Arabidopsis (HXK1 and HXK2) and yeast (HXK2) hexokinases overexpressed in transgenic lines are characterized by different catalytic properties

Three different Arabidopsis thaliana transgenic lines overexpressing AtHXK1, AtHXK2 and yeast HXK2 genes, respectively, were analyzed with regard to their hexokinase activity. The methodology employed was a non-denaturing electrophoresis followed by activity-staining. Three distinct hexokinase activities were detected and named, respectively, AtHXK1, AtHXK2 and YHXK2. Arabidopsis HXK1 and HXK2 showed high affinity for glucose, minor affinity for mannose, and very low specificity for fructose. In comparison, YHXK2 exhibited a higher affinity for both mannose and fructose. Galactose, 3-O-methyl glucose and 6-deoxyglucose showed no influence on hexokinase activity, while glucosamine and 2-deoxyglucose produced a clear inhibition at high concentrations. Trehalose did not cause inhibition. Instead, trehalose 6-phosphate (T6P) was an effective inhibitor of YHXK2 but not of AtHXK1 and AtHXK2. As a whole, these results indicate important differences between the properties of plant and yeast HXKs and provide useful information for a discussion concerning the sugar response of transgenic plants overexpressing AtHXK or YHXK2 genes.