Abstract
Serotonin is important in vertebrates for its crucial roles in regulation of various physiological functions. Investigations on how the biosynthesizing enzymes mediate serotonin production and conversion during biological processes have been active in the past decades. However, a clear-cut picture of these enzymes in molecular evolution is very limited, particularly when the complexity is imaginable in fishes since teleosts had experienced additional whole genome duplication (WGD) event(s) than tetrapods. Since serotonin is the main intermediate product during melatonin biosynthesis from tryptophan, we therefore summarize an overview of recent discoveries about molecular evolution of the four melatonin biosynthesizing enzymes, especially the L-aromatic amino acid decarboxylase (AAAD) for serotonin production and aralkylamine N-acetyltransferase (AANAT) for serotonin conversion in vertebrates. Novel copies of these genes, possibly due to WGD, were discovered in fishes. Detailed sequence comparisons revealed various variant sites in these newly identified genes, suggesting functional changes from the conventional recognition of these enzymes. These interesting advances will benefit readers to obtain new insights into related genomic differences between mammals and fishes, with an emphasis on the potential specificity for AANAT in naturally cave-restricted and deep-sea fishes.
Highlights
Serotonin (5-hydroxytryptamine, 5-HT) acts as a critical neurotransmitter in the central nervous system (CNS) and an important hormone in the peripheral tissues (Keszthelyi et al, 2010)
Tryptophan (Trp) in cells is firstly hydroxylated by tryptophan hydroxylase (TPH) and decarboxylated by L-aromatic amino acid decarboxylase (AAAD) to produce serotonin (Keszthelyi et al, 2010)
NAS is transformed to melatonin via acetylserotonin O-methyltranserase (ASMT)
Summary
Serotonin (5-hydroxytryptamine, 5-HT) acts as a critical neurotransmitter in the central nervous system (CNS) and an important hormone in the peripheral tissues (Keszthelyi et al, 2010). Two sites (T31 and S205) occurring phosphorylation (to be pAANAT) prompt the binding between 14-3-3 proteins and pAANAT (Figure 1A) This action can protect the AANAT against proteasomal proteolysis and lead to a continuous accumulation of AANAT for 5-HT conversion and melatonin synthesis (Gastel et al, 1998; Obsil et al, 2001). Certain species owing a special habitat, such as deep-sea or caved environment (dim or without lights) may lose most of their rhythmicity They are thereby good models for examination of the relationships between molecular mechanisms and melatonin secretion, especially for understanding of serotonin synthesis related enzymes. We provide an overview to summarize recent discoveries about molecular evolution of the four biosynthesizing enzyme genes (especially the aaad for serotonin production and aanat for serotonin conversion) in vertebrates, with a focus on genomic differences between mammals and fishes
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