Effect of substrate concentration, pH, and temperature on the activity of the complex glucose–fructose oxidoreductase/glucono-δ-lactonase present in calcium alginate-immobilized Zymomonas mobilis cells

The action of the enzymes glucose–fructose oxidoreductase (GFOR) and glucono-δ-lactonase (GL), present in calcium alginate-immobilized Zymomonas mobilis cells, was characterized in relation to substrate concentration (0.05–2.0 mol L−1), pH (5.2–9.7), and temperature (34–59 °C). Higher enzymatic activities were obtained at pH 7.8 and 8.2 and at 47 and 50 °C, which were 80% higher than the conditions presented in previously defined conditions for free cells, namely at pH 6.4 and 39 °C. Further analysis indicated that these findings are related to the diffusional barrier resulting from the calcium alginate beads, which hinder the transport of gluconic acid from the inner space of the beads to the external medium. This behavior was reproduced during the initial moments of bioconversion performed at pH 7.8 and 47 °C. Nevertheless, during the last hours of the process, the reaction stopped because of inadequate pH levels inside the beads. The results suggest that a variable pH – from 7.8 to 6.4 – and a constant temperature of about 47 °C are the best conditions for achieving good conversion yields and productivities.