Unique arylalkylamine N-acetyltransferase-2 polymorphism in salmonids and profound variations in thermal stability and catalytic efficiency conferred by two residues.

Melatonin contributes to synchronizing major biological and behavioral functions with cyclic changes in the environment. Arylalkylamine N-acetyltransferase (AANAT) is responsible for a daily rhythm in melatonin secretion. Teleost possess two enzyme forms, AANAT1 and AANAT2, preferentially expressed in the retina and the pineal gland, respectively. The concomitant action of light and temperature shapes the daily and seasonal changes in melatonin secretion: the former controls duration while the latter modulates amplitude. Investigating the respective roles of light and temperature is particularly relevant in the context of global warming, which is likely to affect the way fish decode and anticipate seasonal changes, with dramatic consequences on their physiology and behavior. Here we investigated the impact of temperature on pineal melatonin secretion of a migratory species, the Arctic charr (Salvelinus alpinus), the northernmost living and cold-adapted salmonid. We show that temperature directly impacts melatonin production in cultured pineal glands. We also show that one organ expresses two AANAT2 transcripts displaying high similarity between them and with trout Oncorhynchus mykiss AANAT2, differing by only two amino acid sites. We compared the kinetics and 3D models of these enzymes as well as of a chimeric construct, particularly with regard to their response to temperature. Our study brings interesting and new information on the evolutionary diversity of AANAT enzymes in teleosts and the role played by specific residues in the catalytic properties of the enzymes.

Functional diversity of Teleost arylalkylamine N-acetyltransferase-2: is the timezyme evolution driven by habitat temperature?

Arylalkylamine N-acetyltransferase-2 (AANAT2) is the enzyme responsible for the rhythmic production of the time-keeping hormone melatonin. It plays a crucial role in the synchronization of biological functions with changes in the environment. Annual and daily fluctuations in light are known to be key environmental factors involved in such synchronization. Previous studies have demonstrated that AANAT2 activity is also markedly influenced by temperature but the mechanisms through which it impacts the enzyme activity need to be further deciphered. We investigated AANAT2 primary to tertiary structures (3D models) and kinetics in relation to temperature for a variety of Teleost species from tropical to Arctic environments. The results extend our knowledge on the catalytic mechanisms of AANAT enzymes and bring strong support to the idea that AANAT2 diversification was limited by stabilizing selection conferring to the enzyme well conserved secondary and tertiary structures. Only a few changes in amino acids appeared sufficient to induce different enzyme activity patterns. It is concluded that AANAT2 evolution is mainly driven by phylogenetic relationships although catalytic properties (enzyme turnover and substrate affinity) are also under the influence of the respective species normal habitat temperature.