Structural and functional studies of Arabidopsis thaliana glutamate dehydrogenase isoform 2 demonstrate enzyme dynamics and identify its calcium binding site

Glutamate dehydrogenase (GDH) is an enzyme at the crossroad of plant nitrogen and carbon metabolism. GDH catalyzes the conversion of 2-oxoglutarate into glutamate (2OG-* Glu), utilizing ammonia as cosubstrate and NADH as coenzyme. The GDH reaction is reversible, meaning that the NAD+-dependent reaction (Glu-* 2OG) releases ammonia. In Arabidopsis thaliana, three GDH isoforms exist, AtGDH1, AtGDH2, and AtGDH3. The subject of this work is AtGDH2. Previous reports have suggested that enzymes homologous to AtGDH2 contain a calcium-binding EF-hand motif located in the coenzyme binding domain. Here, we show that while AtGDH2 indeed does bind calcium, the binding occurs elsewhere and the region predicted to be the EF-hand motif has a completely different structure. As the true calcium binding site is > 20 & ANGS; away from the active site, it seems to play a structural, rather than catalytic role. We also performed comparative kinetic characterization of AtGDH1 and AtGDH2 using spectroscopic methods and isothermal titration calorimetry, to note that the isoenzymes generally exhibit similar behavior, with calcium having only a minor effect. However, the spatial and temporal changes in the gene expression profiles of the three AtGDH genes point to AtGDH2 as the most prevalent isoform.

Automated summary

Glutamate dehydrogenase (GDH) is an enzyme that plays a crucial role in the nitrogen and carbon metabolism of plants. This study focuses on AtGDH2, one of the three isoforms found in Arabidopsis thaliana. The researchers discovered that the calcium binding site in AtGDH2 is different from what was previously predicted, suggesting a structural rather than catalytic role. Comparative analysis of AtGDH1 and AtGDH2 showed similar behavior, with calcium having only a minor effect. However, gene expression profiles indicate that AtGDH2 is the most prevalent isoform.