Polyethylene terephthalate (PET) is a commodity polymer known to globally contaminate marine and terrestrial environments. Today, around 80 bacterial and fungal PET-active enzymes (PETases) are known, originating from four bacterial and two fungal phyla. In contrast, no archaeal enzyme had been identified to degrade PET. Here we report on the structural and biochemical characterization of PET46 (RLI42440.1), an archaeal promiscuous feruloyl esterase exhibiting degradation activity on semi-crystalline PET powder comparable to IsPETase and LCC (wildtypes), and higher activity on bis-, and mono-(2-hydroxyethyl) terephthalate (BHET and MHET). The enzyme, found by a sequence-based metagenome search, is derived from a non-cultivated, deep-sea Candidatus Bathyarchaeota archaeon. Biochemical characterization demonstrated that PET46 is a promiscuous, heat-adapted hydrolase. Its crystal structure was solved at a resolution of 1.71 A. It shares the core alpha/beta-hydrolase fold with bacterial PETases, but contains a unique lid common in feruloyl esterases, which is involved in substrate binding. Thus, our study widens the currently known diversity of PET-hydrolyzing enzymes, by demonstrating PET depolymerization by a plant cell wall-degrading esterase.
Polyethylene terephthalate (PET) is a widely used synthetic polymer and known to contaminate marine and terrestrial ecosystems. Only few PET-active microorganisms and enzymes (PETases) are currently known and it is debated whether degradation activity for PET originates from promiscuous enzymes with broad substrate spectra that primarily act on natural polymers or other bulky substrates, or whether microorganisms evolved their genetic makeup to accepting PET as a carbon source. Here, we present a predicted diene lactone hydrolase designated PET40, which acts on a broad spectrum of substrates, including PET. It is the first esterase with activity on PET from a GC-rich Gram-positive Amycolatopsis species belonging to the Pseudonocardiaceae (Actinobacteria). It is highly conserved within the genera Amycolatopsis and Streptomyces. PET40 was identified by sequence-based metagenome search using a PETase-specific Hidden Markov Model (HMM). Besides acting on PET, PET40 has a versatile substrate spectrum, hydrolyzing delta-lactones, beta-lactam antibiotics, the polyester-polyurethane Impranil(a) DLN, and various para-nitrophenyl (pNP) ester substrates. Molecular docking suggests that the PET degradative activity is likely a result of the promiscuity of PET40, as potential binding modes were found for substrates encompassing mono(2-hydroxyethyl) terephthalate (MHET), bis(2-hydroxyethyl) terephthalate (BHET), and a PET trimer (PET(3) ). We also solved the crystal structure of the inactive PET40 variant S178A to 1.60 A resolution. PET40 is active throughout a wide pH (pH 4-10) and temperature range (4-65 degreesC) and remarkably stable in the presence of 5% sodium dodecyl sulfate (SDS), making it a promising enzyme as a starting point for further investigations and optimization approaches.
Pseudomonas aeruginosa is a wide-spread opportunistic human pathogen and a high-risk factor for immunodeficient people and patients with cystic fibrosis. The extracellular lipase A belongs to the virulence factors of P. aeruginosa. Prior to the secretion, the lipase undergoes folding and activation by the periplasmic foldase LipH. At this stage, the enzyme is highly prone to aggregation in mild and high salt concentrations typical for the sputum of cystic fibrosis patients. Here, we demonstrate that the periplasmic chaperone Skp of P. aeruginosa efficiently prevents misfolding of the lipase A in vitro. In vivo experiments in P. aeruginosa show that the lipase secretion is nearly abolished in absence of the endogenous Skp. Small-angle X-ray scattering elucidates the trimeric architecture of P. aeruginosa Skp and identifies two primary conformations of the chaperone, a compact and a widely open. We describe two binding modes of Skp to the lipase, with affinities of 20 nM and 2 microM, which correspond to 1:1 and 1:2 stoichiometry of the lipase:Skp complex. Two Skp trimers are required to stabilize the lipase via the apolar interactions, which are not affected by elevated salt concentrations. We propose that Skp is a crucial chaperone along the lipase maturation and secretion pathway that ensures stabilization and carry-over of the client to LipH.
Certain members of the Actinobacteria and Proteobacteria are known to degrade polyethylene terephthalate (PET). Here, we describe the first functional PET-active enzymes from the Bacteroidetes phylum. Using a PETase-specific Hidden-Markov-Model- (HMM-) based search algorithm, we identified several PETase candidates from Flavobacteriaceae and Porphyromonadaceae. Among them, two promiscuous and cold-active esterases derived from Aequorivita sp. (PET27) and Kaistella jeonii (PET30) showed depolymerizing activity on polycaprolactone (PCL), amorphous PET foil and on the polyester polyurethane Impranil((a)) DLN. PET27 is a 37.8 kDa enzyme that released an average of 174.4 nmol terephthalic acid (TPA) after 120 h at 30 degreesC from a 7 mg PET foil platelet in a 200 microl reaction volume, 38-times more than PET30 (37.4 kDa) released under the same conditions. The crystal structure of PET30 without its C-terminal Por-domain (PET30deltaPorC) was solved at 2.1 A and displays high structural similarity to the IsPETase. PET30 shows a Phe-Met-Tyr substrate binding motif, which seems to be a unique feature, as IsPETase, LCC and PET2 all contain Tyr-Met-Trp binding residues, while PET27 possesses a Phe-Met-Trp motif that is identical to Cut190. Microscopic analyses showed that K. jeonii cells are indeed able to bind on and colonize PET surfaces after a few days of incubation. Homologs of PET27 and PET30 were detected in metagenomes, predominantly aquatic habitats, encompassing a wide range of different global climate zones and suggesting a hitherto unknown influence of this bacterial phylum on man-made polymer degradation.
