From the confidence level of variables, it is apparent that
glycerol, glutamic acid, yeast extract, and K2HPO4 are the most positive
significant variables affecting c-PGA synthesis. The positive
significant effect of K2HPO4 could be attributed to the mechanism
of c-PGA synthetase in B. licheniformis, as proposed by Troy (1973)
and Gardner and Troy (1979). In the mechanism, the activation of
c-PGA synthetase is accompanied by the cleavage of ATP into AMP,
and consequently produces more c-PGA. In a similar way, K2HPO4
acts as the main source of phosphate in the medium that promotes
the activation of c-PGA synthetase. Du et al. (2005) reported that
the enhanced production of c-PGA was observed when glycerol
as a carbon source was added to the culture medium containing
citric acid. Although citric acid showed negative response in this
study, the addition of both glycerol and citric acid showed high
c-PGA production. This result is in agreement with the observations
reported by Du et al. (2005)