A plasmid encoding enzymes for nylon oligomer degradation: nucleotide sequence and analysis of pOAD2

Small molecule substrates for the rapid quantification of acyl transfer activity of nylon hydrolase NylCA

The widespread use of polyamides such as nylons has led to the accumulation of nylon waste, which is particularly resistant to decomposition due to the intrinsic stability of the amide bond. New methods are required for the true recycling of these waste materials by depolymerization. Enzymes that are capable of hydrolyzing polyamides have been proposed as biocatalysts that may be suitable for this application. NylC is an enzyme that can mediate the hydrolysis of aminohexanoic acid oligomers, and to some extent, bulk polymers. However, current assays to characterize the activity of this enzyme require long reaction times and/or rely on secondary reactions to quantify hydrolysis. Herein, we have designed structurally-optimized small molecule chromogenic esters that serve as substrate analogues for monitoring NylC acyltransferase activity in a continuous manner. This assay can be performed in minutes at room temperature, and the substrate N-acetyl-GABA-pNP ester (kcat = 0.37 s-1, KM = 256 μM) shows selectivity for NylC in complex biological media. We also demonstrate that activity towards this substrate analogue correlates with amide hydrolysis, which is the primary activity of this enzyme. Furthermore, our screening of substrate analogues provides insight into the substrate specificity of NylC, which is relevant to biocatalytic applications.

Isolation and genomic analysis of 11-aminoundecanoic acid-degrading bacterium Pseudomonas sp. JG-B from nylon 11 enrichment culture

Nylon 11 is a polymer synthesized from 11-aminoundecanoic acid, and widely used in commercial manufacturing. In this study, we describe the isolation of the first organism capable of metabolizing 11-aminoundecanoic acid from nylon 11 enrichment culture. The strain shows rapid growth on 11-aminoundecanoic acid as a sole source of carbon, nitrogen, and energy. Furthermore, the genome sequence of strain JG-B was deciphered and shown to belong to genus Pseudomonas. Many genes encoding putative extracellular hydrolases, as well as homologues of nylon 6 hydrolases (NylB and NylA) were identified, suggesting the metabolic versatility and possibility that this organism could also depolymerase nylon 11 polymers.

Structural basis of the correct subunit assembly, aggregation, and intracellular degradation of nylon hydrolase

Nylon hydrolase (NylC) is initially expressed as an inactive precursor (36 kDa). The precursor is cleaved autocatalytically at Asn266/Thr267 to generate an active enzyme composed of an α subunit (27 kDa) and a β subunit (9 kDa). Four αβ heterodimers (molecules A-D) form a doughnut-shaped quaternary structure. In this study, the thermostability of the parental NylC was altered by amino acid substitutions located at the A/D interface (D122G/H130Y/D36A/L137A) or the A/B interface (E263Q) and spanned a range of 47 °C. Considering structural, biophysical, and biochemical analyses, we discuss the structural basis of the stability of nylon hydrolase. From the analytical centrifugation data obtained regarding the various mutant enzymes, we conclude that the assembly of the monomeric units is dynamically altered by the mutations. Finally, we propose a model that can predict whether the fate of the nascent polypeptide will be correct subunit assembly, inappropriate protein-protein interactions causing aggregation, or intracellular degradation of the polypeptide.

Draft Genome Sequence of the Nylon Oligomer-Degrading Bacterium Arthrobacter sp. Strain KI72

We report here the 4.6-Mb genome sequence of a nylon oligomer-degrading bacterium, Arthrobacter sp. strain KI72. The draft genome sequence of strain KI72 consists of 4,568,574 bp, with a G+C content of 63.47%, 4,372 coding sequences (CDSs), 54 tRNAs, and six rRNAs.

Unraveling the degradation of artificial amide bonds in nylon oligomer hydrolase: from induced-fit to acylation processes

To unravel the factor that provides the ability to degrade non-biological amide bond with nylon oligomer hydrolase, we investigated the process from induced-fit to acylation by a combination of different theoretical methods.

