Biological catalysts are a quintessential part of the chemistry of life that play key roles in the production of cellular components, the chemical interactions between these molecules and the transformation and use of energy. A large subclass of these catalytic enzymes is utilizing redox reactions, i.e. reactions that transfer electrons from one molecule (reduction) to another (oxidation). Such redox reactions are often used as direct means of cellular signal transduction, but they can also control the function of proteins by altering their conformation, stability, activity, or interaction with other molecules.
Our lab focuses on the redox enzymes that orchestrates the response to oxidative stress – an imbalance between cellular oxidants and antioxidants. Oxidative stress leads to impaired signaling as well as damage to cellular components and is linked to many human diseases and is a key factor in the aging process. Thus, we seek to understand how cellular components are modified by redox reactions and how the intrinsic response system to oxidative stress executes its function in the cell.
Redox Regulation of Membrane Enzymes
Conformational flexibility and structural plasticity (i.e. the ability to change secondary elements and tertiary contacts to remodel the structure) are essential for proteins’ function. Between 10-20% of any genome is composed of intrinsically disordered proteins (IDPs), which lack stable secondary and tertiary structure, and function in signaling and regulation.
Despite their large numbers there is only a handful of IDPs with known enzymatic activity. Most likely this is connected to the need of such enzymes to ‘search’ for the proper conformation necessary for substrate recognition and catalysis, a process that potentially renders such enzymes inefficient. Thus, it was surprising when two of the membrane-bound IDPs were identified as enzymes. These two selenoproteins, SELENOS and SELENOK, have come to define a yet uncharacterized family of eukaryotic membrane proteins that are involved in oxidative health. While both proteins appear to be central to the regulation of membrane complexes in terms of their stability, trafficking, and recycling, neither their substrates nor their precise cellular roles are known. Specifically, we aim to answer the following questions:
•Why is the redox enzymatic activity necessary for resolving oxidative stress?
•Do the two membrane-bound IDPs have specific protein partners or are they promiscuous?
•How do the redox reactions of the two proteins regulate their conformation, and how, in turn, does this play into their function?
The Oxidative Stress Response of Cells
When cysteine reacts with reactive oxygen species (ROS), it forms the transient intermediate sulfenic acid (R-SOH). Once formed, sulfenic acids can either lead to the formation of a stable modification or inactivate a protein if they further oxidized to sulfinic (RSO2H) and sulfonic acids (RSO3H). Because of these abilities sulfenic acids play an important role in cellular signaling and regulation. This tight link between their chemical properties and their biological function sparks great interest in quantifying their sites of formation, lifetime and reactivity. In analogy, selenenic acids (RSeOH) are also much more than mere reaction intermediates of enzymes. They are again the starting point for forming chemical structures and bonds that impact protein conformation and function. This includes further oxidation into seleninic acid (RSeO2H), inter – and intramolecular selenylsulfide (Se-S) bonds and the intramolecular selenylamide (Se-N). These chemical forms, in turn, are all manifestation of how selenoproteins transmit information to the cell about the level of oxidants.
We are particularly interested in the following questions:
• How does the environment of a protein stabilize sulfenic and selenenic acids?
• What factors control the reactivity of sulfenic and selenenic acids?
• How are selenoproteins’ oxidation and conformational changes employed in the cell to detect oxidative stress?