Selective reduction of methylsulfinyl-containing compounds by mammalian MsrA suggests a strategy for improved drug efficacy

Redox Pioneer: Professor Vadim N. Gladyshev

Professor Vadim N. Gladyshev is recognized here as a Redox Pioneer, because he has published an article on antioxidant/redox biology that has been cited more than 1000 times and 29 articles that have been cited more than 100 times. Gladyshev is world renowned for his characterization of the human selenoproteome encoded by 25 genes, identification of the majority of known selenoprotein genes in the three domains of life, and discoveries related to thiol oxidoreductases and mechanisms of redox control. Gladyshev's first faculty position was in the Department of Biochemistry, the University of Nebraska. There, he was a Charles Bessey Professor and Director of the Redox Biology Center. He then moved to the Department of Medicine at Brigham and Women's Hospital, Harvard Medical School, where he is Professor of Medicine and Director of the Center for Redox Medicine. His discoveries in redox biology relate to selenoenzymes, such as methionine sulfoxide reductases and thioredoxin reductases, and various thiol oxidoreductases. He is responsible for the genome-wide identification of catalytic redox-active cysteines and for advancing our understanding of the general use of cysteines by proteins. In addition, Gladyshev has characterized hydrogen peroxide metabolism and signaling and regulation of protein function by methionine-R-sulfoxidation. He has also made important contributions in the areas of aging and lifespan control and pioneered applications of comparative genomics in redox biology, selenium biology, and aging. Gladyshev's discoveries have had a profound impact on redox biology and the role of redox control in health and disease. He is a true Redox Pioneer. Antioxid. Redox Signal. 25, 1-9.

Regulation of protein function by reversible methionine oxidation and the role of selenoprotein MsrB1

SIGNIFICANCE

Protein structure and function can be regulated via post-translational modifications by numerous enzymatic and nonenzymatic mechanisms. Regulation involving oxidation of sulfur-containing residues emerged as a key mechanism of redox control. Unraveling the participants and principles of such regulation is necessary for understanding the biological significance of redox control of cellular processes.

RECENT ADVANCES

Reversible oxidation of methionine residues by monooxygenases of the Mical family and subsequent reduction of methionine sulfoxides by a selenocysteine-containing methionine sulfoxide reductase B1 (MsrB1) was found to control the assembly and disassembly of actin in mammals, and the Mical/MsrB pair similarly regulates actin in fruit flies. This finding has opened up new avenues for understanding the use of stereospecific methionine oxidation in regulating cellular processes and the roles of MsrB1 and Micals in regulation of actin dynamics.

CRITICAL ISSUES

So far, Micals have been the only known partners of MsrB1, and actin is the only target. It is important to identify additional substrates of Micals and characterize other Mical-like enzymes.

FUTURE DIRECTIONS

Oxidation of methionine, reviewed here, is an emerging but not well-established mechanism. Studies suggest that methionine oxidation is a form of oxidative damage of proteins, a modification that alters protein structure or function, a tool in redox signaling, and a mechanism that controls protein function. Understanding the functional impact of reversible oxidation of methionine will require identification of targets, substrates, and regulators of Micals and Msrs. Linking the biological processes, in which these proteins participate, might also lead to insights into disease conditions, which involve regulation of actin by Micals and Msrs.

The discovery of methionine sulfoxide reductase enzymes: An historical account and future perspectives

L-Methionine (L-Met) is the only sulphur-containing proteinogenic amino acid together with cysteine. Its importance is highlighted by it being the initiator amino acid for protein synthesis in all known living organisms. L-Met, free or inserted into proteins, is sensitive to oxidation of its sulfide moiety, with formation of L-Met sulfoxide. The sulfoxide could not be inserted into proteins, and the oxidation of L-Met in proteins often leads to the loss of biological activity of the affected molecule. Key discoveries revealed the existence, in rats, of a metabolic pathway for the reduction of free L-Met sulfoxide and, later, in Escherichia coli, of the enzymatic reduction of L-Met sulfoxide inserted in proteins. Upon oxidation, the sulphur atom becomes a new stereogenic center, and two stable diastereoisomers of L-Met sulfoxide exist. A fundamental discovery revealed the existence of two unrelated families of enzymes, MsrA and MsrB, whose members display opposite stereospecificity of reduction for the two sulfoxides. The importance of Msrs is additionally emphasized by the discovery that one of the only 25 selenoproteins expressed in humans is a Msr. The milestones on the road that led to the discovery and characterization of this group of antioxidant enzymes are recounted in this review.

