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Norcoclaurine synthase (NCS) (EC 4.2.1.78) catalyzes the Pictet–Spengler condensation of dopamine and an aldehyde, forming a substituted (S)-tetrahydroisoquinoline, a pharmaceutically important moiety. This unique activity has led to NCS being used for both in vitro biocatalysis and in vivo recombinant metabolism. Future engineering of NCS activity to enable the synthesis of diverse tetrahydroisoquinolines is dependent on an understanding of the NCS mechanism and kinetics. We assess two proposed mechanisms for NCS activity: (a) one based on the holo X-ray crystal structure and (b) the ‘dopamine-first’ mechanism based on computational docking. Thalictrum flavum NCS variant activities support the dopamine-first mechanism. Suppression of the non-enzymatic background reaction reveals novel kinetic parameters for NCS, showing it to act with low catalytic efficiency. This kinetic behaviour can account for the ineffectiveness of recombinant NCS in in vivo systems, and also suggests NCS may have an in planta role as a metabolic gatekeeper. The amino acid substitution L76A, situated in the proposed aldehyde binding site, results in the alteration of the enzyme's aldehyde activity profile. This both verifies the dopamine-first mechanism and demonstrates the potential for the rational engineering of NCS activity.

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A one-pot synthesis of tetrahydroisoquinoline alkaloids in a phosphate buffer has been achieved, and a reaction mechanism proposed. The utilisation of mild reaction conditions readily afforded a range of isoquinolines, including norcoclaurine.

The versatility and potential of a norcoclaurine synthase (NCS) from Coptis japonica NCS2 has been investigated, together with the development and application of a novel fluorescence-based high-throughput assay using nearly forty amines/aldehydes. The stereocontrol exerted by CjNCS2 on selected non-natural substrates has been determined, where the tetrahydroisoquinolines (THIAs) were formed as the (1S)-isomer in > 95% ee, as observed with the natural product norcoclaurine. Docking calculations involving THIA mechanism intermediates, utilising the reported Thalictrum flavum NCS X-ray crystallographic structure, were carried out and com-bined with the CjNCS2 screening results to further understand the mode of action of NCS. These findings suggested that in addition to the key active-site residues K122 and E110, D141 is also mechanistically essential for the enzymatic transformation. The exceptional tolerance of NCS towards aldehyde substrates is furthermore supported by our proposed mechanism in which the aldehydes protrude out of the enzymatic pocket.