Human pyridoxal kinase: Overexpression and properties of the recombinant enzyme

Pyridoxal kinase catalyses the phosphorylation of the vitamin B₆. A human brain pyridoxal kinase cDNA was isolated, and the recombinant enzyme was overexpressed in E. coli as a fusion protein with maltose binding protein (MBP). Pure pyridoxal kinase exhibits a molecular mass of about 40 kDa when examined by SDS-PAGE and FPLC gel filtration. The recombinant enzyme is a monomer endowed with catalytic activity, indicating that the native quaternary structure of pyridoxal kinase is not a prerequisite for catalytic function. Zn²⁺ is the most effective divalent cation in the phosphorylation of pyridoxal, and the human enzyme has maximum catalytic activity in the narrow pH range of 5.5-6.0. The K_m values for two substrates pyridoxal and ATP are 97 μM and 12 μM, respectively. In addition, the unfolding processes of the recombinant enzyme were monitored by circular dichroism. The values of the free energy change of unfolding (ΔG₀ = 1.2 kcal·mol^-1·K^-1) and the midpoint transition (1 M) suggested that the enzyme is more stable than ovine pyridoxal kinase against denaturation by guanidine hydrochloride. Intrinsic fluorescence spectra of the human enzyme from red-edge excitation and fluorescence quenching experiments showed that the tryptophanyl residues are not completely exposed and more accessible to neutral acrylamide than to the negatively charged iodide. The first complete set of catalytic and structural properties of human pyridoxal kinase provide valuable information for further biochemical studies on this enzyme.