Improved triglyceride transesterification by circular permutedCandida antarcticalipase B

Engineering of a Baeyer-Villiger monooxygenase to Improve Substrate Scope, Stereoselectivity and Regioselectivity

Baeyer-Villiger monooxygenases belong to a family of flavin-binding proteins that catalyze the Baeyer-Villiger (BV) oxidation of ketones to produce lactones or esters, which are important intermediates in pharmaceuticals or sustainable materials. Phenylacetone monooxygenase (PAMO) from Thermobifida fusca with moderate thermostability catalyzes the oxidation of aryl ketone substrates, but is limited by high specificity and narrow substrate scope. In the present study, we applied loop optimization by loop swapping followed by focused saturation mutagenesis in order to evolve PAMO mutants capable of catalyzing the regioselective BV oxidation of cyclohexanone and cyclobutanone derivatives with formation of either normal or abnormal esters or lactones. We further modulated PAMO to increase enantioselectivity. Crystal structure studies indicate that rotation occurs in the NADP-binding domain and that the high B-factor region is predominantly distributed in the catalytic pocket residues. Computational analyses further revealed dynamic character in the catalytic pocket and reshaped hydrogen bond interaction networks, which is more favorable for substrate binding. Our study provides useful insights for studying enzyme-substrate adaptations.

How Does Solvation Layer Mobility Affect Protein Structural Dynamics?

Solvation is critical for protein structural dynamics. Spectroscopic studies have indicated relationships between protein and solvent dynamics, and rates of gas binding to heme proteins in aqueous solution were previously observed to depend inversely on solution viscosity. In this work, the solvent-compatible enzyme Candida antarctica lipase B, which functions in aqueous and organic solvents, was modeled using molecular dynamics simulations. Data was obtained for the enzyme in acetonitrile, cyclohexane, n-butanol, and tert-butanol, in addition to water. Protein dynamics and solvation shell dynamics are characterized regionally: for each α-helix, β-sheet, and loop or connector region. Correlations are seen between solvent mobility and protein flexibility. So, does local viscosity explain the relationship between protein structural dynamics and solvation layer dynamics? Halle and Davidovic presented a cogent analysis of data describing the global hydrodynamics of a protein (tumbling in solution) that fits a model in which the protein's interfacial viscosity is higher than that of bulk water's, due to retarded water dynamics in the hydration layer (measured in NMR τ2 reorientation times). Numerous experiments have shown coupling between protein and solvation layer dynamics in site-specific measurements. Our data provides spatially-resolved characterization of solvent shell dynamics, showing correlations between regional solvation layer dynamics and protein dynamics in both aqueous and organic solvents. Correlations between protein flexibility and inverse solvent viscosity (1/η) are considered across several protein regions and for a rather disparate collection of solvents. It is seen that the correlation is consistently higher when local solvent shell dynamics are considered, rather than bulk viscosity. Protein flexibility is seen to correlate best with either the local interfacial viscosity or the ratio of the mobility of an organic solvent in a regional solvation layer relative to hydration dynamics around the same region. Results provide insight into the function of aqueous proteins, while also suggesting a framework for interpreting and predicting enzyme structural dynamics in non-aqueous solvents, based on the mobility of solvents within the solvation layer. We suggest that Kramers' theory may be used in future work to model protein conformational transitions in different solvents by incorporating local viscosity effects.

Engineering carboxypeptidase G2 circular permutations for the design of an autoinhibited enzyme

Carboxypeptidase G2 (CPG2) is an Food and Drug Administration (FDA)-approved enzyme drug used to treat methotrexate (MTX) toxicity in cancer patients receiving MTX treatment. It has also been used in directed enzyme-prodrug chemotherapy, but this strategy has been hampered by off-site activation of the prodrug by the circulating enzyme. The development of a tumor protease activatable CPG2, which could be achieved using a circular permutation of CPG2 fused to an inactivating 'prodomain', would aid in these applications. We report the development of a protease accessibility-based screen to identify candidate sites for circular permutation in proximity of the CPG2 active site. The resulting six circular permutants showed similar expression, structure, thermal stability, and, in four cases, activity levels compared to the wild-type enzyme. We rationalize these results based on structural models of the permutants obtained using the Rosetta software. We developed a cell growth-based selection system, and demonstrated that when fused to periplasm-directing signal peptides, one of our circular permutants confers MTX resistance in Escherichia coli with equal efficiency as the wild-type enzyme. As the permutants have similar properties to wild-type CPG2, these enzymes are promising starting points for the development of autoinhibited, protease-activatable zymogen forms of CPG2 for use in therapeutic contexts.

