Oxygen-linked S-nitrosation in fish myoglobins: a cysteine-specific tertiary allosteric effect

NO and Heme Proteins: Cross-Talk between Heme and Cysteine Residues

Heme proteins are a diverse group that includes several unrelated families. Their biological function is mainly associated with the reactivity of the heme group, which-among several other reactions-can bind to and react with nitric oxide (NO) and other nitrogen compounds for their production, scavenging, and transport. The S-nitrosylation of cysteine residues, which also results from the reaction with NO and other nitrogen compounds, is a post-translational modification regulating protein activity, with direct effects on a variety of signaling pathways. Heme proteins are unique in exhibiting this dual reactivity toward NO, with reported examples of cross-reactivity between the heme and cysteine residues within the same protein. In this work, we review the literature on this interplay, with particular emphasis on heme proteins in which heme-dependent nitrosylation has been reported and those for which both heme nitrosylation and S-nitrosylation have been associated with biological functions.

The greening reaction of skipjack tuna (Katsuwonus pelamis) metmyoglobin promoted by free cysteine during thermal treatment

Background

Tuna muscle greening is a problem that occurs after heating. A hypothesis has been postulated to address this problem, involving a conserved Cys residue at position 10 (Cys-10) present on tuna myoglobin (Mb) that is exposed during the thermic treatment, forming a disulfide bond with free cysteine (Cys) in the presence of trimethylamine oxide (TMAO), resulting in the greening of the tuna Mb.

Methods

We present a study using skipjack tuna (Katsuwonus pelamis) metmyoglobin (MbFe(III)-H₂O) where the effect of free Cys (1-6 mM), TMAO (1.33 mM), and catalase on the greening reaction (GR) was monitored by UV-vis spectrometry during thermal treatment at 60 °C for 30 min. Moreover, the participation of Cys-10 on the GR was evaluated after its blocking with N-ethymaleimide.

Results

The GR occurred in tuna MbFe(III)-H₂O after heat treatment with free Cys, forming sulfmyoglobin (MbFe(II)-S) as the responsible pigment for the tuna greening. However, the rate constants of MbFe(II)-S production depended on Cys concentration (up to 4 mM) and occurred regardless of the TMAO presence. We postulate that two consecutive reactions involve an intermediate ferrylmyoglobin (promoted by H₂O₂) species with a subsequent MbFe(II)-S formation since the presence of catalase fosters the reduction of the rate reaction. Moreover, GR occurred even with blocked Cys-10 residues in tuna Mb and horse Mb (without Cys in its sequence).

Discussion

We found that GR is not exclusive to tuna Mb´s, and it can be promoted in other muscle systems. Moreover, Cys and thermal treatment are indispensable for promoting this pigmentation anomaly.

Nitric Oxide Production and Regulation in the Teleost Cardiovascular System

Nitric Oxide (NO) is a free radical with numerous critical signaling roles in vertebrate physiology. Similar to mammals, in the teleost system the generation of sufficient amounts of NO is critical for the physiological function of the cardiovascular system. At the same time, NO amounts are strictly controlled and kept within basal levels to protect cells from NO toxicity. Changes in oxygen tension highly influence NO bioavailability and can modulate the mechanisms involved in maintaining the NO balance. While NO production and signaling appears to have general similarities with mammalian systems, the wide range of environmental adaptations made by fish, particularly with regards to differing oxygen availabilities in aquatic habitats, creates a foundation for a variety of in vivo models characterized by different implications of NO production and signaling. In this review, we present the biology of NO in the teleost cardiovascular system and summarize the mechanisms of NO production and signaling with a special emphasis on the role of globin proteins in NO metabolism.

Functional roles of globin proteins in hypoxia-tolerant ectothermic vertebrates

Globins are heme-containing proteins ubiquitously expressed in vertebrates, where they serve a broad range of biological functions, directly or indirectly related to the tight control of oxygen levels and its toxic products in vivo. Perhaps the most investigated of all proteins, hemoglobin and myoglobin are primarily involved in oxygen transport and storage, but also in facilitating arterial vasodilation, suppressing mitochondrial respiration, and preventing tissue oxidative damage via accessory redox enzymatic activities during hypoxia. By contrast, the more recently discovered neuroglobin and cytoglobin do not seem to function as reversible oxygen carriers and are instead involved in redox activities, although their exact biological roles remain to be clarified. In this context, hypoxia-tolerant ectotherms, such as freshwater turtles and members of the carp family that survive winter in extreme hypoxia, have proven as excellent models to appreciate the diversity of biological functions of globin proteins. Unraveling physiological roles of globin proteins in these extreme animals will clarify an important part of the adaptive mechanisms for surviving extreme fluctuations of oxygen availability that are prohibitive to mammals.

Molecular evolution of globin genes in Gymnotiform electric fishes: relation to hypoxia tolerance

BACKGROUND

Nocturnally active gymnotiform weakly electric fish generate electric signals for communication and navigation, which can be energetically taxing. These fish mainly inhabit the Amazon basin, where some species prefer well-oxygenated waters and others live in oxygen-poor, stagnant habitats. The latter species show morphological, physiological, and behavioral adaptations for hypoxia-tolerance. However, there have been no studies of hypoxia tolerance on the molecular level. Globins are classic respiratory proteins. They function principally in oxygen-binding and -delivery in various tissues and organs. Here, we investigate the molecular evolution of alpha and beta hemoglobins, myoglobin, and neuroglobin in 12 gymnotiforms compared with other teleost fish.

RESULTS

The present study identified positively selected sites (PSS) on hemoglobin (Hb) and myoglobin (Mb) genes using different maximum likelihood (ML) methods; some PSS fall in structurally important protein regions. This evidence for the positive selection of globin genes suggests that the adaptive evolution of these genes has helped to enhance the capacity for oxygen storage and transport. Interestingly, a substitution of a Cys at a key site in the obligate air-breathing electric eel (Electrophorus electricus) is predicted to enhance oxygen storage of Mb and contribute to NO delivery during hypoxia. A parallel Cys substitution was also noted in an air-breathing African electric fish (Gymnarchus niloticus). Moreover, the expected pattern under normoxic conditions of high expression of myoglobin in heart and neuroglobin in the brain in two hypoxia-tolerant species suggests that the main effect of selection on these globin genes is on their sequence rather than their basal expression patterns.

CONCLUSION

Results indicate a clear signature of positive selection in the globin genes of most hypoxia-tolerant gymnotiform fishes, which are obligate or facultative air breathers. These findings highlight the critical role of globin genes in hypoxia tolerance evolution of Gymnotiform electric fishes.

Hypoxia tolerance, nitric oxide, and nitrite: lessons from extreme animals

Among vertebrates able to tolerate periods of oxygen deprivation, the painted and red-eared slider turtles (Chrysemys picta and Trachemys scripta) and the crucian carp (Carassius carassius) are the most extreme and can survive even months of total lack of oxygen during winter. The key to hypoxia survival resides in concerted physiological responses, including strong metabolic depression, protection against oxidative damage and-in air-breathing animals-redistribution of blood flow. Each of these responses is known to be tightly regulated by nitric oxide (NO) and during hypoxia by its metabolite nitrite. The aim of this review is to highlight recent work illustrating the widespread roles of NO and nitrite in the tolerance to extreme oxygen deprivation, in particular in the red-eared slider turtle and crucian carp, but also in diving marine mammals. The emerging picture underscores the importance of NO and nitrite signaling in the adaptive response to hypoxia in vertebrate animals.