The methionine sulfoxide reduction system: selenium utilization and methionine sulfoxide reductase enzymes and their functions

SIGNIFICANCE

Selenium is utilized in the methionine sulfoxide reduction system that occurs in most organisms. Methionine sulfoxide reductases (Msrs), MsrA and MsrB, are the enzymes responsible for this system. Msrs repair oxidatively damaged proteins, protect against oxidative stress, and regulate protein function, and have also been implicated in the aging process. Selenoprotein forms of Msrs containing selenocysteine (Sec) at the catalytic site are found in bacteria, algae, and animals.

RECENT ADVANCES

A selenoprotein MsrB1 knockout mouse has been developed. Significant progress in the biochemistry of Msrs has been made, which includes findings of a novel reducing system for Msrs and of an interesting reason for the use of Sec in the Msr system. The effects of mammalian MsrBs, including selenoprotein MsrB1 on fruit fly aging, have been investigated. Furthermore, it is evident that Msrs are involved in methionine metabolism and regulation of the trans-sulfuration pathway.

CRITICAL ISSUES

This article presents recent progress in the Msr field while focusing on the physiological roles of mammalian Msrs, functions of selenoprotein forms of Msrs, and their biochemistry.

FUTURE DIRECTIONS

A deeper understanding of the roles of Msrs in redox signaling, the aging process, and metabolism will be achieved. The identity of selenoproteome of Msrs will be sought along with characterization of the identified selenoprotein forms. Exploring new cellular targets and new functions of Msrs is also warranted.

Recharging oxidative protein repair: catalysis by methionine sulfoxide reductases towards their amino acid, protein, and model substrates

The sulfur-containing amino acid methionine (Met) in its free and amino acid residue forms can be readily oxidized to the R and S diastereomers of methionine sulfoxide (MetO). Methionine sulfoxide reductases A (MSRA) and B (MSRB) reduce MetO back to Met in a stereospecific manner, acting on the S and R forms, respectively. A third MSR type, fRMSR, reduces the R form of free MetO. MSRA and MSRB are spread across the three domains of life, whereas fRMSR is restricted to bacteria and unicellular eukaryotes. These enzymes protect against abiotic and biotic stresses and regulate lifespan. MSRs are thiol oxidoreductases containing catalytic redox-active cysteine or selenocysteine residues, which become oxidized by the substrate, requiring regeneration for the next catalytic cycle. These enzymes can be classified according to the number of redox-active cysteines (selenocysteines) and the strategies to regenerate their active forms by thioredoxin and glutaredoxin systems. For each MSR type, we review catalytic parameters for the reduction of free MetO, low molecular weight MetO-containing compounds, and oxidized proteins. Analysis of these data reinforces the concept that MSRAs reduce various types of MetO-containing substrates with similar efficiency, whereas MSRBs are specialized for the reduction of MetO in proteins.

Analyses of methionine sulfoxide reductase activities towards free and peptidyl methionine sulfoxides

There have been insufficient kinetic data that enable a direct comparison between free and peptide methionine sulfoxide reductase activities of either MsrB or MsrA. In this study, we determined the kinetic parameters of mammalian and yeast MsrBs and MsrAs for the reduction of both free methionine sulfoxide (Met-O) and peptidyl Met-O under the same assay conditions. Catalytic efficiency of mammalian and yeast MsrBs towards free Met-O was >2000-fold lower than that of yeast fRMsr, which is specific for free Met-R-O. The ratio of free to peptide Msr activity in MsrBs was 1:20-40. In contrast, mammalian and yeast MsrAs reduced free Met-O much more efficiently than MsrBs. Their k(cat) values were 40-500-fold greater than those of the corresponding MsrBs. The ratio of free to peptide Msr activity was 1:0.8 in yeast MsrA, indicating that this enzyme can reduce free Met-O as efficiently as peptidyl Met-O. In addition, we analyzed the in vivo free Msr activities of MsrBs and MsrAs in yeast cells using a growth complementation assay. Mammalian and yeast MsrBs, as well as the corresponding MsrAs, had apparent in vivo free Msr activities. The in vivo free Msr activities of MsrBs and MsrAs agreed with their in vitro activities.