Aminopeptidase M (AMP) inhibition is of interest for several diseases, such as highly vascularized cancer types. AMP can be inhibited by linear pentapeptides isolated from Microcystis aeruginosa LTPNA08 (MG7XX). Porcine AMP inhibition—a model for human AMP—activity was spectrophotometrically measured by the formation of p-nitroanilide from L-leucine-p-nitroanilide substrate by AMP. AMP inhibition by MG770 exhibited comparable inhibition levels to amastatin (IC50 values: 1.20 ± 0.1 μM and 0.98 ± 0.1 μM, respectively), while MG756 was slightly less potent (with IC50 values of 3.26 ± 0.5 μM). Molecular modelling suggests a potential binding mode, based on the interaction with the Zn2+ cofactor, where MG770′s extra methyl group contributes to the disturbance of the Zn2+ cofactor complex and highlights the importance of hydrophobicity for the site.