Streptococcus mutans has an oxygen-sensitive enzyme, pyruvate formate-lyase (PFL), which is a key enzyme in anaerobic sugar fermentation. We have shown that S. mutans has an activating system, including a PFL-activating enzyme (PFL-activase) and an electron transport system; the latter transfers an electron from NADPH to PFL-activase, as occurs in Escherichia coli. NADPH was a physiological electron donor for the electron transport system and as little as 0.02 mM NADPH activated over 80% of PFL of S. mutans. The optimum pH of the PFL-activating system was around 6.8, whereas the optimum of the E. coli system is at alkaline pH. In addition, small dialyzable molecules in cell-free extracts participated in keeping PFL active in S. mutans. These results suggest that, in dental plaque under anaerobic conditions where sugar supply is often limited or pH frequently falls below neutrality, S. mutans always keeps PFL active through the activating system.
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