Na+/K+ exchange switches the catalytic apparatus of K-dependent plant L-asparaginase

Plant L-asparaginases, which release ammonia during conversion of L-asparagine to L-aspartate, are important in plant nitrogen management. They also decompose aberrant asparagine beta-peptides arising spontaneously with time, e.g. in seeds. The enzymatic activity of some of the plant enzymes has been known to depend on potassium but the mechanism of this dependence has been obscure. In a series of crystal structure analyses of a K-dependent enzyme from common bean in complex with different alkali metal cations, Magda Bejger was able to show that potassium coordination in a special activation loop rearranges a set of key residues (called the catalytic switch) so that a proper substrate binding site can be formed. The binding of L-Asn [here modeled by its L-Asp isostere (black), which is the reaction product] is enabled by a proper conformation of Arg224, which is part of the catalytic switch. In contrast, when sodium is coordinated in the activation loop, Arg224 swings away from the active site and substrate binding is disabled. The activation loop is shown in pink. The other metal-binding site (red) is the stabilization loop.

Dr. Bejger's results were published in Acta Crystallographica (Acta Cryst. D70, 1854-1872, 2014).

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Last update: January 22, 2017 (MJ)