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Celiac Sprue, or gluten-sensitive enteropathy, is an inheritable human disease of the small intestine that is triggered by the dietary intake of gluten. Recently, several Pro- and Gln-rich peptide sequences (most notably PQPQLPY and analogs) have been identified from gluten with potent immunogenic activity toward CD4+ T cells from small intestinal biopsies of Celiac Sprue patients. These peptides have three unusual properties. First, they are relatively stable toward further proteolysis by gastric, pancreatic, and intestinal enzymes. Second, they are recognized and deamidated by human tissue transglutaminase (tTGase) with high selectivity. Third, tTGase-catalyzed deamidation enhances their affinity for HLA-DQ2, the disease-specific class II major histocompatibility complex heterodimer. In an attempt to seek a mechanistic explanation for these properties, we undertook secondary structural studies on PQPQLPY and its analogs. Circular dichroism studies on a series of monomeric and dimeric analogs revealed a strong polyproline II helical propensity in a subset of them. Two-dimensional nuclear magnetic resonance spectroscopic analysis confirmed a polyproline II conformation of PQPQLPY, and was also used to elucidate the secondary structure of the most helical variant, (d-P)QPQLPY. Remarkably, a strong correlation was observed between polyproline II content of naturally occurring gluten peptides and the specificity of human tTGase toward these substrates. Analogs with up to two d-amino acid residues retained both polyproline II helical content and transglutaminase affinity. Since the Michaelis constant (K m) is the principal determinant of tTGase specificity for naturally occurring gluten peptides and their analogs, our results suggest that the tTGase binding site may have a preference for polyproline II helical substrates. If so, these insights could be exploited for the design of selective small molecule inhibitors of this pharmacologically important enzyme.

High-resolution solid state 17 O NMR spectra of 17 O-enriched compounds were observed with multiple-quantum magic-angle-spinning (MQMAS) experiments. The resolution of the 17 O MQMAS spectra is approximately 30- to 150-fold higher than that found in conventional 17 O MAS spectra, making it possible to detect crystallographically distinct oxygen sites. It is shown that 17 O MQMAS studies are feasible for systems with 17 O quadrupole coupling constants up to 7 MHz, provided that sufficiently high radio-frequency field strengths (> 120 kHz and spinning speeds (∼ 20 kHz) can be achieved.