johnson.38 Haasnoot's program CAGPLUS, which relates vicinal coupling constants to torsion angles between protons,34 was used to determine the conformation of the diazepine ring.Registry No. 2, 98720-84-4. Supplementary Material Available:Tables of anisotropic temperature parameters and a list of observed and calculated structure amplitudes (8 pages). Ordering information is given on any current masthead page. (37) Ahmed, F. R.; Hall, S. R.; Pippy, M. E.; Huber, C. P.Abstract: Model dipeptides t-BuCO-L-Pro-Xaa-NHMe (Xaa = L-or D-Leu, Val, Cys, Met, Phe, and Tyr) with an aliphatic, aromatic, or weakly polar Xaa side chain have been investigated in solution by IR and 'H NMR spectroscopies and in the solid state by X-ray diffraction. The heterochiral dipeptides L-Pro-DXaa are found to accommodate the same 011-turn conformation in both states. The homochiral dipeptides L-Pro-L-Xaa experience more conformational freedom since the 0-turn conformation is shown to be of the 01 type in solution and of the 011 type in the crystal. This conformational change probably arises from the molecular packing forces and essentially from an intermolecular hydrogen bond between the Xaa NH and C'O groups of two neighboring molecules. This shows that the PII-turn conformation is a stable disposition of a LL-dipeptide sequence provided there is some energy compensation through an intermolecular interaction in oligopeptides or a long-range interaction in larger peptides. @-Folded regions or @-turns are conformational units of primary importance for the three-dimensional structure of peptides and protein~.~J Four consecutive amino acid residues indexed from Registry No. t-BuCO-L-Pro-1

Leucine-enkephalin (Try1-Gly2-Gly3-Phe4-Leu5) has been crystallized as a trihydrate from water solution. X-ray diffraction reveals a tightly folded molecular conformation with two fused beta III- (Gly2-Gly3) and beta I- (Gly3-Phe4) turns. The Tyr1 and Phe4 aromatic rings have a close orthogonal arrangement analogous to the tyramine and cyclohexenyl rings in morphine. This suggests that the conformation found in the trihydrate crystal structure could be required for recognition by mu-receptor sites.

The analogue gamma-(dimethylsila)-proline, denoted silaproline (Sip), was synthesized in both enantiomerically pure forms by diastereoselective alkylation of a chiral glycine equivalent with use of Schöllkopf's bis-lactim ether method. The effect of replacing a proline residue in model peptides by this new proline surrogate has been examined in the crystal state by X-ray diffraction and in solution by IR absorption and NMR techniques. Silaproline and proline-containing sequences exhibit very similar conformational properties. Silaproline was also substituted for proline in a neurotensin (8-13) analogue that retained biological activity and exhibited enhanced resistance to biodegradation.

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The X‐ray diffraction experiments on peptides and related molecules which have been carried out in Western Europe, except Italy, in the last eight years are reviewed. The crystal structures of some bioactive peptides such as Leu‐enkephalin (a neurotransmitter), cyclosporin A (an immunomodulator in both the free and protein‐bound state), balhimycin (an antibiotic) and octreotide (a somatostatin analogue) are briefly presented. Crystallized N‐ and C‐protected model peptides have given an insight into the folding tendency and folding modes depending on the peptide sequences. The crystal structures of various pseudopeptide molecules reveal how the three‐dimensional structure of peptide analogues can be modulated by substituting non‐peptide groups for the peptide bond. A few examples of structural mimetics of the β‐ and γ‐turns, and of templates for α‐helix induction are also presented. © 1996 John Wiley & Sons, Inc.