Arginine Kinase Activates Arginine for Phosphorylation by Pyramidalization and Polarization

Arginine phosphorylation plays numerous roles throughout biology. Arginine kinase (AK) catalyzes the delivery of an anionic phosphoryl group (PO₃^–) from ATP to a planar, trigonal nitrogen in a guanidinium cation. Density functional theory (DFT) calculations have yielded a model of the transition state (TS) for the AK-catalyzed reaction. They reveal a network of over 50 hydrogen bonds that delivers unprecedented pyramidalization and out-of-plane polarization of the arginine guanidinium nitrogen (Nη2) and aligns the electron density on Nη2 with the scissile P–O bond, leading to in-line phosphoryl transfer via an associative mechanism. In the reverse reaction, the hydrogen-bonding network enforces the conformational distortion of a bound phosphoarginine substrate to increase the basicity of Nη2. This enables Nη2 protonation, which triggers PO₃^– migration to generate ATP. This polarization–pyramidalization of nitrogen in the arginine side chain is likely a general phenomenon that is exploited by many classes of enzymes mediating the post-translational modification of arginine.