Three-dimensional structure of nylon hydrolase and mechanism of nylon-6 hydrolysis

We performed x-ray crystallographic analyses of the 6-aminohexanoate oligomer hydrolase (NylC) from Agromyces sp. at 2.0 Å-resolution. This enzyme is a member of the N-terminal nucleophile hydrolase superfamily that is responsible for the degradation of the nylon-6 industry byproduct. We observed four identical heterodimers (27 kDa + 9 kDa), which resulted from the autoprocessing of the precursor protein (36 kDa) and which constitute the doughnut-shaped quaternary structure. The catalytic residue of NylC was identified as the N-terminal Thr-267 of the 9-kDa subunit. Furthermore, each heterodimer is folded into a single domain, generating a stacked αββα core structure. Amino acid mutations at subunit interfaces of the tetramer were observed to drastically alter the thermostability of the protein. In particular, four mutations (D122G/H130Y/D36A/E263Q) of wild-type NylC from Arthrobacter sp. (plasmid pOAD2-encoding enzyme), with a heat denaturation temperature of T(m) = 52 °C, enhanced the protein thermostability by 36 °C (T(m) = 88 °C), whereas a single mutation (G111S or L137A) decreased the stability by ∼10 °C. We examined the enzymatic hydrolysis of nylon-6 by the thermostable NylC mutant. Argon cluster secondary ion mass spectrometry analyses of the reaction products revealed that the major peak of nylon-6 (m/z 10,000-25,000) shifted to a smaller range, producing a new peak corresponding to m/z 1500-3000 after the enzyme treatment at 60 °C. In addition, smaller fragments in the soluble fraction were successively hydrolyzed to dimers and monomers. Based on these data, we propose that NylC should be designated as nylon hydrolase (or nylonase). Three potential uses of NylC for industrial and environmental applications are also discussed.

X-ray crystallographic analysis of the 6-aminohexanoate cyclic dimer hydrolase: catalytic mechanism and evolution of an enzyme responsible for nylon-6 byproduct degradation

We performed x-ray crystallographic analyses of the 6-aminohexanoate cyclic dimer (Acd) hydrolase (NylA) from Arthrobacter sp., an enzyme responsible for the degradation of the nylon-6 industry byproduct. The fold adopted by the 472-amino acid polypeptide generated a compact mixed alpha/beta fold, typically found in the amidase signature superfamily; this fold was especially similar to the fold of glutamyl-tRNA(Gln) amidotransferase subunit A (z score, 49.4) and malonamidase E2 (z score, 44.8). Irrespective of the high degree of structural similarity to the typical amidase signature superfamily enzymes, the specific activity of NylA for glutamine, malonamide, and indoleacetamide was found to be lower than 0.5% of that for Acd. However, NylA possessed carboxylesterase activity nearly equivalent to the Acd hydrolytic activity. Structural analysis of the inactive complex between the activity-deficient S174A mutant of NylA and Acd, performed at 1.8 A resolution, suggested the following enzyme/substrate interactions: a Ser(174)-cis-Ser(150)-Lys(72) triad constitutes the catalytic center; the backbone N in Ala(171) and Ala(172) are involved in oxyanion stabilization; Cys(316)-S(gamma) forms a hydrogen bond with nitrogen (Acd-N(7)) at the uncleaved amide bond in two equivalent amide bonds of Acd. A single S174A, S150A, or K72A substitution in NylA by site-directed mutagenesis decreased the Acd hydrolytic and esterolytic activities to undetectable levels, indicating that Ser(174)-cis-Ser(150)-Lys(72) is essential for catalysis. In contrast, substitutions at position 316 specifically affected Acd hydrolytic activity, suggesting that Cys(316) is responsible for Acd binding. On the basis of the structure and functional analysis, we discussed the catalytic mechanisms and evolution of NylA in comparison with other Ser-reactive hydrolases.

Two alternative modes for optimizing nylon-6 byproduct hydrolytic activity from a carboxylesterase with a beta-lactamase fold: X-ray crystallographic analysis of directly evolved 6-aminohexanoate-dimer hydrolase

