Methionine Sulfoxide Reductase a Is a Stereospecific Methionine Oxidase
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Methionine sulfoxide reductase A is a stereospecific methionine oxidase Jung Chae Lim, Zheng You, Geumsoo Kim, and Rodney L. Levine1 Laboratory of Biochemistry, National Heart, Lung, and Blood Institute, Bethesda, MD 20892-8012 Edited by Irwin Fridovich, Duke University Medical Center, Durham, NC, and approved May 10, 2011 (received for review February 10, 2011) Methionine sulfoxide reductase A (MsrA) catalyzes the reduction Results of methionine sulfoxide to methionine and is specific for the S epi- Stoichiometry. Branlant and coworkers have studied in careful mer of methionine sulfoxide. The enzyme participates in defense detail the mechanism of the MsrA reaction in bacteria (17, 18). against oxidative stresses by reducing methionine sulfoxide resi- In the absence of reducing agents, each molecule of MsrA dues in proteins back to methionine. Because oxidation of methio- reduces two molecules of MetO. Reduction of the first MetO nine residues is reversible, this covalent modification could also generates a sulfenic acid at the active site cysteine, and it is function as a mechanism for cellular regulation, provided there reduced back to the thiol by a fast reaction, which generates a exists a stereospecific methionine oxidase. We show that MsrA disulfide bond in the resolving domain of the protein. The second itself is a stereospecific methionine oxidase, producing S-methio- MetO is then reduced and again generates a sulfenic acid at the nine sulfoxide as its product. MsrA catalyzes its own autooxidation active site. Because the resolving domain cysteines have already as well as oxidation of free methionine and methionine residues formed a disulfide, no further reaction forms. The mass of the þ16 in peptides and proteins. When functioning as a reductase, MsrA protein has increased by 14 Da, Da from the sulfenic oxygen −2 fully reverses the oxidations which it catalyzes. and Da from the disulfide. When the mouse MsrA was incubated with MetO for 30 min, the stoichiometry of the Met formation appeared at first examination to be identical to that ethionine is an essential amino acid for most mammals for the bacterial enzymes (Fig. 1A). However, when the incuba- including humans. Approximately 99% of cellular methio- M tion time was lengthened, it became clear that three MetO are nine is found in protein, with the remainder in small molecules reduced by each MsrA (Fig. 1B) and that the mass of protein such as free methionine and S-adenosyl-methionine (1). Methio- increased by 12 Da, not 14 Da (from 23,648.6 Da to 23,660.6 Da, nine, as cysteine (Cys), the other sulfur-containing amino acid, is with an error of ∼0.5 Da for each mass). We therefore measured relatively easily oxidized to yield methionine sulfoxide. Oxidation the mass change of MsrA during the reaction and identified four by hydrogen peroxide, hypochlorous acid, and other reactive major species (Fig. 1C). species creates a chiral center at the sulfur so that the methionine sulfoxide (MetO) produced is a mixture of the S and R epimers Characterization of the MsrA Species. The species with different (2). Reduction of the sulfoxide back to methionine is catalyzed by masses could be purified by taking samples at different times of methionine sulfoxide reductases (Msr), enzymes found in almost incubation with MetO and purifiying them by reverse phase chro- all organisms from microbes to humans. Two distinct classes of matography (Fig. S1). These were treated with iodoacetamide to reductases are required, one to reduce the S epimer [methionine alkylate cysteine thiols, digested with trypsin, and the peptides sulfoxide reductase A (MsrA)] and one to reduce the R epimer mapped by reverse phase HPLC and tandem mass spectrometry. [methionine sulfoxide reductase B (MsrB)]. MsrA was described Peptides that were modified during the reaction with MetO were many years before MsrB and as a consequence most studies identified and their posttranslational modifications established focused on MsrA. Although the enzyme is almost ubiquitous (Table 1). Similar to the bacterial MsrA, the reduction of the first and a variety of studies establish its importance, its actual roles MetO of the mouse MsrA leads to disulfide formation in the −2 in cellular function remain unclear. Knocking out MsrA caused resolving domain, with a net mass change of Da. The second MetO further decreases the mass to −4 Da from the native, a increased susceptibility to oxidative stress in mice (3), yeast (4), þ14 and bacteria (5–7). Conversely, overexpressing MsrA conferred distinct difference from the Da observed in the bacterial form. Although the additional decrease of 2 Da could have increased resistance to Drosophila (8), Saccharomyces (9), Arabi- resulted from the formation of a second disulfide bond, it did not. dopsis (10), PC-12 cells (11), and human T cells (9). Overexpres- We then considered that a sulfenic acid did form on the active site sion in Drosophila doubled the lifespan of the flies (8). Cys72 but then reacted with the nitrogen of an adjacent residue’s There is reasonable evidence that cyclic oxidation and reduc- peptide bond to form a sulfenamide, a well-documented reac- tion of methionine residues in proteins act to scavenge reactive tion of protein sulfenic acids (19, 20). Sulfenamide formation oxygen and nitrogen species (12). Methionine residues can thus −4 “ ” would generate a modified protein of Da from the native, function as macromolecular bodyguards, preventing the oxida- and because the electrospray ionization technique is dehydrating, tion of critical residues in structural proteins and enzymes (13). sulfenamide formation would be favored in the mass spectro- Because methionine oxidation is a reversible covalent modifica- meter. Dimedone and 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole tion, investigators have frequently suggested that it may have (NBD) both react with sulfenic acid to give a stable derivative. a role in cellular signaling and regulation but the experimental evidence to support such a role is limited (14–16). Although MsrA is a stereospecific MetO reductase, no stereospecific Author contributions: J.C.L., Z.Y., G.K., and R.L.L. designed research; J.C.L., Z.Y., G.K., and R.L.L. performed research; J.C.L., Z.Y., G.K., and R.L.L. analyzed data; and J.C.L. and R.L.L. methionine oxidase has yet been described. If such an oxidase wrote the paper. exists, it would complete the enzymatic requirements for a rever- The authors declare no conflict of interest. sible covalent modification. We now show that MsrA is a dual- This article is a PNAS Direct Submission. function enzyme with the ability to stereospecifically catalyze 1To whom correspondence should be addressed. E-mail: [email protected]. both the oxidation of methionine in proteins to S-MetO as well This article contains supporting information online at www.pnas.org/lookup/suppl/ as reduction of S-MetO back to methionine. doi:10.1073/pnas.1101275108/-/DCSupplemental. 10472–10477 ∣ PNAS ∣ June 28, 2011 ∣ vol. 108 ∣ no. 26 www.pnas.org/cgi/doi/10.1073/pnas.1101275108 Downloaded by guest on October 2, 2021 A NBD reacted with the unoxidized Cys of each form. Dimedone 2 did not react with the native nor −2 Da forms. Both dimedone and NBD reacted with the −4 Da form to give the derivative expected from a sulfenic acid (Table 1). Thus, except for the ol of MsrA 1 secondary formation of a sulfenamide by the mouse MsrA, its catalytic cycle thus far was the same as that observed for the bacterial enzyme. 0 Reduction of the third MetO generated an MsrA species which was þ12 Da heavier than the native form. Because this species Mol of Met/mo was þ16 Da from the −4 Da species, it probably resulted from −4 0 1020304050 the addition of an oxygen to the Da form. The most likely explanation would be the addition of another oxygen to the active MetO (µM) site Cys72, generating either a sulfinic acid or a more complex derivative such as an S-oxide (21). However, the þ12 Da form B 3 still reacted with dimedone and NBD to give sulfenic acid deri- vatives. In addition, the þ12 Da form as well as the −2 Da and f MsrA −4 2 Da forms was converted back to the native mass upon addi- tion of a reducing agent such as dithiothreitol or tris(2-carbox- yethyl)phosphine (TCEP). 1 These characteristics indicated that the oxygen was on a resi- due other than the active site Cys72. Peptide mapping and 0 l of Met/mol sequencing by mass spectrometry demonstrated that the modifi- cation giving rise to the þ12 Da form was conversion of Met229 Mol to methionine sulfoxide, resulting in a 16 Da increase in the mass 0 30 60 90 120 150 180 of the tryptic peptide (Fig. 2 and Table S1). Thus, the mouse Incubation Time (min) MsrA can autooxidize a methionine residue. After reduction of C 100 the second MetO, the sulfenic acid on Cys72 oxidizes Met229, e and is itself reduced back to a thiol which allows it to react with a third MetO. Oxidation of Met229 is relatively slow (Fig. 1B). 75 16 16 When wild-type MsrA was incubated with Met OinH2 O, the protein mass increased by 12 Da. When incubated with 50 16 18 Met OinH2 O, the mass increased by 14 Da (Table 2). Thus 25 the oxygen in Met229O is not transferred from the substrate MetO ve abundance but is derived from water. When the þ12 Da form was incubated with NBD, the mass of the derivative was 4 Da higher when 0 18 16 carried out in H2 O rather than H2 O, demonstrating that both % relati the oxygen of Met229O and that of the sulfenic acid on Cys72 are 0306090120150180 from water.