Structural analysis of K + dependence in L-asparaginases from Lotus japonicus

The molecular features responsible for the existence in plants of K+-dependent asparaginases have been investigated. For this purpose, two different cDNAs were isolated in Lotus japonicus, encoding for K+-dependent (LjNSE1) or K+-independent (LjNSE2) asparaginases. Recombinant proteins encoded by these cDNAs have been purified and characterized. Both types of asparaginases are composed by two different subunits, α (20 kDa) and β (17 kDa), disposed as (αβ)2 quaternary structure. Major differences were found in the catalytic efficiency of both enzymes, due to the fact that K+ is able to increase by tenfold the enzyme activity and lowers the Km for asparagine specifically in LjNSE1 but not in LjNSE2 isoform. Optimum LjNSE1 activity was found at 5—50 mM K+, with a Km for K+ of 0.25 mM. Na+ and Rb+ can, to some extent, substitute for K+ on the activating effect of LjNSE1 more efficiently than Cs+ and Li+ does. In addition, K+ is able to stabilize LjNSE1 against thermal inactivation. Protein homology modelling and molecular dynamics studies, complemented with site-directed mutagenesis, revealed the key importance of E248, D285 and E286 residues for the catalytic activity and K+ dependence of LjNSE1, as well as the crucial relevance of K+ for the proper orientation of asparagine substrate within the enzyme molecule. On the other hand, LjNSE2 but not LjNSE1 showed β-aspartylhydrolase activity (Km = 0.54 mM for β-Asp-His). These results are discussed in terms of the different physiological significance of these isoenzymes in plants.