Promiscuous 6-aminohexanoate-linear dimer (Ald)-hydrolytic activity originally obtained in a carboxylesterase with a beta-lactamase fold was enhanced about 80-fold by directed evolution using error-prone PCR and DNA shuffling. Kinetic studies of the mutant enzyme (Hyb-S4M94) demonstrated that the enzyme had acquired an increased affinity (K(m) = 15 mM) and turnover (k(cat) = 3.1 s(-1)) for Ald, and that a catalytic center suitable for nylon-6 byproduct hydrolysis had been generated. Construction of various mutant enzymes revealed that the enhanced activity in the newly evolved enzyme is due to the substitutions R187S/F264C/D370Y. Crystal structures of Hyb-S4M94 with bound substrate suggested that catalytic function for Ald was improved by hydrogen-bonding/hydrophobic interactions between the Ald--COOH and Tyr370, a hydrogen-bonding network from Ser187 to Ald--NH(3) (+), and interaction between Ald--NH(3) (+) and Gln27-O(epsilon) derived from another subunit in the homo-dimeric structure. In wild-type Ald-hydrolase (NylB), Ald-hydrolytic activity is thought to be optimized by the substitutions G181D/H266N, which improve an electrostatic interaction with Ald--NH(3) (+) (Kawashima et al., FEBS J 2009; 276:2547-2556). We propose here that there exist at least two alternative modes for optimizing the Ald-hydrolytic activity of a carboxylesterase with a beta-lactamase fold.

Genetic organization of nylon-oligomer-degrading enzymes from alkalophilic bacterium, Agromyces sp. KY5R

A 15-kb gene locus including nylon-oligomer-degrading genes from the chromosome of an alkalophilic bacterium, Agromyces sp. KY5R, was cloned and sequenced. The genetic organization was similar to the DNA region flanked by directly repeated IS6100 sequences on the nylon-oligomer-degradative plasmid pOAD2. However, we found several genetic rearrangements between the two DNA regions. Here, we discuss the possible mechanisms underlying the genetic rearrangements.

6-Aminohexanoate oligomer hydrolases from the alkalophilic bacteria Agromyces sp. strain KY5R and Kocuria sp. strain KY2

Alkalophilic, nylon oligomer-degrading strains, Agromyces sp. and Kocuria sp., were isolated from the wastewater of a nylon-6 factory and from activated sludge from a sewage disposal plant. The 6-aminohexanoate oligomer hydrolases (NylC) from the alkalophilic strains had 95.8 to 98.6% similarity to the enzyme in neutrophilic Arthrobacter sp. but had superior thermostability, activity under alkaline conditions, and affinity for nylon-related substrates, which would be advantageous for biotechnological applications.

A review of biodegradation of synthetic plastic and foams

Synthetic polymeric foams have pervaded every aspect of modern life. Although foams provide numerous benefits, they also cause a significant environmental litter problem because of their recalcitrant and xenobiotic nature. Biodegradation may provide solution to the problem, but not enough is known about the biodegradation process of synthetic plastic and plasticbased foams. This review has been written to provide an overview of the current state of plastic foam biodegradation. Several biodegradation pathways of a few select synthetic polymers are also presented along with a discussion on some of the physico-chemical factors that can influence the biodegradation of plastic foams.

Mutational analysis of 6-aminohexanoate-dimer hydrolase: Relationship between nylon oligomer hydrolytic and esterolytic activities

Carboxylesterase (EII') from Arthrobacter sp. KI72 has 88% homology to 6-aminohexanoate-dimer hydrolase (EII) and possesses ca. 0.5% of the level of 6-aminohexanoate-linear dimer (Ald)-hydrolytic activity of EII. To study relationship between Ald-hydrolytic and esterolytic activities, random mutations were introduced into the gene for Hyb-24 (an EII/EII' hybrid with the majority of the sequence deriving for EII' and possessing an EII'-like level of Ald-hydrolytic activity). Either a G181D or a D370Y substitution in Hyb-24 increased the Ald-hydrolytic activity ca. 10-fold, and a G181D/D370Y double substitution increased activity ca. 100-fold. On the basis of kinetic studies and the three-dimensional structure of the enzyme, we suggest that binding of Ald is improved by these mutations. D370Y increased esterolytic activity for glycerylbutyrate ca. 30-50-fold, whereas G181D decreased the activity to 30% of the parental enzyme.

