Marine bacterial enzyme degrades polyamide 4 into gamma-aminobutyric acid oligomers

Polyamide 4 (PA4) is a biobased plastic that has superior mechanical properties. PA4 shows good biodegradability in terrestrial and marine environments; however, the detailed interaction between PA4 and PA4-degrading enzymes from PA4-degrading microorganisms remains unclear. In this study, we isolated 11 novel strains of a PA4-degrading bacterium belonging to Pseudoalteromonas from a marine environment. Pseudoalteromonas sp. Y-5, which showed the highest PA4-degrading activity among the isolates, produced maximum PA4-degrading activity in culture supernatant after 4 days of incubation at 15 degrees C in Marine Broth 2216 containing 0.5 w/v% PA4 powder. We purified an enzyme that hydrolyzes PA4 at amide bonds into gamma-aminobutyric acid oligomers (2-3 mers) from the culture supernatant. Based on a homology search using the amino acid sequence and predicted 3D structure of the enzyme, it was predicted to be composed of a substrate binding domain (SBD), linker domain, and catalytic domain (CD). The enzyme is a homodimer consisting of 75-kDa subunits, and the SBD is digested by autolysis in the absence of Ca2+. Interestingly, enzymatic properties, such as optimum temperature, pH, and NaCl concentration for high activity, differed between the full-size enzyme and autolyzed enzyme. In the predicted 3D structure of the enzyme, the structure of the CD overlapped with that of 6-amino-hexanoate-dimer hydrolase, with a root mean square deviation of 1.084 A. To our knowledge, this is the first report on the enzymatic and structural properties of a marine PA4-degrading enzyme. Our findings contribute to the understanding of the mechanisms of PA4 degradation in the marine environment.