Crystal structure and functional mapping of human ASMT, the last enzyme of the melatonin synthesis pathway

Abstract

Melatonin is a synchronizer of many physiological processes. Abnormal melatonin signaling is associated with human disorders related to sleep, metabolism, and neurodevelopment. Here, we present the X-ray crystal structure of human N-acetyl serotonin methyltransferase (ASMT), the last enzyme of the melatonin biosynthesis pathway. The polypeptide chain of ASMT consists of a C-terminal domain, which is typical of other SAM-dependent O-methyltransferases, and an N-terminal domain, which intertwines several helices with another monomer to form the physiologically active dimer. Using radioenzymology, we analyzed 20 nonsynonymous variants identified through the 1000 genomes project and in patients with neuropsychiatric disorders. We found that the majority of these mutations reduced or abolished ASMT activity including one relatively frequent polymorphism in the Han Chinese population (N17K, rs17149149). Overall, we estimate that the allelic frequency of ASMT deleterious mutations ranges from 0.66% in Europe to 2.97% in Asia. Mapping of the variants on to the 3-dimensional structure clarifies why some are harmful and provides a structural basis for understanding melatonin deficiency in humans.

Genetic Variation in Melatonin Pathway Enzymes in Children with Autism Spectrum Disorder and Comorbid Sleep Onset Delay

“…The involvement of the ASMT gene in relation to ASD etiology has been studied extensively (Toma et al 2007; Cai et al 2008; Melke et al 2008; Jonsson et al 2010; Wang et al 2013). An ASD-risk haplotype was reported that includes two SNPs located in the promoter region, rs4446909 and rs5989681, and a third SNP, rs6644635, located in the 5′-untranslated region (UTR) of the only known functional isoform of ASMT (Melke et al 2008; Botros et al 2012). Further statistical studies have failed to replicate the associations of these common variants in ASMT with ASD risk (Toma et al 2007; Wang et al 2013).…”

ASMT gene expression correlates with cognitive impairment in patients with recurrent depressive disorder

“…ASMT is the limiting enzyme in the synthesis of melatonin [25]. ASTM gene is located in the pseudoautosomal region of the X chromosome [26], which is a candidate region for the development of mental illness [27]. …”

Catalytic mechanism of humanN-acetylserotonin methyltransferase: a theoretical investigation

“…Moreover, the crystal structures of each enzyme involved in this pathway have been identified except for ASMT. Recently, the crystal structure of human ASMT has been successfully determined by Botros et al [8] and they confirmed its presence in the human pineal gland. To further elucidate the impact of ASMT mutations on melatonin synthesis, the crystal structures of ASMT in complex with SAM and SAM + NAS were also deposited in the protein data bank.…”

“…On the carboxylate group of E311 [8]. A proton of H255 is abstracted by the E311, which enhances the possibility of the proton transferring from the hydroxyl of NAS to H255.…”

“…The bound SAM and NAS molecules are directed by their respective binding sites so that the methyl group of the SAM moiety and the hydroxyl group of NAS are aligned and close to each other, which is favourable for methyl transfer. On the basis of the crystal structure, Botros et al [8] proposed the following mechanism that the methyl transfer is activated by the abstraction of a proton from the hydroxyl of NAS to the imidazole group of H255 assisted by the carboxylate group of E311, which is similar to the mechanism for Chalcone O-methyltransferase (ChOMT) [9].…”