Cytochrome b5-like reductase; part of the gene cluster that mediates the biosynthesis of the cyclic tetrapeptide apicidin F (APF) (PubMed:25058475). The non-ribosomal peptide synthetase apf1 incorporates four different amino acids to produce apicidin F: L-phenylalanine, D-pipecolic acid (D-pip), N-methoxy-L-tryptophan and L-2-aminooctanedioic acid (PubMed:25058475). L-Phenylalanine is the only proteinogenic amino acid directly used by apf1 (PubMed:23825955, PubMed:25058475). The 3 other apf1 substrates are non-proteinogenic and have to be modified by other enzymes of the cluster (PubMed:25058475). Lysine is converted to delta-1-pyrroline-5-carboxylate (P5C) which is reduced to L-pipecolic acid (L-pip) by apf3 (PubMed:25058475). L-pip is epimerized to D-pip, probably by apf1 activity, prior to incorporation (PubMed:25058475). L-Tryptophan is N-oxidyzed by one of the cytochrome P450 monooxygenases (apf7 or apf8), and further methylated at the hydroxy group by the O-methyltransferase apf6 to yield N-methoxy-L-tryptophan (PubMed:25058475). The synthesis of the fourth apf1 substrate is more complex (PubMed:25058475). The fatty acid synthase apf5 is involved in the synthesis of the octanoic acid backbone of L-2-aminooctanedioic acid by fixing one acetyl-CoA unit and three malonyl-CoA units (PubMed:25058475). Then one of the cytochrome P450 monooxygenases (apf7 or apf8) may oxidize this backbone to 2-oxooctanoic acid (PubMed:25058475). The aminotransferase apf4 is predicted to catalyze the exchange of the keto group with an amino group (PubMed:25058475). The next step would be the oxidation of 2-aminooctanoic acid by one of the cytochrome P450 monooxygenases (apf7 or apf8). The last step is the oxidation of 2-amino-8-hydroxyoctanoic acid to 2-aminooctanedioic acid is catalyzed by the FAD-dependent monooxygenase apf9 (PubMed:25058475). Secondary metabolite biosynthesis. Expression is positively regulated by the apicidin F cluster-specific transcription factor apf2 that binds to the eight-base-pair motif 5'-TGACGTGA-3' called the 'Api-box' that is found in all promoters of the apicidin F cluster except in the promoter region of apf2 itself (PubMed:25058475). Apicidin F, like the other known apicidins, is a cyclic tetrapeptides with anti-malarial properties via histone deacetylase inhibitory activity (PubMed:24195442). Belongs to the flavoprotein pyridine nucleotide cytochrome reductase family.