Sahng Ha Lee

Synergic effects of nanofiber alignment and electroactivity on myoblast differentiation

The interactions between cells and materials play critical roles in the success of new scaffolds for tissue engineering, since chemical and physical properties of biomaterials regulate cell adhesion, proliferation, migration, and differentiation. We have developed nanofibrous substrates that possess both topographical cues and electroactivity. The nanofiber scaffolds were fabricated through the electrospinning of polycaprolactone (PCL, a biodegradable polymer) and polyaniline (PANi, a conducting polymer) blends. We investigated the ways in which those properties influenced myoblast behaviors. Neither nanofiber alignment nor PANi concentration influenced cell growth and proliferation, but cell morphology changed significantly from multipolar to bipolar with the anisotropy of nanofibers. According to our analyses of myosin heavy chain expression, multinucleate myotube formation, and the expression of differentiation-specific genes (myogenin, troponin T, MHC), the differentiation of myoblasts on PCL/PANi nanofibers was strongly dependent on both nanofiber alignment and PANi concentration. Our results suggest that topographical cues and the electroactivity of nanofibers synergistically stimulate muscle cell differentiation to make PCL/PANi nanofibers a suitable scaffold material for skeletal tissue engineering.

Rational Design and Engineering of Quantum‐Dot‐Sensitized TiO2 Nanotube Arrays for Artificial Photosynthesis

Redox enzymes can catalyze complex synthesis reactions under mild conditions but conventional catalysts rarely accomplish this task. Despite the high potential of redox enzymes for the synthesis of valuable compounds (e.g., chiral alcohols and drug intermediates), [ 1–5 ] their application is hampered by the high cost of enzyme-specifi c cofactors that are required as a redox equivalent, such as nicotinamide adenine dinucleotide (NAD(P) H) and fl avin adenine dinucleotide (FADH). Thus, numerous efforts have been made over the past decades to accomplish in situ cofactor regeneration from their oxidized counterpart. [ 6–9 ]

Coupling Photocatalysis and Redox Biocatalysis Toward Biocatalyzed Artificial Photosynthesis

In green plants, solar-energy utilization is accomplished through a cascade of photoinduced electron transfer, which remains a target model for realizing artificial photosynthesis. We introduce the concept of biocatalyzed artificial photosynthesis through coupling redox biocatalysis with photocatalysis to mimic natural photosynthesis based on visible-light-driven regeneration of enzyme cofactors. Key design principles for reaction components, such as electron donors, photosensitizers, and electron mediators, are described for artificial photosynthesis involving biocatalytic assemblies. Recent research outcomes that serve as a proof of the concept are summarized and current issues are discussed to provide a future perspective.

Eosin Y-sensitized artificial photosynthesis by highly efficient visible-light-driven regeneration of nicotinamide cofactor.

Dye‐sensitized photosynthesis: Eosin Y (EY), a dye photosensitizer, works efficiently as a molecular photoelectrode by catalyzing the visible‐light‐driven electron‐transfer reaction for efficient regeneration of NADH through a photosensitizer–electron relay dyad. Injection of the photosensitized electron resulted in highly accelerated regeneration of NADH, which can be used by glutamate dehydrogenase for the photosynthesis of L‐glutamate.

Electrochemical Regeneration of NADH Enhanced by Platinum Nanoparticles

This work was supported by the Korea Energy Management Corporation (2005-C-CD11-P-04) and the Korea Research Foundation (KRF-2006-331-D00113).

Cofactor-Free Light-Driven Whole-Cell Cytochrome P450 Catalysis†

Cytochromes P450 can catalyze various regioselective and stereospecific oxidation reactions of non-functionalized hydrocarbons. Here, we have designed a novel light-driven platform for cofactor-free, whole-cell P450 photo-biocatalysis using eosin Y (EY) as a photosensitizer. EY can easily enter into the cytoplasm of Escherichia coli and bind specifically to the heme domain of P450. The catalytic turnover of P450 was mediated through the direct transfer of photoinduced electrons from the photosensitized EY to the P450 heme domain under visible light illumination. The photoactivation of the P450 catalytic cycle in the absence of cofactors and redox partners is successfully conducted using many bacterial P450s (variants of P450 BM3) and human P450s (CYPs 1A1, 1A2, 1B1, 2A6, 2E1, and 3A4) for the bioconversion of different substrates, including marketed drugs (simvastatin, lovastatin, and omeprazole) and a steroid (17β-estradiol), to demonstrate the general applicability of the light-driven, cofactor-free system.