Bacterial lipolytic enzymes of family IV are homologs of the mammalian hormone-sensitive lipases (HSL) and have been successfully used for various biotechnological applications. The broad substrate specificity and ability for enantio-, regio-, and stereoselective hydrolysis are remarkable features of enzymes from this class. Many crystal structures are available for esterases and lipases, but structures of enzyme-substrate or enzyme-inhibitor complexes are less frequent although important to understand the molecular basis of enzyme substrate interaction and to rationalize biochemical enzyme characteristics. Here, we report on the structures of a novel family IV esterase isolated from a metagenomic screen which shows a broad substrate specificity. We solved the crystal structures in the apo form and with a bound substrate analogue at 1.35 and 1.81 resolution, respectively. This enzyme named PtEst1 hydrolyzed more than 60 out 96 structurally different ester substrates thus being substrate promiscuous. Its broad substrate specificity is in accord with a large active site cavity, which is covered by an alpha-helical cap domain. The substrate analogue methyl 4-methylumbelliferyl hexylphosphonate was rapidly hydrolyzed by the enzyme leading to a complete inactivation caused by covalent binding of phosphinic acid to the catalytic serine. Interestingly, the alcohol leaving group 4-methylumbelliferone was found remaining in the active site cavity and additionally, a complete inhibitor molecule was found at the cap domain next to the entrance of the substrate tunnel. This unique situation allowed gaining valuable insights into the role of the cap domain for enzyme-substrate interaction of esterases belonging to family IV.
Biodegradation of synthetic polymers, in particular polyethylene terephthalate (PET), is of great importance, since environmental pollution with PET and other plastics has become a severe global problem. Here, we report on the polyester degrading ability of a novel carboxylic ester hydrolase identified in the genome of the marine hydrocarbonoclastic bacterium Pseudomonas aestusnigri VGXO14T. The enzyme, designated PE-H, belongs to the type IIa family of PET hydrolytic enzymes as indicated by amino acid sequence homology. It was produced in Escherichia coli, purified and its crystal structure was solved at 1.09 A resolution representing the first structure of a type IIa PET hydrolytic enzyme. The structure shows a typical alpha/beta-hydrolase fold and high structural homology to known polyester hydrolases. PET hydrolysis was detected at 30C with amorphous PET film (PETa), but not with PET film from a commercial PET bottle (PETb). A rational mutagenesis study to improve the PET degrading potential of PE-H yielded variant PE-H (Y250S) which showed improved activity, ultimately also allowing the hydrolysis of PETb. The crystal structure of this variant solved at 1.35 A resolution allowed to rationalize the improvement of enzymatic activity. A PET oligomer binding model was proposed by molecular docking computations. Our results indicate a significant potential of the marine bacterium P. aestusnigri for PET degradation.
Poly(ethylene terephthalate) (PET) is one of the most widely applied synthetic polymers, but its hydrophobicity is challenging for many industrial applications. Biotechnological modification of PET surface can be achieved by PET hydrolyzing cutinases. In order to increase the adsorption towards their unnatural substrate, the enzymes are fused to carbohydrate-binding modules (CBMs) leading to enhanced activity. In this study, we identified novel PET binding CBMs and characterized the CBM-PET interplay. We developed a semi-quantitative method to detect CBMs bound to PET films. Screening of eight CBMs from diverse families for PET binding revealed one CBM that possesses a high affinity towards PET. Molecular dynamics (MD) simulations of the CBM-PET interface revealed tryptophan residues forming an aromatic triad on the peptide surface. Their interaction with phenyl rings of PET is stabilized by additional hydrogen bonds formed between amino acids close to the aromatic triad. Furthermore, the ratio of hydrophobic to polar contacts at the interface was identified as an important feature determining the strength of PET binding of CBMs. The interaction of CBM tryptophan residues with PET was confirmed experimentally by tryptophan quenching measurements after addition of PET nanoparticles to CBM. Our findings are useful for engineering PET hydrolyzing enzymes and may also find applications in functionalization of PET.
        
Title: Structural basis for recognition and ring-cleavage of the Pseudomonas quinolone signal (PQS) by AqdC, a mycobacterial dioxygenase of the alpha/beta-hydrolase fold family Wullich SC, Kobus S, Wienhold M, Hennecke U, Smits SHJ, Fetzner S Ref: J Struct Biol, 207:287, 2019 : PubMed
The cofactor-less dioxygenase AqdC of Mycobacteroides abscessus catalyzes the cleavage and thus inactivation of the Pseudomonas quinolone signal (PQS, 2-heptyl-3-hydroxy-4(1H)-quinolone), which plays a central role in the regulation of virulence factor production by Pseudomonas aeruginosa. We present here the crystal structures of AqdC in its native state and in complex with the PQS cleavage product N-octanoylanthranilic acid, and of mutant AqdC proteins in complex with PQS. AqdC possesses an alpha/beta-hydrolase fold core domain with additional helices forming a cap domain. The protein is traversed by a bipartite tunnel, with a funnel-like entry section leading to an elliptical substrate cavity where PQS positioning is mediated by a combination of hydrophobic interactions and hydrogen bonds, with the substrate's C4 carbonyl and C3 hydroxyl groups tethered by His97 and the catalytic His246, respectively. The side chain of the AqdC-bound product extends deeper into the "alkyl tail section" of the tunnel than PQS, tentatively suggesting product exit via this part of the tunnel. AqdC prefers PQS over congeners with shorter alkyl substituents at C2. Kinetic data confirmed the strict requirement of the active-site base His246 for catalysis, and suggested that evolution of the canonical nucleophile/His/Asp catalytic triad of the hydrolases to an Ala/His/Asp triad is favorable for catalyzing dioxygenolytic PQS ring cleavage.