Expression, purification, and biochemical characterization of an NAD-dependent homoserine dehydrogenase from the symbiotic Polynucleobacter necessarius subsp. necessarius.

Homoserine dehydrogenase (HSD), encoded by the hom gene, is a key enzyme in the aspartate pathway, which reversibly catalyzes the conversion of l-aspartate β-semialdehyde to l-homoserine (l-Hse), using either NAD(H) or NADP(H) as a coenzyme. In this work, we presented the first characterization of the HSD from the symbiotic Polynucleobacter necessaries subsp. necessarius (PnHSD) produced in Escherichia coli. Sequence analysis showed that PnHSD is an ACT domain-containing monofunctional HSD with 436 amnio acid residues. SDS-PAGE and Western blot demonstrated that PnHSD could be overexpressed in E. coli BL21(DE3) cell as a soluble form by using SUMO fusion technique. It could be purified to apparent homogeneity for biochemical characterization. Size-exclusion chromatography revealed that the purified PnHSD has a native molecular mass of ∼160 kDa, indicating a homotetrameric structure. The oxidation activity of PnHSD was studied in this work. Kinetic analysis revealed that PnHSD displayed an up to 1460-fold preference for NAD over NADP, in contrast to its homologs. The purified PnHSD displayed maximal activity at 35 °C and pH 11. Similar to its NAD-dependent homolog, neither NaCl and KCl activation nor L-Thr inhibition on the enzymatic activity of PnHSD was observed. These results will contribute to a better understanding of the coenzyme specificity of the HSD family and the aspartate pathway of P. necessarius.