X-ray Crystallographic Analysis of 6-Aminohexanoate-Dimer Hydrolase

6-Aminohexanoate-dimer hydrolase (EII), responsible for the degradation of nylon-6 industry by-products, and its analogous enzyme (EII') that has only approximately 0.5% of the specific activity toward the 6-aminohexanoate-linear dimer, are encoded on plasmid pOAD2 of Arthrobacter sp. (formerly Flavobacterium sp.) KI72. Here, we report the three-dimensional structure of Hyb-24 (a hybrid between the EII and EII' proteins; EII'-level activity) by x-ray crystallography at 1.8 A resolution and refined to an R-factor and R-free of 18.5 and 20.3%, respectively. The fold adopted by the 392-amino acid polypeptide generated a two-domain structure that is similar to the folds of the penicillin-recognizing family of serine-reactive hydrolases, especially to those of d-alanyl-d-alanine-carboxypeptidase from Streptomyces and carboxylesterase from Burkholderia. Enzyme assay using purified enzymes revealed that EII and Hyb-24 possess hydrolytic activity for carboxyl esters with short acyl chains but no detectable activity for d-alanyl-d-alanine. In addition, on the basis of the spatial location and role of amino acid residues constituting the active sites of the nylon oligomer hydrolase, carboxylesterase, d-alanyl-d-alanine-peptidase, and beta-lactamases, we conclude that the nylon oligomer hydrolase utilizes nucleophilic Ser(112) as a common active site both for nylon oligomer-hydrolytic and esterolytic activities. However, it requires at least two additional amino acid residues (Asp(181) and Asn(266)) specific for nylon oligomer-hydrolytic activity. Here, we propose that amino acid replacements in the catalytic cleft of a preexisting esterase with the beta-lactamase fold resulted in the evolution of the nylon oligomer hydrolase.

Crystallization and X-ray diffraction analysis of 6-aminohexanoate-dimer hydrolase from Arthrobacter sp. KI72

To investigate the structure-function relationship between 6-aminohexanoate-dimer hydrolase (EII) from Arthrobacter sp. and a cryptic protein (EII') which shows 88% sequence identity to EII, a hybrid protein (named Hyb-24) of EII and EII' was overexpressed, purified and crystallized using the sitting-drop vapour-diffusion method with ammonium sulfate as a precipitant in MES buffer pH 6.5. The crystal belongs to space group P3(1)21 or P3(2)21, with unit-cell parameters a = b = 96.37, c = 113.09 A. Diffraction data were collected from native and methylmercuric chloride derivative crystals to resolutions of 1.75 and 1.80 A, respectively.

Bacterial succession in a petroleum land treatment unit

Bacterial community dynamics were investigated in a land treatment unit (LTU) established at a site contaminated with highly weathered petroleum hydrocarbons in the C(10) to C(32) range. The treatment plot, 3,000 cubic yards of soil, was supplemented with nutrients and monitored weekly for total petroleum hydrocarbons (TPH), soil water content, nutrient levels, and aerobic heterotrophic bacterial counts. Weekly soil samples were analyzed with 16S rRNA gene terminal restriction fragment (TRF) analysis to monitor bacterial community structure and dynamics during bioremediation. TPH degradation was rapid during the first 3 weeks and slowed for the remainder of the 24-week project. A sharp increase in plate counts was reported during the first 3 weeks, indicating an increase in biomass associated with petroleum degradation. Principal components analysis of TRF patterns revealed a series of sample clusters describing bacterial succession during the study. The largest shifts in bacterial community structure began as the TPH degradation rate slowed and the bacterial cell counts decreased. For the purpose of analyzing bacterial dynamics, phylotypes were generated by associating TRFs from three enzyme digests with 16S rRNA gene clones. Two phylotypes associated with Flavobacterium and Pseudomonas were dominant in TRF patterns from samples during rapid TPH degradation. After the TPH degradation rate slowed, four other phylotypes gained dominance in the community while Flavobacterium and Pseudomonas phylotypes decreased in abundance. These data suggest that specific phylotypes of bacteria were associated with the different phases of petroleum degradation in the LTU.

Sequence analysis of a cryptic plasmid from Flavobacterium sp. KP1, a psychrophilic bacterium

A cryptic plasmid found at high copy number was isolated from Flavobacterium sp. KP1, a psychrophilic Gram-negative bacterium, cloned, and sequenced. The sequence will appear in the DDBJ/EMBL/GenBank databases under the accession number AB007196. The pFL1 plasmid is 2311 nucleotides in length with 32.7% GC content, and shows a distinctive nucleotide sequence without homology to other plasmids of similar length. The plasmid contains two open reading frames of significant length, ORFI and ORFII. ORFI encodes a protein similar to the replication proteins found in Gram-negative bacterial plasmids, Bacteroides fragilis plasmid pBI143 and Zymomonas mobilis plasmid pZM2. The putative translation product of ORFII shows homologies with plasmid recombination proteins found mainly in Gram-positive bacterial plasmids such as Staphylococcus aureus plasmid pT181.