Solar energy in production of L-glutamate through visible light active photocatalyst-redox enzyme coupled bioreactor

A new potentially promising visible-light driven photobioreactor synthesizes fine chemical via photobiocatalysis by generating NADH in a non-enzymatic light-driven process and coupling it to the enzymatic dark reaction catalysis.

Cytochrome P450-Catalyzed O-Dealkylation Coupled With Photochemical NADPH Regeneration

Cytochrome P450 monooxygenases are multifunctional enzymes with potential applications in chemoenzymatic synthesis of complex chemicals as well as in studies of metabolism and xenobiotics. Widespread application of cytochrome P450s, however, is encumbered by the critical need for redox equivalents in their catalytic function. To overcome this limitation, we studied visible light‐driven regeneration of NADPH for P450‐catalyzed O‐dealkylation reaction; we used eosin Y as a photosensitizing dye, triethanolamine as an electron donor, and [Cp*Rh(bpy)H2O] as an electron mediator. We analyzed catalytic activity of cell‐free synthesized P450 BM3 monooxygenase variant (Y51F/F87A, BM3m2) in the presence of key components for NADPH photoregeneration. The P450‐catalyzed O‐dealkylation reaction sustainably maintained its turnover with the continuous supply of photoregenerated NADPH. Visible light‐driven, non‐enzymatic NADPH regeneration provides a new route for efficient, sustainable utilization of P450 monooxygenases. Biotechnol. Bioeng. 2013; 110: 383–390.

Biocatalytic Photosynthesis with Water as an Electron Donor

Efficient harvesting of unlimited solar energy and its conversion into valuable chemicals is one of the ultimate goals of scientists. With the ever-increasing concerns about sustainable growth and environmental issues, numerous efforts have been made to develop artificial photosynthetic process for the production of fuels and fine chemicals, thus mimicking natural photosynthesis. Despite the research progress made over the decades, the technology is still in its infancy because of the difficulties in kinetic coupling of whole photocatalytic cycles. Herein, we report a new type of artificial photosynthesis system that can avoid such problems by integrally coupling biocatalytic redox reactions with photocatalytic water splitting. We found that photocatalytic water splitting can be efficiently coupled with biocatalytic redox reactions by using tetracobalt polyoxometalate and Rh-based organometallic compound as hole and electron scavengers, respectively, for photoexcited [Ru(bpy)3](2+). Based on these results, we could successfully photosynthesize a model chiral compound (L-glutamate) using a model redox enzyme (glutamate dehydrogenase) upon in situ photoregeneration of cofactors.

Cofactor-Free, Direct Photoactivation of Enoate Reductases for the Asymmetric Reduction of C=C Bonds

Abstract Enoate reductases from the family of old yellow enzymes (OYEs) can catalyze stereoselective trans‐hydrogenation of activated C=C bonds. Their application is limited by the necessity for a continuous supply of redox equivalents such as nicotinamide cofactors [NAD(P)H]. Visible light‐driven activation of OYEs through NAD(P)H‐free, direct transfer of photoexcited electrons from xanthene dyes to the prosthetic flavin moiety is reported. Spectroscopic and electrochemical analyses verified spontaneous association of rose bengal and its derivatives with OYEs. Illumination of a white light‐emitting‐diode triggered photoreduction of OYEs by xanthene dyes, which facilitated the enantioselective reduction of C=C bonds in the absence of NADH. The photoenzymatic conversion of 2‐methylcyclohexenone resulted in enantiopure (ee>99 %) (R)‐2‐methylcyclohexanone with conversion yields as high as 80–90 %. The turnover frequency was significantly affected by the substitution of halogen atoms in xanthene dyes.