Poly(aspartic acid) (PAA) is a green alternative to nonbiodegradable poly(carboxylates) and has applications in both industrial and biomedical settings. PAA is synthesized by heating monomeric aspartic acid to yield a polysuccinamide that can be ring-opened to yield thermal PAA composed of 30% α-amide and 70% β-amide linkages. Here, we report the first X-ray crystal structure of a PAA hydrolase from the bacteria Sphingomonas sp. KT-1 (PahZ1 KT-1 ) which functions to degrade synthetic PAA to oligo(aspartic acid) by selective cleavage of β-amide linkages. The structure was solved to 2.45 Å and shows a dimeric assembly where each monomer maintains an α/β hydrolase fold with a prominent, positively lined trough responsible for binding the anionic polymeric substrate. The putative catalytic sites of each monomer lie at the surface of the enzyme on opposite faces. The dimeric interface, as supported by small-angle X-ray scattering/multi-angle light scattering data, is primarily hydrophobic and is further stabilized by flanking hydrogen bonds. Molecular dynamics simulations support the previously determined specific cleavage of only the β-amide linkage through a conformational change that aligns the substrate with the active site Ser. These data provide a scaffold for further understanding the mechanism of PAA hydrolysis and opens the opportunity for using protein engineering to catalyze the biodegradation of other xenobiotics.

Polymers are ubiquitous and essential to modern society, which is why the American Chemical Society has mandated inclusion of polymer chemistry in the undergraduate curriculum. To meet this requirement, we have chosen to weave a polymeric theme through multiple laboratory courses beginning with organic chemistry, where students use aspartic acid to synthesize poly(aspartic acid), an ecofriendly alternative to nonbiodegradable poly(carboxylates). Subsequently, these student-synthesized polymers serve as substrates for the enzyme poly(aspartic acid) hydrolase-1 in our biochemistry course. This experiment introduces the concept of biodegradation through a gel assay that allows students to visualize enzyme-mediated polymer degradation. Students learn the difference between mono-and polydisperse polymers, how biodegradation affects the size of a polymer through analysis of mobility shifts in an agarose gel, and how to use densitometry software to calculate enzyme activity. Finally, keeping the same polymeric theme provides a source of continuity in our curriculum while expanding students' understanding of polymer chemistry from the viewpoint of different chemistry disciplines.

Poly(aspartic acid) (PAA) is a biodegradable synthetic polymer that is easily produced through heating aspartic acid followed by the subsequent addition of sodium hydroxide. Polymer applications range from drug delivery and biomimetics to hygiene products. Partial biodegradation of PAA is accomplished using poly(aspartic acid) hydrolase‐1 (PAAH‐1) which selectively cleaves a specific cross‐link. E. coli expressions of recombinant PAAH‐1 showed the production of significant amounts of protein which could be purified using a single Ni‐NTA column. Analysis of the PAAH‐1 sequence using the Phyre2 Protein Fold Recognition Server resulted in a 33% similarity to a “putative” poly(3‐hydroxybutyrate) depolymerase belonging to a family of enzymes that hydrolyze carboxylic ester bonds; however, no known structure of PAAH‐1 has been reported. Protein crystallization screening at the Hauptman‐Woodward Research Institute resulted in numerous protein crystallization hits being identified. Crystallization experiments were optimized in‐house and the structure was determined at 2.45Å using molecular replacement. The progress of PAAH‐1 characterization and its structure will be discussed.Support or Funding InformationNSF‐IUSE Grant: DUE 1611988This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

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