Under physiological conditions, proteins may be the main target for free radicals and oxidants, to which biological systems are constantly exposed. That is because proteins are essential components of most biological systems and react quickly with many oxidants. The accumulation of oxidized proteins has been associated with aging and several diseases. However, oxidative changes do not always mean damage. In some cases, changes in the redox state of proteins are involved in signaling pathways in response to metabolic or environmental changes. Oxidized proteins could also be markers for pathology diagnosis and monitoring.

“The systematization of the study of protein oxidation is 40 years old. I searched on PubMed and found 115,000 articles with data and 12,000 reviews about it. Specifically, on oxidative modifications, there are 28 thousand articles and 3,500 reviews in the last decade. So I decided to explore the theme, trying to integrate knowledge about protein oxidation aimed at human pathophysiology”, says researcher Marilene Demasi, from the Instituto Butantan and a member of the RIDC Redoxoma, who coordinated an extensive review article published in the journal Antioxidants & Redox Signaling. Several Redoxoma researchers and Uruguayan researchers involved in the study of protein oxidation are co-authors of the article.

Proteins are the main structural component of all cells, being the most abundant molecules in the body, except for water. They are formed by a set of 20 amino acids, arranged in different specific sequences, and participate in practically all cellular processes. Proteins act as catalysts, in the case of enzymes; help with muscle contraction - actin and myosin; protect the organism in the form of antibodies; act as pigments, opsins, for example; and guarantee the transport of gases as hemoglobin does. Also, several hormones, such as insulin, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) are proteins.

Exposure of biological molecules to oxidants is inevitable. Biological oxidants, such as the superoxide radical, hydrogen peroxide, nitric oxide, peroxynitrite, hypochlorous acid, among others, are formed in our organism either deliberately, for example, to kill invading pathogens or as intermediates in enzymatic reactions, or unintentionally, for example, via electron leakage from electron transport chain, drug metabolism, exposure to chemicals, pollutants and radiation. The formation of these oxidants and their reactions are partly limited by our antioxidant defense system. Even so, oxidants can cause damage to all components of biological systems, including lipids, proteins, and DNA.

Oxidations are responsible for changes in the structure, function, and turnover of proteins, resulting in loss or gain in activity. Although the extent of the damage and its biological importance vary, it can be said that most modifications are harmful. There are many examples in the literature associating protein oxidation with pathologies such as Alzheimer’s disease, cardiovascular disease, and cataract. Besides, there is ample evidence that, with aging, there is an increase in the oxidation of proteins, with a decrease in activity and degradation. In the article, the researchers addressed all the deleterious changes described so far, focusing on the final product and not on the etiology of oxidation.

Reversible oxidations have also been extensively analyzed. They occur in sulfur-containing amino acids, such as cysteine ​​and methionine, which are involved in catalysis and cell signaling. “Reversible oxidations may be modulating a cellular moment in which there is a loss of reducing capacity and alteration of metabolism. When the cell reestablishes homeostasis and this protein returns to its native state, some of the changes have an aspect of activity modulators”, explains Demasi.

However, the authors claim that quantitative data are lacking that correlate specific oxidized proteins with the development and progression of pathological conditions and emphasize the need to understand the mechanisms underlying these modifications, as well as the fate of the modified proteins.

Oxidized proteins can have two destinations: they are degraded and eliminated in a process called proteolysis, or, if they are not eliminated, they form protein aggregates. Proteolysis would be the best pathway for cellular homeostasis maintenance, whereas aggregation is associated with human pathologies.

Perspectives

A biomarker is any substance, structure, or process that can be measured in the body or in its products to diagnose and monitor the evolution of a disease. Oxidized proteins may have this role, but, for now, there are few results in the literature when it comes to studies in humans. In the article, the researchers show a detailed survey of changes in proteins related to human pathologies.

However, the possibility of using oxidized proteins as a disease marker faces technical challenges. According to Professor Ohara Augusto, from the Instituto de Química at USP and the RIDC Redoxoma and co-author of the article, the methodologies for detecting, identifying, and quantifying oxidized proteins are still limited, and only recently scientists are using mass spectrometry for this purpose. “The difficulty is how to apply these methods to a sick person. For oxidized proteins to be a disease marker we need non-invasive methods to measure and monitor the evolution of a particular disease.”

Still, on methodologies, another topic discussed in the article is the possibility of using tools to deal with large databases, still little explored. “Redox biology today has a lot of data. Concerning proteins, these systems biology tools, as well as artificial intelligence now, are not yet tools used in this approach,” says Demasi.

The good news is the emergence of a new therapeutic technique, called proteolysis-targeting chimeras (PROTACs), which allows you to choose the protein to be degraded. This technique has already been used in the treatment of cancer and there is an expectation that it will be effective in treating neurodegenerative diseases. According to the researchers, several studies have demonstrated the use of this technique to degrade proteins prone to form aggregates. In principle, this approach can be extended to the degradation of any protein.

For the future, researchers believe in studies that deepen basic knowledge, with an emphasis on quantification, and employ emerging approaches to establish more reliable connections between redox biology and specific pathological conditions.

The article Oxidative Modification of Proteins: From Damage from Catalysis, Signaling and Beyond, by Marilene Demasi, Ohara Augusto, Etelvino JH Bechara, Renata N. Bicev, Fernanda M. Cerqueira, Fernanda M. da Cunha, Ana Denicola, Fernando Gomes, Sayuri Miyamoto, Luis ES Netto, Lía M. Randall, Cassius V. Stevani, and Leonor Thomson, can be read here.