Influence of pH conditions on metabolic regulations in serine alkaline protease production by Bacillus licheniformis

Abstract

The effects of the controlled and uncontrolled pH conditions, as well as of the value of initial pH in the range of 7–7.5, on serine alkaline protease (SAP) production by Bacillus licheniformis were investigated on a defined medium with the single carbon source glucose in batch bioreactors. Besides uncontrolled pH operations, growth phase, production phase, and entire process control strategies were also applied. The concentrations of the product (SAP) and by-products, i.e. neutral protease, amylase, amino acids, and organic acids were determined in addition to SAP activities. For SAP production, uncontrolled pH operation was more favourable than the controlled pH operations; and pH 0=7.25 was optimum for SAP production where maximum enzyme activity was obtained as 390 U cm −3, while, the highest cell concentration was obtained at pH 0=7 in uncontrolled operation. Thereafter, by using the experimental data obtained at pH 0=7, 7.25 and 7.5 uncontrolled pH operations, perturbation effects of pH on the intracellular flux distributions were investigated for the growth (0< t<20 h) and SAP production (20< t≤43 h) periods. In the growth phase the fluxes of the glycolysis pathway and the TCA cycle increased with the increase in pH 0; further, due to the difference in proton electrochemical gradients generated, the lowest and the highest energies were produced at pH 0=7 and 7.5 conditions, respectively. In the SAP production period, at pH 0=7.5 and 7 the glycolysis pathway fluxes were, respectively, the lowest and the highest. However, the TCA cycle fluxes, amino acid synthesis fluxes and SAP synthesis flux were the highest at pH 0=7.25 condition. The diversions in the pathways and certain metabolic reactions and potential strategies for improving SAP production are also discussed.