Expression and characterisation of human glycerol kinase: the role of solubilising agents and molecular chaperones

Background: Glycerol kinase (GK; EC 2.7.1.30) facilitates the entry of glycerol into pathways of glucose and triglyceride metabolism and may play a potential role in Type 2 diabetes mellitus (T2DM). However, the detailed regulatory mechanisms and structure of the human GK are unknown. Methods: The human GK gene was cloned into the pET-24a(+) vector and over-expressed in Escherichia coli BL21 (DE3). Since the protein was expressed as inclusion bodies (IBs), various culture parameters and solubilising agents were used but they did not produce bioactive His-GK; however, co-expression of His-GK with molecular chaperones, specifically pKJE7, achieved expression of bioactive His-GK. The overexpressed bioactive His-GK was purified using coloumn chromatography and characterised using enzyme kinetics. Results: The overexpressed bioactive His-GK was purified apparently to homogeneity (∼295-fold) and characterised. The native His-GK was a dimer with a monomeric molecular weight of ∼55 kDa. Optimal enzyme activity was observed in TEA buffer (50 mM) at 7.5 pH. K(+) (40 mM) and Mg(2+) (2.0 mM) emerged as prefered metal ions for His-GK activity with specific activity 0.780 U/mg protein. The purified His-GK obeyed standard Michaelis–Menten kinetics with K(m) value of 5.022 µM (R(2)=0.927) for its substrate glycerol; whereas, that for ATP and PEP was 0.767 mM (R(2)=0.928) and 0.223 mM (R(2)=0.967), respectively. Other optimal parameters for the substrate and co-factors were also determined. Conclusion: The present study demonstrates that co-expression of molecular chaperones assists with the expression of bioactive human GK for its characterisation.