Involved in the biosynthesis of albonoursin (cyclo[(alpha,beta-dehydro-Phe)-(alpha,beta-dehydro-Leu)]), an antibacterial peptide. Catalyzes the formation of alpha,beta-dehydro-Phe (DPhe) and alpha,beta-dehydro-Leu (DLeu) residues during the biosynthesis of albonoursin. The catalytic reaction of cyclo(L-Phe-L-Leu) occurs in a two-step sequential alpha-beta-dehydrogenation leading first to cyclo(alpha,beta-dehydro-Phe-L-Leu) and finally to albonoursin. Can also use cyclo(L-Phe-L-His), cyclo(L-Trp-L-Trp), cyclo(L-Leu-L-Ala), cyclo(L-Phe-Gly), cyclo(L-Leu-Gly), cyclo(L-Ser-Gly) and cyclo(L-Glu-Gly) as substrate suggesting that the diketopiperazine ring is essential for the enzymatic reaction. cyclo(L-phenylalanyl-L-leucyl) + 2 O2 = albonoursin + 2 H2O2 kcat is 0.453 and 0.69 sec(-1) for cyclo(L-Phe-L-His) and cyclo(L-Phe-L-Leu), respectively. Optimum pH is 8. Optimum temperature is 60 degrees Celsius. The activity increases quite linearly with temperature and is nearly twice as high at 60 degrees Celsius as compared to 30 degrees Celsius. Above 60 degrees Celsius, the enzyme activity dramatically decreases, probably due to thermal inactivation. Homomer. The N-terminus is blocked. Belongs to the nitroreductase family.