Characterization of porphobilinogen synthase from an aerobic photosynthetic bacterium, erythrobacter sp. strain OCh 114

Porphobilinogen synthase (formerly 5-aminolevulinic acid dehydratase, EC 4.2.1.24) was purified 7,405-fold from an aerobic photosynthetic bacterium, Erythrobacter sp. strain OCh 114. The molecular weight of the enzyme was determined to be 260,000 by Sephadex G-200 gel filtration. The enzyme had a single pH optimum at 8.0 and showed no requirement for metal ion and thiol compound for its maximum activity. The K_m value for 5-aminolevulinic acid was 0.29 mM. 4,5-Dioxovaleric acid and levulinic acid were found to be competitive inhibitors of the enzyme, with K_i values of 0.65 and 0.80 mM, respectively. The enzyme was extremely labile in acidic pH and almost completely lost its activity within 1 h at pH 6.0 and 30°C. This Erythrobacter enzyme seems to be similar to the enzyme from the anaerobic photosynthetic bacterium Rhodobacter capsulatus in its molecular and catalytic properties.