Protein expression and purification. The various enzymes studied here were prepared as described in our previous work. Briefly, genes were cloned into a pET24-b with resistance to kanamycin and subsequently transformed into E. coli BL21 (DE3). The proteins were purified by affinity chromatography taken advantage of a His-tag attached at the C-terminus. Protein stock solutions were prepared by exhaustive dialysis. Protein concentrations were determined spectrophotometrically using known values of the extinction coefficient.