Abstract
The decarboxylation of pyruvate is a central reaction in the carbon metabolism of all organisms. It is catalyzed by the pyruvate:ferredoxin oxidoreductase (PFOR) and the pyruvate dehydrogenase (PDH) complex. Whereas PFOR reduces ferredoxin, the PDH complex utilizes NAD+. Anaerobes rely on PFOR, which was replaced during evolution by the PDH complex found in aerobes. Cyanobacteria possess both enzyme systems. Our data challenge the view that PFOR is exclusively utilized for fermentation. Instead, we show, that the cyanobacterial PFOR is stable in the presence of oxygen in vitro and is required for optimal photomixotrophic growth under aerobic and highly reducing conditions while the PDH complex is inactivated. We found that cells rely on a general shift from utilizing NAD(H)- to ferredoxin-dependent enzymes under these conditions. The utilization of ferredoxins instead of NAD(H) saves a greater share of the Gibbs-free energy, instead of wasting it as heat. This obviously simultaneously decelerates metabolic reactions as they operate closer to their thermodynamic equilibrium. It is common thought that during evolution, ferredoxins were replaced by NAD(P)H due to their higher stability in an oxidizing atmosphere. However, the utilization of NAD(P)H could also have been favored due to a higher competitiveness because of an accelerated metabolism.
Highlights
IntroductionLife evolved under anaerobic conditions in an environment that was reducing and replete with iron and sulfur
FeS clusters, pyruvate:ferredoxin oxidoreductase and ferredoxinsLife evolved under anaerobic conditions in an environment that was reducing and replete with iron and sulfur
In line with this we found that the cyanobacterial pyruvate:ferredoxin oxidoreductase (PFOR) is stable in the presence of oxygen in vitro
Summary
Life evolved under anaerobic conditions in an environment that was reducing and replete with iron and sulfur. Prebiotic redox reactions that took place on the surfaces of FeS minerals, are at present mimicked by catalytic FeS clusters in a plethora of enzymes and redox carriers [3, 4]. That are small, soluble proteins containing 4Fe4S, 3Fe4S or 2Fe2S clusters and shuttle electrons between redox reactions. They display a wide range of redox potentials between -240 mV to -680 mV and are involved in a variety of metabolic pathways [5]. Ferredoxins are among the earliest proteins on Earth and are present in all three kingdoms of life [6]. FeS enzymes are especially widespread in anaerobes [7]
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