L-asparaginases (EC 3.5.1.1) are hypothesized to play an important role in nitrogen supply to sink tissues, especially in legume-developing seeds. Two plant L-asparaginase subtypes were previously identified according to their K(+)-dependence for catalytic activity. An L-asparaginase homologous to Lupinus K(+)-independent enzymes with activity towards beta-aspartyl dipeptides, At5g08100, has been previously characterized as a member of the N-terminal nucleophile amidohydrolase superfamily in Arabidopsis. In this study, a K(+)-dependent L-asparaginase from Arabidopsis, At3g16150, is characterized. The recombinants At3g16150 and At5g08100 share a similar subunit structure and conserved autoproteolytic pentapeptide cleavage site, commencing with the catalytic Thr nucleophile, as determined by ESI-MS. The catalytic activity of At3g16150 was enhanced approximately tenfold in the presence of K(+). At3g16150 was strictly specific for L-Asn, and had no activity towards beta-aspartyl dipeptides. At3g16150 also had an approximately 80-fold higher catalytic efficiency with L-Asn relative to At5g08100. Among the beta-aspartyl dipeptides tested, At5g08100 had a preference for beta-aspartyl-His, with catalytic efficiency comparable to that with L-Asn. The phylogenetic analysis revealed that At3g16150 and At5g08100 belong to two distinct subfamilies. The transcript levels of At3g16150 and At5g08100 were highest in sink tissues, especially in flowers and siliques, early in development, as determined by quantitative RT-PCR. The overlapping spatial patterns of expression argue for a partially redundant function of the enzymes. However, the high catalytic efficiency suggests that the K(+)-dependent enzyme may metabolize L-Asn more efficiently under conditions of high metabolic demand for N.