Table 2.

Summary of variables that affect the susceptibility of actin to oxidation and the outcome of this oxidation.

Variable	Susceptibility/Effect of oxidation			Notes
Variable	High	Medium	Low	Notes
Type of actin	α, β, γ			α-, β-, γ-actin are all readily oxidized, but β-, γ-actin have an additional cysteine (272) that can be oxidized which is lacking in α-actin.
Form of actin	Globular		Filamentous	Filamentous actin has been described as less accessible to oxidation [80] and greater amounts of oxidized G-actin have been observed compared to F-actin, yet this could be due to the fact that once actin is oxidized it promotes the globular form (F-actin depolymerizes and G-actin is slow to polymerize) [70].
Cysteine identity	Cys374	Cys217, Cys257, Cys272, Cys285	Cys17	Cys374 is highly reactive and is even oxidized upon air exposure [67]. Cys17 is only oxidized in very specific conditions [70, 79, 414]. In conditions where cysteines other than Cys374 are highly susceptible to oxidation, the functional consequences are not as great. For example, when Cys272 is described to be oxidized before Cys374, it has no effect on polymerization until C374 is oxidized [70]. Similarly, Cys257 is highly reactive with 7-dimethylamino-4-methyl-(N-maleimidyl) coumarin, but this doesn’t affect polymerization [80].
Methionine identity	Met44	Met47		Actin is oxidized at Met44 and Met47 by a family of proteins called MICALs (Molecule Interacting with CasL), but Met44 oxidation is the key residue for regulation of actin disassembly [77]. Other methionines can also be oxidized under extreme oxidative stress [415, 424].
Divalent ion bound	Ca²⁺		Mg²⁺	Unlike Ca²⁺-bound G-actin, Mg²⁺-bound G-actin is resistant to H₂O₂ [81].
Calcium concentration	Low Ca²⁺		High Ca²⁺	More residues become available for oxidation at low Ca²⁺, and high Ca²⁺ (>10μM) shields Cys374 from H₂O₂ oxidation [70].
Protein bound	Cys10 of Myosin bound-actin		Cys10 of Myosin free-actin	Actin Cys10 is more reactive when myosin II is bound [79], yet this effect may be specific to this cysteine as others have found that myosin II blocks reactivity of SH groups in actin [425].