PHOSPHOMANNOSE ISOMERASE OF XANTHOMONAS-CAMPESTRIS - A ZINC ACTIVATEDENZYME

Phosphomannose isomerase (pmi, EC 5.3.1.8) was purified to homogeneity from a wild strain of Xanthomonas campestris. The apparent molecular weight as determined by SDS-PAGE and Sephadex G-100 Superfine was foun d to be 58 kDa. The purified enzyme showed a single band on acrylamide gel electrophocusing with pI = 5.25. The optimum pH was 7.0 and the K m for D-mannose-6-phosphate was 2 mM. Pmi can be activated by bivalent cations with the order of Co2+>Zn2+>Mn2+>Ni2+>Ca2+. Addition of low c oncentration of ZnCl2 (2 x 10(-7) M) in the growth medium resulted in the enhancement of pmi activity to around 2.5 x fold. The half life of pmi, as it was measured by the addition of chloramphenicol, was 110 m in, whereas in the medium supplemented with ZnCl2 was 270 min. Chemica l modification experiments implied the existence of one histidyl resid ue located at or near the active site.