Protein Nitration

Protein nitration involves the addition of a nitro group (–NO₂) to susceptible amino acids, mainly tyrosine (Tyr) and tryptophan (Trp), although most studies focus on tyrosine nitration. Nitration is generally considered irreversible and often impairs protein function, though neutral or positive effects have been reported. Importantly, tyrosine nitration is a selective, non-random process influenced by protein structure and cellular context (e.g., soluble vs. membrane-bound).

Mechanisms of Nitration

Tyrosine and tryptophan nitration mainly occur via peroxynitrite (ONOO⁻) or nitrogen dioxide radicals (•NO₂). Peroxynitrite is rapidly formed from nitric oxide (•NO) and superoxide (O₂•⁻) near sites of superoxide production and is highly reactive at physiological pH. Nitration proceeds indirectly through radical intermediates. Alternatively, •NO₂-mediated nitration can arise from reactions between hydrogen peroxide (H₂O₂) and nitrite (NO₂⁻) in the presence of hemoperoxidases.

Simple model of protein tyrosine and tryptophan nitration via peroxynitrite and nitrogen dioxide

Fig. 1. Simple model of protein tyrosine and tryptophan nitration. Two main nitrating reactions have been proposed: (a) by peroxynitrite (ONOO⁻) formed from nitric oxide (•NO) and superoxide (O₂•⁻); (b) by •NO₂ produced from hydrogen peroxide (H₂O₂) and nitrite (NO₂⁻) in the presence of hemoperoxidase (Corpas et al., 2021).

Reversibility and Cellular Fate

For tyrosine nitration to regulate signaling independently of phosphorylation, it must be reversible. Although once considered irreversible, denitration mechanisms have been identified in animals, while in plants no denitrase has yet been confirmed, leaving reversibility uncertain. Alternative outcomes include reduction of nitrotyrosine to aminotyrosine or removal of nitrated proteins via proteasome-mediated degradation. Evidence suggests that nitration often increases protein susceptibility to proteasomal degradation, including in plants.

Functional Effects and Phosphorylation

Functionally, protein tyrosine nitration lowers the pKa of tyrosine, increases hydrophobicity, and can induce structural and steric changes. In plants, nitration generally leads to loss of protein function, though exceptions exist. In animals, nitration can either activate, inactivate, or have no effect on protein activity, and functional changes are not always directly caused by tyrosine modification alone.

Tyrosine nitration can also positively or negatively affect tyrosine phosphorylation, thereby influencing cell signaling. While this relationship is well documented in non-plant systems, evidence in plants remains limited. Nonetheless, disruptions in microtubule organization and protein stability suggest a functional interplay between tyrosine nitration and phosphorylation, potentially through competition for the same tyrosine residues (Kolbert et al., 2017).

Fates and consequences of protein tyrosine nitration (PTN): mechanisms and functional outcomes

Fig. 2. Fates and consequences of PTN. Possible mechanisms regulating nitrated protein pool (left) and possible functional, signalling consequences of tyrosine nitration (right). The nitro group in tyrosine residue can be reduced, but neither enzymatic nor non-enzymatic reductants have been identified in plants or in animals. Denitrase activity has been characterized in animals but not in plants; consequently the reversibility of tyrosine nitration is still questionable. Nitrated proteins can be targeted for polyubiquitination and for proteasomal degradation. Nitration may cause structural and consequently functional modifications (inactivation, activation) in proteins. In plants, evidence is available for PTN-triggered functional loss or unaffected activity. Moreover, nitration of tyrosine may interfere with phosphorylation (Kolbert et al., 2017).

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References

Corpas FJ, González-Gordo S, Palma JM Protein nitration: A connecting bridge between nitric oxide (NO) and plant stress.. Plant Stress 2, 100026 (2021). https://doi.org/10.1016/j.stress.2021.100026
Kolbert Zs, Feigl G, Bordé Á, Molnár Á, Erdei L Protein tyrosine nitration in plants: Present knowledge, computational prediction and future perspectives.. Plant Physiology and Biochemistry 113, 56-63 (2017). https://doi.org/10.1016/j.plaphy.2017.01.028

Protein Nitration

References

Corpas FJ, González-Gordo S, Palma JM Protein nitration: A connecting bridge between nitric oxide (NO) and plant stress.. Plant Stress 2, 100026 (2021). https://doi.org/10.1016/j.stress.2021.100026

Kolbert Zs, Feigl G, Bordé Á, Molnár Á, Erdei L Protein tyrosine nitration in plants: Present knowledge, computational prediction and future perspectives.. Plant Physiology and Biochemistry 113, 56-63 (2017). https://doi.org/10.1016/j.plaphy.2017.01.028