Peijun Ji

Specific immobilization of D-amino acid oxidase on hematin-functionalized support mimicking multi-enzyme catalysis

D-Amino acid oxidase (DAAO) catalyzes oxidative deamination of D-amino acids to yield corresponding α-keto acids, producing hydrogen peroxide (H2O2). D-Amino acid oxidase was genetically modified by fusion to an elastin-like polypeptide (ELP). For enzyme immobilization, multi-walled carbon nanotubes (MWCNTs) were adopted as the model support. MWCNTs were functionalized with hematin. ELP-DAAO was immobilized on the functionalized CNTs by coupling to the hematin. The specific immobilization enabled ELP-DAAO in proximity to the hematin at a molecular distance. The molecular-distance proximity facilitated the immediate decomposition of H2O2 catalyzed by the hematin. The evolved oxygen was efficiently utilized to oxidize the reduced cofactor FDA of DAAO, and H2O2 was produced. The forming of a H2O2 → O2 → H2O2 circle between the DAAO and hematin has been demonstrated to be the driving force to accelerate the deamination reaction. The enzyme kinetics has shown that the ELP-DAAO/hematin-CNT conjugate exhibited a catalytic efficiency more than three times that of free ELP-DAAO, demonstrating its ability in mimicking multi-enzyme catalysis. The methodology for highly specific enzyme immobilization is not restricted to carbon nanotubes, and can be extended easily to other micro and nanomaterials as supports for specific immobilization of oxidases.

Molecular mechanism of the interactions between inhibitory tripeptides and angiotensin-converting enzyme.

Angiotensin I-converting enzyme (ACE) is a key therapeutic target for combating hypertension and related cardiovascular diseases. ACE inhibitory peptides offer the prospect of enhanced potency, high specificity, and no or low side effect. The ACE inhibitory tripeptides LKP and IKP differ from each other by one amino acid but their inhibitory potencies for ACE differ significantly. To uncover the molecular mechanism underlying this phenomenon, we have investigated the tripeptide/ACE complexes through molecular dynamics simulations coupled with quantum mechanical simulations. Comparative structural analysis has identified a hydrophobic subsite in the active site of cACE comprising hydrophobic residues Val379, Val380, Phe457, Phe527, and Ala418. The interactions of the side chains of Leu and Ile with the hydrophobic residues determine the binding positions of N-terminal residues of the tripeptides, that influence the interaction of the residues of tripeptides with the active site of cACE. This work presents the molecular mechanism of the interactions between the inhibitory tripeptides and ACE, and deciphers the structural basis for the high affinity LKP inhibition of ACE.

Sodium hexadecyl sulfate as an interfacial substance adjusting the adsorption of a protein on carbon nanotubes.

Carbon nanotubes (CNTs) were functionalized with sodium hexadecyl sulfate (SHS). The lysozyme adsorbed on the SHS-CNTs exhibited a higher activity than that immobilized on the nonfunctionalized CNTs. To explain the experimental results and explore the mechanism of lysozyme adsorption, large-scale molecular dynamics simulations have been performed for a four-component system, including lysozyme, SHS, CNTs in explicit water. It has been found that the assembled SHS molecules form a soft layer on the surface of CNTs. The interactions between lysozyme and SHS induce the rearrangement of SHS molecules, forming a saddle-like structure on the CNT surface. The saddle-like structure fits the shape of the lysozyme, and the active-site cleft of the lysozyme is exposed to the water phase. Whereas, for the lysozyme adsorbed on the nonfunctionalized CNT, due to the hydrophobic interactions, the active-site cleft of the enzyme tends to face the wall of the CNT. The results of this work demonstrate that the SHS molecules as the interfacial substance have a function of adjusting the lysozyme with an appropriate orientation, which is favorable for the lysozyme having a higher activity.

A two‐enzyme immobilization approach using carbon nanotubes/silica as support

Multiple enzyme mixtures are attractive for the production of many compounds at an industrial level. We report a practical and novel approach for coimmobilization of two enzymes. The system consists of a silica microsphere core coated with two layers of individually immobilized enzymes. The model enzymes α‐amylase (AA) and glucoamylase (GluA) were individually immobilized on carbon nanotubes (CNTs). A CNT‐GluA layer was formed by adsorbing CNT‐GluA onto silica microsphere. A sol‐gel layer with entrapped CNT‐AA was then formed outside the CNT‐GluA/silica microsphere conjugate. The coimmobilized α‐amylase and glucoamylase exhibited 95.1% of the activity of the mixture of free α‐amylase and glucoamylase. The consecutive use exhibited a good stability of the coimmobilized enzymes. The developed approach demonstrates advantages, including controlling the ratio of coimmobilized enzymes in an easy way, facilitating diffusion of small molecules in and out of the matrix, and preventing the leaching of enzymes.

Immobilization of R-ω-transaminase on MnO2 nanorods for catalyzing the conversion of (R)-1-phenylethylamine

R-ɷ-transaminases transfer an amino group from an amino donor (e.g. (R)-1-phenylethylamine) onto an amino acceptor (e.g. pyruvate), resulting a co-product (e.g. d-alanine). This work intends to immobilize R-ɷ-Transaminase on MnO2 nanorods to achieve multienzyme catalysis. R-ɷ-Transaminase (RTA) and d-amino acid oxidase (DAAO) have been fused to an elastin-like polypeptide (ELP) separately through genetic engineering of the enzymes. ELP-RTA and ELP-DAAO have been separately immobilized on polydopamine-coated MnO2 nanorods. When the two immobilized enzymes were used together in one pot, the transformation of (R)-1-phenylethylamine was catalyzed by the immobilized ELP-RTA, and the co-product d-alanine was converted back to pyruvate under the catalysis of the immobilized ELP-DAAO, achieving the recycling of pyruvate in situ. Thus pyruvate was maintained at a low concentration in order to reduce its negative effect. On the other hand, the generated H2O2 of ELP-DAAO was decomposed by the MnO2 nanorods, and the evolved oxygen oxidized the reduced cofactors of ELP-DAAO. Forming the circles of hydrogen peroxide→oxygen→hydrogen peroxide accelerated the deamination reaction. The highly efficient conversion of the co-product d-alanine back to pyruvate accelerated the forming of the pyruvate→d-alanine→pyruvate cycle between the two immobilized enzymes. The coordination of the pyruvate→d-alanine→pyruvate and hydrogen peroxide→oxygen→hydrogen peroxide cycles accelerated the transformation of (R)-1-phenylethylamine. As a result, As a result, the immobilized enzymes achieved a conversion of 98±1.8% in comparison to 69.6±1.2% by free enzymes.