Potential Role of Methionine Sulfoxide in the Inactivation of the Chaperone GroEL by Hypochlorous Acid (HOCl) and Peroxynitrite (ONOO–)
GroEL is anEscherichia colimolecular chaperone that functionsin vivoto fold newly synthesized polypeptides as well as to bind and refold denatured proteins during stress. This protein is a suitable model for its eukaryotic homolog, heat shock protein 60 (Hsp60), due to the high number of conserved amino acid sequences and similar function. Here, we will provide evidence that GroEL is rather insensitive to oxidants produced endogenously during metabolism, such as nitric oxide (·NO) or hydrogen peroxide (H2O2), but is modified and inactivated by efficiently reactive species generated by phagocytes, such as peroxynitrite (ONOO–) and hypochlorous acid (HOCl). For the exposure of 17.5 μmGroEL to 100–250 μmHOCl, the major pathway of inactivation was through the oxidation of methionine to methionine sulfoxide, established through mass spectrometric detection of methionine sulfoxide and the reactivation of a significant fraction of inactivated GroEL by the enzyme methionine sulfoxide reductase B/A (MsrB/A). In addition to the oxidation of methionine, HOCl caused the conversion of cysteine to cysteic acid and this product may account for the remainder of inactivated GroEL not recoverable through MsrB/A. In contrast, HOCl produced only negligible yields of 3-chlorotyrosine. A remarkable finding was the conversion of Met111and Met114to Met sulfone, which suggests a rather low reduction potential of these 2 residues in GroEL. The high sensitivity of GroEL toward HOCl and ONOO–suggests that this protein may be a target for bacterial killing by phagocytes.