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MICHIGAN STATEUNIVERSITY
College of Natural Science
Department of Chemistry

Kevin D. Walker

Photo of Kevin D. Walker

Research

Constructing Enzyme Cascade Reactions toward Bioactive and Commodity Compounds

(Research Description PDF)

A More Complete Research Description (click here)

We use interdisciplinary methods to evaluate enzyme catalysts from various sources, such as bacteria, plants, and yeast, with non-natural substrates. Our vision is to transform natural compounds or synthetically-derived chemicals into novel products. Transfer of the genes encoding native or bioengineered enzymes into a chassis organism can potentially make various bioactive or commodity molecules in vivo or in vitro.

Semibiocatalysis of Ethyl Dimethylpyrazines from Hydroxyketones made via Carboligation Chemistry Catalyzed by a Pyruvate Decarboxylase 

A yeast pyruvate decarboxylase assembled regioisomerically enriched mixtures of hydroxyketones that, when reacted with an alkyl diamine, a spontaneous coupling reaction produces an ethyl mixture of dimethylpyrazines enriched in one isomer over the other.  

EDMP Graphic

 

More on the Biocatalysis of Arylserines and Arylisoserines from Epoxides by an MIO α/β-Aminomutase and a Mild Amine-Group Donor

An MIO-dependent aminomutase from Taxus canadensis (TcPAM) was repurposed to transfer the amino group irreversibly from (2S)-styryl-α-alanine to exogenously supplied trans-3-phenylglycidate enantiomers. TcPAM converted (2S,3R)-3-phenylglycidate to (2S)-anti-phenylserine predominantly (89%) and (2R,3S)-3-phenylglycidate to (2R)-anti-phenylserine (88%) over their antipodes, with inversion of the configuration at Cα in each case.

Bifurcation path TcPAM with epoxides

Recycling CoA in the Biocatalysis of Surrogate Taxanes to Access New Generation Bioactive Compounds

 

Biocatalysis of Arylserines and Arylisoserines from Epoxides by an MIO α/β-Aminomutase and a Mild Amine-Group Donor (more here)

Epoxide Isomers Docked in TcPAM 

We diverged from a fixed program of using MIO-aminomutase/lyases to aminate cinnamic acids and describe a first account of employing an MIO-aminomutase (TcPAM) from Taxus plants to convert previously untested epoxy acid substrates to bifunctional hydroxy amino acids. Arylserines were made more abundantly over the arylisoserines when styrylalanine was used instead of ammonium salts, to selectively transaminate the MIO group of TcPAM. This mild amine-group delivery to the oxirane avoided nonenzymatic ring-opening of the epoxides.

Taxane analogues (docetaxel, paclitaxel, cabazitaxel, paclitaxel C, and tesetaxel) are used 1) for breast, ovarian, and prostate cancers, 2) to stem complications from stent implants in heart surgery, and 3) to work potentially as neuroprotectants against stroke. Current methods to make docetaxel still use an 11 to 12-step semisynthesis, which involves protecting group chemistry that compromises yields and reduces atom economy.

We use regioselective biocatalysts (Taxus Acyltransferases (AT) and Bacterial CoA Ligases) to bypass protecting group chemistry to make the drug docetaxel.

Streamlined 3-Step Biocatalysis of Docetaxel: An alternative to make docetaxel:

Coupling acyltransferases with CoA ligases (above) provides a Green source of docetaxel and its drug analogues.

Paclitaxel (Taxol) Pathway Aminomutase

A Taxus phenylalanine aminomutase (TcPAM) converts (2S)-α-phenylalanine ((2S)-α-Phe) to (3R)-β-Phe and lies on the paclitaxel (Taxol™) biosynthetic pathway in Taxus plants.

To understand how to use TcPAM chemistry to biocatalyze β-amino acids, it is necessary to understand the subtleties of its mechanism. 

The aminomutase forms a transient MIO-NH2 adduct with a finite lifetime. The lifetime of adduct was unknown for TcPAM or any of the several enzymes in this family until we used stopped-flow monitoring of product release to measure the exponential burst phase at presteady state.

Andrimid Pathway Aminomutase

TcPAM converts (2S)-α-Phe to (3R)-β-Phe, while PaPAM on the andrimid biosynthetic pathway converts the same substrate to (3S)-β- Phe. We used the TcPAM structure in complex with (E)-cinnamate, which functions as both a substrate and an intermediate, and the PaPAM structure to account for the distinct β-amino acid stereochemistries. TcPAM must rotate/flip the cinnamate skeleton 180° before exchange and PaPAM must hold the intermediate stationary before rebinding of the NH2/H pair to the cinnamate.

 

Phe455 (spheres) in PaPAM shown displacing the phenylpropanoate ligand (green), preventing a bidentate linkage (magenta) with Arg323. This trajectory may explain the different product stereochemistries of the two enzymes—TcPAM forms a bidentate complex with its substrate.

In vivo Biocatalysis of β-Amino Acids

Our goal is to use this mechanistic information to repurpose these aminomutases to produce value-added phenylpropanoids.

PaPAM biocatalyzed various phenyl- (and heteryl-) β-amino acids from their corresponding α-amino acids.

A new graduate student can embark on studies involving organic chemistry synthesis of novel surrogate substrates. Other areas of training include molecular cloning techniques, expression of various enzymes in E. coli, and assay development. Included are basic biochemical applications and molecular engineering approaches related to enzyme kinetics, enzyme purification and characterization, and various analytical techniques (such as NMR, GC/ MS, LC-MS(/MS), and X-ray crystallography).