Glucose provides energy and components for the cells in our body. Scientists have known for many years that the normal breakdown of glucose is disrupted under oxidative stress as can arise, for example, in inflammatory or toxic processes. The reason for this is that one of the key enzymes in glucose breakdown, GAPDH (glycerinaldehyde 3-phosphate dehydrogenase), is oxidized extremely rapidly and efficiently by hydrogen peroxide (H2O2) and is inactivated in the process. In chronic inflammatory reactions, immune cells permanently release H2O2 – a characteristic of oxidative stress.
But why is it that GAPDH is inactivated by hydrogen peroxide much more easily and rapidly than other enzymes? And what does the interruption of the glucose breakdown mean for the cell? “Until now, scientists have believed that the oxidative inactivation of GAPDH is just an inevitable side effect of its generally high reactivity,” says Tobias Dick from the German Cancer Research Center (Deutsches Krebsforschungszentrum, DKFZ).”To break down glucose efficiently, the GAPDH enzyme has a highly reactive center, which reacts non-specifically with H2O2 and thereby inhibits itself,” says Dick, describing the commonly used explanation of this phenomenon. Thus, it has been presumed until now that for the cell to produce energy from glucose efficiently, it has to put up with the fact that the glucose metabolism is disrupted in the case of oxidative stress.
In a collaboration with a team headed by Frauke Gräter from the Heidelberg Institute for Theoretical Studies (HITS) and colleagues from the National Institute of Oncology in Budapest, Dick’s working group has now shown that the contrary is the case: The scientists have discovered a previously unknown mechanism that specifically induces the reaction of GAPDH with H2O2.
Using laboratory experiments and computer simulations, the researchers found out that the high sensitivity of GAPDH to H2O2 is not a side effect of GAPDH’s general reactivity, as scientists have believed until now. Instead, GAPDH accelerates its own oxidative inactivation in a specific process that is independent of its activity in the glucose metabolism.
“We were surprised to discover that this special mechanism can be found in the GAPDH of almost all life forms, from bacteria to man. All this suggested that it plays a fundamental role for survival under stress conditions,” Dick explains.
The scientists generated a genetically modified GAPDH that fully retains its normal glycolytic activity without being sensitive to inhibition by H2O2. In yeast strains, they replaced the normal enzyme by the oxidation-insensitive variant. No differences were observed under normal conditions, i.e., glucose metabolism and cell growth were the same with both variants.
Under oxidative stress, however, the cells with the normal oxidation-sensitive GAPDH had a significant advantage for growth: As the researchers demonstrated, the oxidative blocking of GAPDH led to an alternative utilization of glucose. This alternative path primarily promoted the formation of NADPH, a molecule that counteracts oxidation and helps the cell cope with oxidative stress. Thus, the disruption of the normal glucose metabolism in the cell generates a key advantage for survival. This also explains why the mechanism of oxidative inactivation of GAPDH formed early in the evolution of organisms and has been conserved to the present day.
In a next step, the researchers plan to investigate whether cancer cells may also benefit from oxidative inactivation of GAPDH. David Peralta, the first author of the study, explains: “Cancer cells use particularly high amounts of glucose and additionally are under oxidative stress. We therefore presume that they use oxidative inactivation of GAPDH for their own purposes. By switching off this mechanism, we might hit cancer cells extremely hard.”
David Peralta, Agnieszka K Bronowska, Bruce Morgan, Éva Dóka, Koen Van Laer, Péter Nagy, Frauke Gräter, Tobias P Dick (2015). A proton relay enhances H2O2 sensitivity of GAPDH to facilitate metabolic adaptation. Nature Chemical Biology 2015, DOI: 10.1038/nchembio.1720
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