A Combinatorial Approach To Study Cytochrome P450 Enzymes for De Novo Production of Steviol Glucosides in Baker's Yeast.

Biosynthesis of steviol glycosides in planta proceeds via two cytochrome P450 enzymes (CYPs): kaurene oxidase (KO) and kaurenoic acid hydroxylase (KAH). KO and KAH function in succession with the support of a NADPH-dependent cytochrome P450 reductase (CPR) to convert kaurene to steviol. This work describes a platform for recombinant production of steviol glucosides (SGs) in Saccharomyces cerevisiae, demonstrating the full reconstituted pathway from the simple sugar glucose to the SG precursor steviol.

Mining Enzyme Diversity of Transcriptome Libraries through DNA Synthesis for Benzylisoquinoline Alkaloid Pathway Optimization in Yeast.

The ever-increasing quantity of data deposited to GenBank is a valuable resource for mining new enzyme activities. Falling costs of DNA synthesis enables metabolic engineers to take advantage of this resource for identifying superior or novel enzymes for pathway optimization. Previously, we reported synthesis of the benzylisoquinoline alkaloid dihydrosanguinarine in yeast from norlaudanosoline at a molar conversion of 1.

Microbial Factories for the Production of Benzylisoquinoline Alkaloids.

Benzylisoquinoline alkaloids (BIAs) are a family of ∼2500 alkaloids with both potential and realized pharmaceutical value, including most notably the opiates such as codeine and morphine. Only a few BIAs accumulate readily in plants, which limits the pharmaceutical potential of the family. Shifting BIA production to microbial sources could provide a scalable and flexible source of these compounds in the future.

Synthesis of Morphinan Alkaloids in Saccharomyces cerevisiae.

Morphinan alkaloids are the most powerful narcotic analgesics currently used to treat moderate to severe and chronic pain. The feasibility of morphinan synthesis in recombinant Saccharomyces cerevisiae starting from the precursor (R,S)-norlaudanosoline was investigated. Chiral analysis of the reticuline produced by the expression of opium poppy methyltransferases showed strict enantioselectivity for (S)-reticuline starting from (R,S)-norlaudanosoline.

Characterization of a flavoprotein oxidase from opium poppy catalyzing the final steps in sanguinarine and papaverine biosynthesis.

Benzylisoquinoline alkaloids are a diverse class of plant specialized metabolites that includes the analgesic morphine, the antimicrobials sanguinarine and berberine, and the vasodilator papaverine. The two-electron oxidation of dihydrosanguinarine catalyzed by dihydrobenzophenanthridine oxidase (DBOX) is the final step in sanguinarine biosynthesis. The formation of the fully conjugated ring system in sanguinarine is similar to the four-electron oxidations of (S)-canadine to berberine and (S)-tetrahydropapaverine to papaverine.

Selectively weakened binding of methotrexate by human dihydrofolate reductase allows rapid ex vivo selection of mammalian cells.

Ex vivo selection of transduced hematopoietic stem cells (HSC) with drug-resistance genes offers the possibility to enrich transduced cells prior to engraftment, toward increased reconstitution in transplant recipients. We evaluated the potential of highly methotrexate (MTX)-resistant variants of human dihydrofolate reductase (hDHFR) for this application. Two subsets of hDHFR variants with reduced affinity for MTX that had been previously identified in a bacterial system were considered: those with substitutions at positions 31, 34, and/or 35, and those with substitutions at position 115.

Multiple conformers in active site of human dihydrofolate reductase F31R/Q35E double mutant suggest structural basis for methotrexate resistance.

Methotrexate is a slow, tight-binding, competitive inhibitor of human dihydrofolate reductase (hDHFR), an enzyme that provides key metabolites for nucleotide biosynthesis. In an effort to better characterize ligand binding in drug resistance, we have previously engineered hDHFR variant F31R/Q35E. This variant displays a >650-fold decrease in methotrexate affinity, while maintaining catalytic activity comparable to the native enzyme.

2-tier bacterial and in vitro selection of active and methotrexate-resistant variants of human dihydrofolate reductase.

We report a rapid and reliable 2-tier selection and screen for detection of activity as well as drug-resistance in mutated variants of a clinically-relevant drug-target enzyme. Human dihydrofolate reductase point-mutant libraries were subjected to a 1st-tier bacterial complementation assay, such that bacterial propagation served as an indicator of enzyme activity. Alternatively, when selection was performed in the presence of the inhibitor methotrexate (MTX), propagation indicated MTX resistance.

Increasing methotrexate resistance by combination of active-site mutations in human dihydrofolate reductase.

Methotrexate-resistant forms of human dihydrofolate reductase have the potential to protect healthy cells from the toxicity of methotrexate (MTX), to improve prognosis during cancer therapy. It has been shown that synergistic MTX-resistance can be obtained by combining two active-site mutations that independently confer weak MTX-resistance. In order to obtain more highly MTX-resistant human dihydrofolate reductase (hDHFR) variants for this application, we used a semi-rational approach to obtain combinatorial active-site mutants of hDHFR that are highly resistant towards MTX.

Exploitation of the alcohol dehydrogenase-acetone NADP-regeneration system for the enzymatic preparative-scale production of 12-ketochenodeoxycholic acid.

The performance of a new NADP-regeneration system, based on the use of alcohol dehydrogenase (ADH)-acetone, has been investigated for the regioselective oxidation of cholic acid (1) to 12-ketochenodeoxycholic acid (2). Enzymes stabilities and substrate and/or product inhibitory effects under defined synthetic reaction conditions have been evaluated. The optimized system, based on a 4% w/v solution of 1 in a reaction mixture containing 25% v/v acetone, allowed the preparative scale transformation of 1 into 2 with a 92% conversion.