Evolution of cation binding in the active sites of P-loop nucleoside triphosphatases

The activity of cellular nucleoside triphosphatases (NTPases) must be tightly controlled to prevent spontaneous ATP hydrolysis leading to cell death. While most P-loop NTPases require activation by arginine or lysine fingers, some of the apparently ancestral ones are, instead, activated by potassium ions, but not by sodium ions. We combined comparative structure analysis of P-loop NTPases of various classes with molecular dynamics (MD) simulations of Mg-ATP complexes in water and in the presence of potassium, sodium, or ammonium ions. In all analyzed structures, the conserved P-loop motif keeps the triphosphate chains of enzyme-bound NTPs in an extended, catalytically prone conformation, similar to that attained by ATP in water in the presence of potassium or ammonium ions bound between alpha- and gamma-phosphate groups. The smaller sodium ions could not reach both alpha- and gamma-phosphates of a protein-bound extended phosphate chain and therefore are unable to activate most potassium-dependent P-loop NTPases.