January 28, 2004
Findings could aid efforts to harness nature for making drugs
WEST LAFAYETTE, Ind. Chemical engineers at Purdue University have shown how to make yeast cells double the activity and boost productivity of a type of enzyme plants need to create important chemicals such as anticancer compounds.
The work is related to efforts aimed at developing techniques to use plants and microorganisms as natural factories for producing pharmaceuticals. Such techniques would be safer and more environmentally friendly than conventional methods for making drugs, which often require hazardous chemicals and steel "reactors" operated at high pressures and temperatures. The enzymes from plants and other organisms typically function in water near room temperature under ordinary pressure.
The Purdue researchers demonstrated that altering the nutrients and carefully controlling fermentation time caused yeast cultures to produce an enzyme called ferulate 5-hydroxylase that has twice its normal rate of activity, which increases the enzyme's productivity.
"Activity relates to the amount of product that can be synthesized in a given time," said John Morgan, an assistant professor of chemical engineering at Purdue. "So we could make more than twice the amount of product per hour."
Findings are detailed in a paper appearing in the Jan. 20 issue of the journal Biotechnology and Bioengineering, published by John Wiley & Sons Inc. The paper was written by Morgan and Purdue doctoral student Hanxiao Jiang.
The enzyme is a member of a family of enzymes called cytochrome P450, which plants need to produce numerous chemical compounds.
Plants ordinarily produce small quantities of "flavonoids," which are beneficial chemicals known as antioxidants. So researchers are developing ways to boost production of the chemicals by transferring vital enzymes from plants to microorganisms. Because P450 enzymes are "biocatalysts" that enable an organism to produce the beneficial drugs, researchers are trying to develop techniques that cause plants to make greater quantities of the enzymes and enzymes that are more productive.
The method pursued by the Purdue researchers was to focus on a gene responsible for producing ferulate 5-hydroxylase.
Altering the composition of nutrients fed to the yeast cultures and controlling the fermentation time caused the gene to be "expressed," producing 45 percent more of the enzyme while doubling the enzyme's activity.
Increasing the quantity and activity of various cytochrome P450 enzymes might enable scientists to use plants and microorganisms like E. coli and baker's yeast to one day commercially produce pharmaceuticals. More progress is needed, however, before it will be practical to use plants and plant enzymes in microorganisms as natural pharmaceutical factories, Morgan said.
"I wouldn't consider this a major breakthrough, but it does represent significant progress in improving the expression of the enzyme," he said. "I think there is certainly room for greater expression of these P450 enzymes."
The same technique could be used to increase the production of other P450 enzymes, Morgan said.
"The plant kingdom contains a large and relatively untapped diversity of P450s that are needed to create thousands of valuable natural products," he said.
In ongoing work, the Purdue researchers also are trying to develop methods for coaxing the enzymes to make drugs not normally produced by plants.
"We are feeding them what's known as substrate analogs, or compounds that are structurally similar to the compound that this enzyme will normally recognize and react with but are somewhat structurally different," Morgan said. "Therefore, if the enzyme recognizes this compound, it will produce a novel product, or a product that's never been synthesized before.
"From a scientific standpoint, we want to better understand precisely how organisms make certain compounds, and from an engineering standpoint, we want to devise a strategy for manipulating the organism so that it makes the chemicals we want it to make."
Writer: Emil Venere, (765) 494-4709, firstname.lastname@example.org
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Optimization of an In Vitro Plant P450 Monooxygenase System in Saccharomyces cerevisiae
Cytochrome P450s are heme-thiolate oxygenases involved in a wide number of reactions such as epoxidation, hydroxylation, and demethylation. Heterologously expressed eukaryotic P450s are potentially useful biocatalysts for stereospecific oxygenation reactions under mild conditions. Numerous factors, such as intracellular pH, cytochrome P450, cytochrome P450 reductase, NADPH, and oxygen concentration all influence the in vivo activity. To systematically examine these factors, we selected ferulate 5-hydroxylase (F5H), a plant P450, with the Saccharomyces cerevisiae WAT11 strain as an expression host. Two media compositions and two cultivation procedures were investigated to optimize the in vivo activity of F5H. We modified a previously published selective growth medium (Pompon et al.  Methods Enzymol 272: 51-64) that increased the specific growth rate and cell yield of the host strain. A cultivation procedure with separate growth and induction stages that each contain selective media resulted in a 45 percent increase of whole cell F5H specific activity. In a medium designed for simultaneous growth and induction, we observed a 2.6-fold higher specific F5H activity, but substantially lower cell yield. Surprisingly, in this medium the higher specific F5H activity did not correlate with a higher P450 concentration. The effects of addition of the first committed heme precursor, _amino-levulinic acid, and FE (III) at the beginning of induction period were also studied for our two-stage procedure. A small, but significant (P < 0.05) increase in whole cell F5H activity was observed following ALA addition.