Sodium-23 and lanthanum-139 nuclear magnetic resonance studies of cation binding to aqualysin I, a thermostable serine protease.

Aqualysin I has at least two Ca2+-binding sites that have different affinities for Call. The binding of various metal ions to aqualysin I was studied using Na-23- and La-139-NMR spectrometry. Evidence is presented that Ca2+, La3+, and Na+ bind to the low-affinity Ca2+-binding site of aqualysin I, but Mg2+ does not. Our results confirm that binding of metals at the low-affinity Ca2+-binding site is essential for thermostabilization, since the addition of Mg2+ did not result in thermostabilization. La3+ was found to bind to both the low-affinity Ca2+-binding site and an additional metal ion-binding site that can also be involved in the thermostabilization of aqualysin I.