The intracellular sucrase (SacA) of Zymomonas mobilis is not involved in sucrose assimilation

The intracellular sucrase SacA from Zymomonas mobilis was purified to homogeneity from a recombinant E. coli strain containing the SacA gene under an expression system. The protein was monomeric with a molecular mass of 58 kDa. The sucrase activity was maximal at 25 degrees C and thermal stability of the purified protein was low (50% recovery after 30 min at 46 degrees C ). The activation energy was low at 33 kJ mol(-1). Maximum activity was at pH 6.5. Activity was strongly inhibited (> 99%) by SH blocking reagents and reducing agents slightly (10-60%) increased the activity of purified SacA. The sucrase showed a low K-M (42 mM) and k(cat) (125 s(-1)) which indicated its very low efficiency for sucrose hydrolysis. A mutant strain of Z. mobilis not able to grow on sucrose was isolated. This strain (ZM4S) lacked the two sucrases SacB and SacC but SacA was present in the intracellular fraction. Therefore, SacA alone is unable to allow growth Z. mobilis on sucrose.