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Description
Rheumatoid arthritis (RA) is one of the most widespread autoimmune conditions, affecting ~1% of population worldwide. The inflammation of the joints in RA is driven by autoantibodies against citrullinated epitopes (ACPA). Proteins citrullination is a posttranslational modification catalyzed by peptidylarginine deiminases (PAD1, PAD2, PAD3, PAD4 and PAD6) converting positively charged Arg residues to neutral citrulline residues. Although citrullination occurs both in physiological and pathological conditions, ACPA are found exclusively in RA patients and appear in circulation years before clinical onset of the disease. It is accepted that in subjects with specific genetic risk factors immunotolerance breakdown leading to ACPA generation happens on inflamed mucosal surfaces where excessive protein citrullination is carried out by PAD2 or PAD4. However, despite intense research on PADs it is still a mystery how citrullination happens in vivo. PADs activity is calcium-dependent and requires non-physiologically high concentration of Ca2+ (above 5 mM). Up to date, an intense search for PAD cofactors lowering their Ca2+ requirement failed. The groundbreaking character of the project is based on our seminal finding that a group of glycosaminoglycans (GAG), including heparin, heparan sulphate and chondroitin are PAD activators, which lower the Ca2+ requirement of these enzymes to physiological levels, explaining in vivo activity of PADs. The experimental results are fully supported by initial structural docking/modelling studies in silico showing the potential high-affinity heparin binding pockets formed by a cluster of positively charged residues within the PAD2 and PAD4 structure. The aim of the project is to identify and characterize the binding pocket(s) responsible for activation, which will allow targeted design of the novel PAD-specific, allosteric blockers with therapeutic potential, based on previously unidentified activation mechanism.