Tolerance of a Knotted Near-Infrared Fluorescent Protein to Random Circular Permutation

Bacteriophytochrome photoreceptors (BphP) are knotted proteins that have been developed as near-infrared fluorescent protein (iRFP) reporters of gene expression. To explore how rearrangements in the peptides that interlace into the knot within the BphP photosensory core affect folding, we subjected iRFPs to random circular permutation using an improved transposase mutagenesis strategy and screened for variants that fluoresce. We identified 27 circularly permuted iRFPs that display biliverdin-dependent fluorescence in Escherichia coli. The variants with the brightest whole cell fluorescence initiated translation at residues near the domain linker and knot tails, although fluorescent variants that initiated translation within the PAS and GAF domains were discovered. Circularly permuted iRFPs retained sufficient cofactor affinity to fluoresce in tissue culture without the addition of biliverdin, and one variant displayed enhanced fluorescence when expressed in bacteria and tissue culture. This variant displayed a quantum yield similar to that of iRFPs but exhibited increased resistance to chemical denaturation, suggesting that the observed increase in the magnitude of the signal arose from more efficient protein maturation. These results show how the contact order of a knotted BphP can be altered without disrupting chromophore binding and fluorescence, an important step toward the creation of near-infrared biosensors with expanded chemical sensing functions for in vivo imaging.

Protein engineering for development of new hydrolytic biocatalysts

Hydrolytic enzymes play important roles as biocatalysts in chemical synthesis. The chemical versatility and structurally sturdy features of Candida antarctica lipase B has placed this enzyme as a common utensil in the synthetic tool-box. In addition to catalyzing acyl transfer reactions, a number of promiscuous activities have been described recently. Some of these new enzyme activities have been amplified by mutagenesis. Epoxide hydrolases are of interest due to their potential as catalysts in asymmetric synthesis. This current update discusses recent development in the engineering of lipases and epoxide hydrolases aiming to generate new biocatalysts with refined features as compared to the wild-type enzymes. Reported progress in improvements in reaction atom economy from dynamic kinetic resolution or enantioconvergence is also included.

Circular permutation of E. coli EPSP synthase: increased inhibitor resistance, improved catalytic activity, and an indicator for protein fragment complementation

We performed the first circular permutation analysis for E. coli 5-enolpyruvylshikimate-3-phosphate synthase, and identified one circular permutant with notably increased resistance to its specific inhibitor and several others with moderately improved catalytic activity. Valid circular permutation sites can be used as effective split sites of protein fragment complementation.

de novo design and synthesis of Candida antarctica lipase B gene and α-factor leads to high-level expression in Pichia pastoris

Candida antarctica lipase B (CALB) is one of the most widely used and studied enzymes in the world. In order to achieve the high-level expression of CALB in Pichia, we optimized the codons of CALB gene and α-factor by using a de novo design and synthesis strategy. Through comparative analysis of a series of recombinants with different expression components, we found that the methanol-inducible expression recombinant carrying the codon-optimized α-factor and mature CALB gene (pPIC9KαM-CalBM) has the highest lipase production capacity. After fermentation parameters optimization, the lipase activity and protein content of the recombinant pPIC9KαM-CalBM reached 6,100 U/mL and 3.0 g/L, respectively, in a 5-L fermentor. We believe this strategy could be of special interest due to its capacity to improve the expression level of target gene, and the Pichia transformants carrying the codon-optimized gene had great potential for the industrial-scale production of CALB lipase.

Circular permutation in the Ω-loop of TEM-1 β-lactamase results in improved activity and altered substrate specificity

Generating diverse protein libraries that contain improved variants at a sufficiently high frequency is critical for improving the properties of proteins using directed evolution. Many studies have illustrated how random mutagenesis, cassette mutagenesis, DNA shuffling and similar approaches are effective diversity generating methods for directed evolution. Very few studies have explored random circular permutation, the intramolecular relocation of the N- and C-termini of a protein, as a diversity-generating step for directed evolution. We subjected a library of random circular permutations of TEM-1 β-lactamase to selections on increasing concentrations of a variety of β-lactam antibiotics including cefotaxime. We identified two circularly permuted variants that conferred elevated resistance to cefotaxime but decreased resistance to other antibiotics. These variants were circularly permuted in the Ω-loop proximal to the active site. Remarkably, one variant was circularly permuted such that the key catalytic residue Glu166 was located at the N-terminus of the mature protein.

Protein engineering of microbial enzymes

Protein engineering has emerged as an important tool to overcome the limitations of natural enzymes as biocatalysts. Recent advances have mainly focused on applying directed evolution to enzymes, especially important for organic synthesis, such as monooxygenases, ketoreductases, lipases or aldolases in order to improve their activity, enantioselectivity, and stability. The combination of directed evolution and rational protein design using computational tools is becoming increasingly important in order to explore enzyme sequence-space and to create improved or novel enzymes. These developments should allow to further expand the application of microbial enzymes in industry.