Na⁺-site in FIXa protease domain. The Na⁺-site in FIXa involves coordination from four carbonyl groups (184A, 185, 221A and 224) from the protein and two water molecules, W1 and W2. Electron density (2F_obs- F_calc) is contoured at 1σ and sodium is labeled as Na. Note that the Na⁺-site is linked through water to the carboxylate of Asp189. The salt bridge between PBZ and Asp189 at the S1 site is shown. The coordination of the Na⁺-site is similar to that in FXa, FVIIa and APC. It differs from thrombin where two main chain carbonyls and four water molecules are coordinated to Na⁺.