Oxygen- and Growth Rate-Dependent Regulation of Escherichia coli Fumarase (FumA, FumB, and FumC) Activity

Abstract

Escherichia coli contains three biochemically distinct fumarases which catalyze the interconversion of fumarate to L-malate in the tricarboxylic acid cycle. Batch culture studies indicated that fumarase activities varied according to carbon substrate and cell doubling time. Growth rate control of fumarase activities in the wild type and mutants was demonstrated in continuous culture; FumA and FumC activities were induced fourto fivefold when the cell growth rate (k) was lowered from 1.2/h to 0.24/h at 1 and 21% O 2 , respectively. There was a twofold induction of FumA and FumC activities when acetate was utilized instead of glucose as the sole carbon source. However, these fumarase activities were still shown to be under growth rate control. Thus, the activity of the fumarases is regulated by the cell growth rate and carbon source utilization independently. Further examination of FumA and FumC activities in a cya mutant suggested that growth rate control of FumA and FumC activities is cyclic AMP dependent. Although the total fumarase activity increased under aerobic conditions, the individual fumarase activities varied under different oxygen levels. While FumB activity was maximal during anaerobic growth (k ‫؍‬ 0.6/h), FumA was the major enzyme under anaerobic cell growth, and the maximum activity was achieved when oxygen was elevated to 1 to 2%. Further increase in the oxygen level caused inactivation of FumA and FumB activities by the high oxidized state, but FumC activity increased simultaneously when the oxygen level was higher than 4%. The same regulation of the activities of fumarases in response to different oxygen levels was also found in mutants. Therefore, synthesis of the three fumarase enzymes is controlled in a hierarchical fashion depending on the environmental oxygen that the cell encounters.

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“…However, our study has shown that the glucose supplement did not make a significant difference in the growth rates when E. coli was growing in LB medium. These findings are consistent with those of the previous study showing that the growth rate of E. coli is barely affected when glucose is added to LB media (31). In this study, the doubling times were similar: 41 min when wild-type E. coli was grown in LB and 37 min when it was grown in LB supplemented with glucose (40 mM).…”

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“…Fumarase A is the major active E. coli fumarase under microaerophilic conditions (49). E. coli fumarase B has a higher affinity for malate than for fumarate and appears to operate under anaerobic conditions to generate fumarate from malate (49,55). Fumarase C is highly active under aerobic growth conditions (49).…”

“…There are three fumarases in Escherichia coli, fumarase A, fumarase B, and fumarase C, encoded by fumA, fumB, and fumC, respectively (2, 31, 54). Fumarase A is the major active E. coli fumarase under microaerophilic conditions (49). E. coli fumarase B has a higher affinity for malate than for fumarate and appears to operate under anaerobic conditions to generate fumarate from malate (49,55).…”

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“…Fumarate hydratase A, the component of the Kreb's cycle (citric acid cycle) that converts fumarate to malate, is encoded by fumA (46). The citric acid cycle is one of the three metabolic pathways of cellular respiration and is necessary for fuel catabolism and ATP production.…”