The old-dog catalase
has a new trick
Published January 05 2016 Immunohistochemical staining of H. pylori (brown) from a gastric biopsy. IMAGE COURTESY OF WIKIMEDIA
One way the human body protects itself against invading bacteria is by generating reactive oxygen species that harm the bacteria. Helicobacter pylori, a pathogen present in the stomachs of almost half of the world’s human population, has developed mechanisms to resist the damage induced by ROS. For more than 100 years, scientists have known that catalase helps protect the H. pylori against ROS by breaking down harmful hydrogen peroxide. But in a recent Paper of the Week in the Journal of Biological Chemistry, Stéphane Benoit and Robert Maier at the University of Georgia showed that catalase is no one-trick pony: In addition to its enzymatic activity, catalase has another mechanism for protection against harmful ROS that is independent from what was originally thought to be catalase’s only enterprise.
Part two of catalase’s story began 20 years ago with the observation that methionine residues in proteins undergo oxidation in the presence of ROS to form methionine sulfoxide. Scientists hypothesized that these methionine residues were acting as antioxidants to save other sites, such as DNA bases. However, evidence to support this hypothesis was lacking.
A few years ago, researchers from the Maier lab showed that methionine residues in catalase undergo oxidation in the presence of hypochlorous acid, an ROS produced by many white blood cells in response to an infection. This oxidation is reversible. Methionine sulfoxide reductase physically interacts with catalase to reduce methionine sulfoxide back to methionine.
Taken together, this led Benoit and Maier to hypothesize that the methionine residues in catalase serve as recyclable antioxidants to protect the bacteria from the host-generated ROS, something that never had been observed in an organism before. In order to show that catalase’s methionine antioxidant role is independent from its enzymatic function, Benoit and Maier engineered enzymatically inactive catalase, known as apo-catalase. They treated media containing hypochlorous acid, an oxidant that is normally lethal to H. pylori, with apo-catalase before exposing H. pylori to the media. The survival of H. pylori indicated that the apo-catalase protected the bacterium from oxidative stress in vitro. The investigators were able to show that this was also true in vivo by growing H. pylori expressing the apo-catalase on agar media containing hypochlorous acid. The H. pylori expressing the apo-catalase grew just as well as H. pylori expressing enzymatically active catalase, while H. pylori lacking catalase were more sensitive to hypochlorous acid.
Benoit and Maier showed that this process is dependent on methionine oxidation by knocking out the Msr enzyme that recycles methionine sulfoxide back to methionine after oxidation. The apo-catalase/Msr knockout showed pronounced sensitivity to hypochlorous acid when compared to the apo-catalase alone, signifying that the methionines must be recycled for apo-catalase to act effectively as an antioxidant.
Next, Benoit and Maier explored whether H. pylori expressing apo-catalase could colonize the stomachs of laboratory mice. The authors showed that H. pylori expressing apo-catalase was able to colonize the stomachs as efficiently as H. pylori expressing wild-type catalase and was several orders of magnitude better than H. pylori catalase knockouts. This revealed that catalase is conferring a fitness advantage for H. pylori that is completely independent of its enzymatic activity by protecting the bacterium from host-generated ROS in the mice.
The results of this study revealed that catalase is multifaceted, protecting invading H. pylori from host-generated ROS through its ability to break down hydrogen peroxide enzymatically as well as quenching ROS via methionine oxidation. H. pylori infections have been linked to higher rates of stomach ulcers and cancers, and, with the serious threat that antibiotic resistance poses to human health, understanding these protective mechanisms can lead to the design of new drugs.
is aPh.D. candidate at Carnegie Mellon University.