ATP-grasp enzyme; part of the gene cluster that mediates the biosynthesis of the isoquinoline alkaloids fumisoquin A, fumisoquin B and fumisoquin C; as well as small amounts of fumipyrrole as a shunt metabolite (PubMed:25582336, PubMed:27065235). The products of the cluster lead to a brown coloration and are important for growth and conidiation (PubMed:25582336). The nonribosomal peptide synthetase-like protein fsqF, which lacks a canonical condensation domain, is required for addition of a serine-derived dehydroalanine moiety to activated tyrosine but is not essential for the subsequent steps leading to isoquinoline formation (PubMed:27065235). A different enzyme, most likely the ATP-grasp enzyme fsqD, is responsible for activation of tyrosine (Probable). Three additional enzymes encoded by the fsq cluster, the N-methyltransferase fsqC, the phenol 2-monooxygenase fsqG and the FAD-dependent oxidase fsqB, catalyze the formation of the isoquinoline ring system in the fumisoquins (PubMed:27065235, PubMed:30194285). FsqB converts the fspF thiolation domain-bound (2S,4S,5S)-2-amino-6-(3,4-dihydroxyphenyl)-4-hydroxy-5-(methylamino)hexanoyl into isoquinoline (PubMed:27065235, PubMed:30194285). The cyclization most likely proceeds via a two-step mechanism, beginning with FAD-dependent oxidation of the methyl group to an iminium species followed by electrophilic attack on the deprotonated phenol (Probable). Binds 2 magnesium or manganese ions per subunit. Secondary metabolite biosynthesis. Expression is positively regulated by the fumisoquins biosynthesis specific transcription factor fsqA (PubMed:25582336).