Partial characterization and photolabeling of a Rhizobium meliloti polysaccharide methyltransferase with S-adenosylmethionine

Abstract

S-Adenosylmethionine (SAM) has been used to directly cross-link a polysaccharide specific methyltransferase isolated from Rhizobium meliloti HA. This peculiar enzyme transfers a methyl group to the 2-O-galacturonosyl residue of a teichuronic type polysaccharide and was very unstable. The apparent K_m for SAM was 0.46 mM. The Hill coefficient, n, was 1. The enzyme had an optimum pH of 8.2 and requires Mn²⁺ at concentration of 2 mM. The enzyme was inactivated by saline concentrations of 120 mM or greater and was eluted from Superose columns with an apparent molecular weight of 28 kDa. The isoelectric point was close to 7.0. To elucidate the relationship between chemical structure and catalytic function, (³H)SAM was cross-linked to the enzyme and the enzymatic activity was assayed in presence and in absence of commercial substrate analogs. Cross-linking was performed by direct irradiation of enzyme and (³H)SAM. The uptake of radioactivity was linear up to about 20 min and then reached a plateau. This irreversible junction is specific, as shown by a number of different criteria. Several competitive inhibitors were able to affect this photoactivated cross-linkage. As the concentration of inhibitors increased, both, the level of photolabeling and enzyme activity always decreased. The SAM-enzyme adduct was shown to be a single protein band by SDS polyacrylamide gel electrophoresis.