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Greening of tuna metmyoglobin (MetMb) by thermal treatment (TT) and free cysteine is associated with sulfmyoglobin (SulfMb) production. This greening reaction (GR) was once thought to occur only in tuna species. However, recent research has revealed that not all tuna species exhibit this behavior, and it can also occur in horse MetMb. Thus, the present study aimed to compare the GR-reactive (Katsuwonus pelamis and Equus caballus) and GR-unreactive (Sarda chiliensis and Euthynnus lineatus) MetMb using UV-vis spectrometry during TT (60 °C/30 min and free cysteine) to monitor the GR. We used molecular dynamics (MD) simulation to assess the stability of the heme group during TT. We discovered that using GR-unreactive MetMb resulted in an incomplete GR without producing SulfMb. Additionally, our MD simulations indicated that Met85 presence in the heme cavity from GR-unreactive is responsible for the heme group instability and displacement of distal His during TT., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Juan Carlos Ramirez-Suarez reports financial support was provided by Consejo Nacional de Humanidades, Ciencia y Tecnología (CONHACyT) through the project A1-S-44107.. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Myoglobin is the main factor responsible for muscle pigmentation in tuna; muscle color depends upon changes in the oxidative state of myoglobin. The tuna industry has reported muscle greening after thermal treatment involving metmyoglobin (MetMb), trimethylamine oxide (TMAO), and free cysteine (Cys). It has been proposed that this pigmentation change is due to a disulfide bond between a unique cysteine residue (Cys10) found in tuna MetMb and free Cys. However, no evidence has been given to confirm that this reaction occurs. In this review, new findings about the mechanism of this greening reaction are discussed, showing evidence of how free radicals produced from Cys oxidation under thermal treatment participate in the greening of tuna and horse muscle during thermal treatment. In addition, the reaction conditions are compared to other green myoglobins, such as sulfmyoglobin, verdomyoglobin, and cholemyoglobin.