Regulation of cell function by methionine oxidation and reduction

Abstract

Reactive oxygen species (ROS) are generated during normal cellular activity and may exist in excess in some pathophysiological conditions, such as inflammation or reperfusion injury. These molecules oxidize a variety of cellular constituents, but sulfur-containing amino acid residues are especially susceptible. While reversible cysteine oxidation and reduction is part of well-established signalling systems, the oxidation and the enzymatically catalysed reduction of methionine is just emerging as a novel molecular mechanism for cellular regulation. Here we discuss how the oxidation of methionine to methionine sulfoxide in signalling proteins such as ion channels affects the function of these target proteins. Methionine sulfoxide reductase, which reduces methionine sulfoxide to methionine in a thioredoxin-dependent manner, is therefore not only an enzyme important for the repair of age- or degenerative disease-related protein modifications. It is also a potential missing link in the post-translational modification cycle involved in the specific oxidation and reduction of methionine residues in cellular signalling proteins, which may give rise to activity-dependent plastic changes in cellular excitability.

Figure 1. Oxidation and reduction of methionine residues

The amino acid methionine (Met) is easily oxidized to methionine sulfoxide (MetO) in the presence of mild oxidants. A second oxidation step, requiring stronger oxidants such as chloramine-T, results in methionine sulfone (MetO₂). While MetO₂ is stable under physiological conditions, MetO can be reduced back to Met by means of the enzyme peptide methionine sulfoxide reductase (MSRA). To maintain catalytic activity, MSRA needs an electron acceptor. Under physiological conditions MSRA is coupled via thioredoxin, thioredoxin reductase, and NADPH to the cellular redox system. The activity of MSRA may also be subject to cellular regulation by thus far unidentified cofactors.

Figure 2. Regulation of potassium channels by methionine oxidation and reduction

A, schematic diagram of a potassium channel complex indicating the N-terminal ends of three of the four α-subunits that form in some channel types inactivating structures, i.e. protein segments that occlude the permeation pore upon channel activation. Recent studies suggest that the N-terminal segment of the α-subunit may also contain a structure that could be described as a hanging gondola (Gulbis et al. 2000; Kobertz et al. 2000) but for the sake of simplicity the gondola-like structure is not included in the diagram. B, in Shaker C/B channels the amino acid residue at position 3 is methionine. With this residue in the reduced form, the channels exhibit rapid N-type inactivation; upon oxidation to MetO rapid inactivation is impaired (Ciorba et al. 1997). C, superposition of current traces obtained from different Xenopus oocytes after heterologous expression of Shaker C/B channels; depolarization to +40 mV. The time course of inactivation obtained in whole cells shows a very strong scatter indicating cell-to-cell variability of the amount of oxidized methionine. D, upon coexpression with human MSRA the time course of inactivation is much faster and the variability is diminished.

Figure 3. Activity-dependent oxidative regulation

Simplified scheme illustrating activity-dependent regulation of cellular excitability. Increased cellular activity requires a higher ATP consumption and therefore results in an increased production of O²⁻· in the mitochondrial respiratory chain. O²⁻· is dismutated to form H₂O₂ by the superoxide dismutase (SOD). H₂O₂ is degraded by antioxidant enzymes such as catalase (Cat) or glutathione peroxidase. In addition, ·OH radicals are formed from H₂O₂ via the Fenton reaction in the presence of transitional metal ions (Me). Stimulated by Ca²⁺-CaM and the activation of calcineurin, nitric oxide synthase (NOS) generates NO· from L-arginine. NO· is combined with O²⁻· to form the highly reactive peroxynitrite (ONOO⁻). In addition, the highly diffusible gas NO· can affect neighbouring cells and therefore acts, for example, as a retrograde messenger. The reactive oxygen and nitrogen species (ROS, RNS, darker blue shading) can now result in protein oxidation or nitration. Of particular interest are signalling proteins that are coupled to the intracellular Ca²⁺ level or the membrane voltage, i.e. Ca²⁺-permeable ion channels, voltage-gated K⁺ channels, Ca²⁺-ATPases or proteins that are functionally regulated by CaM. Cyclic oxidation and reduction of methionine residues in such proteins may close feedback loops in which increased cellular activity stimulates either increased or decreased cellular excitability.