Purification and properties of NADP‐isocitrate dehydrogenase from the unicellular cyanobacterium Synechocystis sp. PCC 6803

Abstract

NADP-dependent isocitrate dehydrogenase activity has been screened in several cyanobacteria grown on different nitrogen sources; in all the strains tested isocitrate dehydrogenase activity levels were similar in cells grown either on ammonium or nitrate. The enzyme from the unicellular cyanobacterium Synechocystis sp. PCC 6803 has been purified to electrophoretic homogeneity by a procedure that includes Reactive-Red-120-agarose affinity chromatography and phenyl-Sepharose chromatography as main steps. The enzyme was purified about 600-fold, with a yield of 38% and a specific activity of 15.7 U/mg protein. The native enzyme (108 kDa) is composed of two identical subunits with an apparent molecular mass of 57 kDa. Synechocystis isocitrate dehydrogenase was absolutely specific for NADP as electron acceptor. Apparent K_m values were 125, 59 and 12 μM for Mg²⁺, d,l-isocitrate and NADP, respectively, using Mg²⁺ as divalent cation and 4, 5.7 and 6 μM for Mn²⁺, d,l-isocitrate and NADP, respectively, using Mn²⁺ as a cofactor. The enzyme was inhibited non-competitively by ADP (K_i, 6.4 mM) and 2-oxoglutarate, (K_i, 6mM) with respect to isocitrate and in a competitive manner by NADPH (K_i, 0.6 mM). The circular-dichroism spectrum showed a protein with a secondary structure consisting of about 30%α-helix and 36%β-pleated sheet. The enzyme is an acidic protein with an isoelectric point of 4.4 and analysis of the NH₂-terminal sequence revealed 45% identity with the same region of Escherichia coli isocitrate dehydrogenase. The aforementioned data indicate that NADP isocitrate dehydrogenase from Synechocystis resembles isocitrate dehydrogenase from prokaryotes and shows similar molecular and structural properties to the well-known E. coli enzyme.