Gunnar Reiske/Sandbox 102: Difference between revisions

The gluten protein complex is made up of gliadin and glutenin components. Of the complex, gliadin directly affects the induction of an innate immune response via the proline and glutamine peptide sequences. In the small intestine of patients with celiac disease, HLA-DQ2 restricted T-cells are present. After ingestion of a gluten product, the gliadin peptides enter the circulatory system and come into contact with lymphocytes and the gliadin-specific, HLA-DQ2 restricted T-cells, which is the fundamental step in producing the inflammatory response associated with celiac disease.<ref name ="Maiuri">Maiuri, L., Ciacci, C., Ricciardelli, I., Vacca, L., Raia, V., Auricchio, S., . . . Londei, M. (2003). Association between innate response to gliadin and activation of pathogenic T cells in coeliac disease. Lancet, 362(9377), 30-37. doi:10.1016/S0140-6736(03)13803-2</ref>

Human leukocyte antigens (HLA) are responsible for regulation of the immune system. The binding of gliadin peptides to HLA should be the same in celiac and non-celiac patients. However, it is unclear why only specific individuals produce the gliadin-specific, HLA-DQ2 restricted T-cells with pathogenic consequences. These implications suggest an underlying genetic component. <ref name ="Maiuri" />

HLA-DQ2 and HLA-DQ8 proteins are at high concentrations, 95% and 5% of all patients with celiac disease, respectively. The proteins are able to amplify the autoimmune response by binding the gluten complex to the transglutaminase tissue of the small intestine lumen. The new complexes are comprised of three chains, two being MHC class II antigens of alpha helical and beta sheet nature with the third being the gluten peptide.  The MHC class II molecules HLA-DQ2 and HLA-DQ8 are human leukocyte antigens associated with the genetic risk of developing celiac disease and serves as a MHC class II molecule in the immune system of the body. In addition, the complexes have conformations that only expose the gliadin sequence that has gastrointestinal protease resistance. As a result, the body sends out antibodies, which bind the epitopes of the complex thus labeling it as a toxin. The end result is an amplified autoimmune response that attacks the lining of the small intestine to help rid the body of the resistant complex.<ref name="Mellins"> Mellins, E., & Stern, L. (n.d.). HLA-DM and HLA-DO, key regulators of MHC-ll processing and presentation. Current Opinion in Immunology, 26, 115-122. February 2014.

Prolyl endopeptidases (PEPs) are a family of serine protease enzymes that help accelerate the breakdown of proline residues in peptides. Since gluten is a proline-rich complex, these enzymes can be used to help treat individuals with celiac disease. <ref name="shan">Shan, L., I. I. Mathews, and C. Khosla. "Structural and Mechanistic Analysis of Two Prolyl nEndopeptidases: Role of Interdomain Dynamics in Catalysis and Specificity." Proceedings of the National Academy of Sciences 102.10 (2005): 3599-604. Web.</ref>

A study was done to explore the structural features of two bacterial PEPs, one with a bound enzyme and one without. Both PEPs have two domains: one that is the catalytic binding site and one called the propeller domain. <ref name="shan" />

With further investigation of domain features of the PEP isolated from Myxococcus xanthus, interactions were observed that give the enzyme its proline-cleaving properties. Interactions between the domains of the PEP isolated from Myxococcus xanthus and the inhibitor in the binding pocket, which is colored yellow. Residues from the catalytic domain are labeled in black with their hydrogen bonds represented by blue dashed lines and residues from the propeller domain are labeled in gray with the salt bridges represented by yellow dashed lines. <ref name="shan" />

Using the structural components of the open and bound forms of PEP enzymes, a mechanism was proposed in which the incoming proline-rich peptide causes a conformational change that opens the catalytic binding site. This conformation is stabilized by the prolines in the substrate interacting with the arginine and aspartate residues in the binding site. The propeller region does not interact with the bound substrate, but the aspartates and glutamate residues interact with arginine residues in the catalytic region to stabilize the unbound form of the enzyme. <ref name="shan" />

Bacterial PEPs can detoxify immunotoxic proline-rich peptides in gut lumen of celiac patients by breaking down the gliadin before it reaches the small intestines. Bacterial PEPs can be given orally as a therapeutic treatment for those with celiac disease. Further research would need to be done on how we can make these enzymes more acid-stable to withstand the acidic environment of the human intestine. <ref name="shan" />