NMR solution structure of the recombinant tick anticoagulant protein (rTAP), a factor Xa inhibitor from the tick Ornithodoros moubata

The solution structure of the recombinant tick anticoagulant protein (rTAP) was determined by 1H nuclear magnetic resonance (NMR) spectroscopy in aqueous solution at pH 3.6 and 36°C. rTAP is a 60-residue protein functioning as a highly specific inhibitor of the coagulation protease factor Xa, which was originally isolated from the tick Ornithodoros moubata. Its regular secondary structure consists of a two-stranded antiparallel β-sheet with residues 22–28 and 32–38, and an α-helix with residues 51–60. The relative orientation of these regular secondary structure elements has nearly identical counterparts in the bovine pancreatic trypsin inhibitor (BPTI). In contrast, the loop between the β-sheet and the C-terminal α-helix as well as the N-terminal 20-residue segment preceding the β-sheet adopt different three-dimensional folds in the two proteins. These observations are discussed with regard to the implication of different mechanisms of protease inhibition by rTAP and by Kunitz-type protein proteinase inhibitors.