Engineering the Activity of a Newly Identified Arylalkylamine N-Acetyltransferase in the Acetylation of 5-Hydroxytryptamine

Arylalkylamine N-acetyltransferase (AANAT) serves as a key enzyme in the biosynthesis of melatonin by transforming 5-hydroxytryptamine (5-HT) to N-acetyl-5-hydroxytryptamine (NAS), while its low activity may hinder melatonin yield. In this study, a novel AANAT derived from Sus scrofa (SsAANAT) was identified through data mining using 5-HT as a model substrate, and a rational design of SsAANAT was conducted in the quest to improving its activity. After four rounds of mutagenesis procedures, a triple combinatorial dominant mutant M3 was successfully obtained. Compared to the parent enzyme, the conversion of the whole-cell reaction bearing the best variant M3 exhibted an increase from 50 % to 99 % in the transformation of 5-HT into NAS. Additionally, its catalytic efficiency (k_cat/K_m) was enhanced by 2-fold while retaining the thermostability (T_m>45 °C). In the up-scaled reaction with a substrate loading of 50 mM, the whole-cell system incorporating variant M3 achieved a 99 % conversion of 5-HT in 30 h with an 80 % yield. Molecular dynamics simulations were ultilized to shed light on the origin of improved activity. This study broadens the repertoire of AANAT for the efficient biosynthesis of melatonin.