Cheanyeh Cheng - Enzyme-Based Organic Synthesis

To encourage and propel the implementation of green chemistry, the Presidential Green Chemistry Challenge Award was established by the United States government in 1996. The following are several award winners that can be used as models in green chemistry by using enzyme and related microbe for production of chemicals [35].

The winner of year 2011: Production of Basic Chemicals from Renewable Feedstocks at Lower Cost

Genomatica has developed a microbe using sophisticated genetic engineering to make 1,4‐butanediol (BDO) (a high‐volume chemical building block used to make many common polymers, such as spandex) by fermenting sugars. When produced at commercial scale, Genomatica’s Bio‐BDO will be less expensive, require about 60% less energy, and produce 70% less carbon dioxide emissions than BDO made from natural gas.

Merck and Codexis have developed a second‐generation green synthesis of sitagliptin, the active ingredient in Januvia, a treatment for type 2 diabetes. This collaboration has led to an enzymatic process with transaminase that reduces waste, improves yield and safety, and eliminates the need for a metal catalyst. Early research suggests that the new biocatalysts will be useful in manufacturing other drugs as well.

Esters are an important class of ingredients in cosmetics and personal care products. Usually, they are manufactured by harsh chemical methods that use strong acids and potentially hazardous solvents; these methods also require a great deal of energy. Eastman’s new method uses immobilized enzymes to make esters, saving energy, and avoiding both strong acids and organic solvents. This method is so gentle that Eastman can use delicate, natural raw materials to make esters never before available.

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Enzymatic and microbial oxidations can be dated back to 2000 BCE with vinegar production that is based on the oxidation of ethanol by acetic acid bacteria. Enzymes involved in the biocatalyzed oxidations are dehydrogenases and oxidases. The more common coenzymes associated with dehydrogenases are nicotinamide adenine dinucleotide/nicotinamide adenine dinucleotide hydrogen (NAD +/NADH), nicotinamide adenine dinucleotide phosphate/nicotinamide adenine dinucleotide phosphate hydrogen (NADP +/NADPH), and flavin adenine dinucleotide/flavin adenine dinucleotide hydrogen (FAD/FADH 2), whereas oxidases are usually assisted by flavoproteins for transferring electrons to molecular oxygen. Dehydrogenases can be found in aerobic and anaerobic organisms or microorganisms; however, oxidases are not present in strictly anaerobic species. Nowadays, dehydrogenases and oxidases have been extensively used for selective oxidations and as an alternative synthetic strategy for conventional oxidations with the advantages of being environmentally friendly and highly chemo‐, regio‐, and stereoselective. Nevertheless, due to poor stability of the two enzymes at high substrate/product/organic solvent concentration and temperature, large‐scale bio‐oxidation processes were few. The functional groups involved in those bio‐oxidations include hydroxyls of primary and secondary alcohols, carbonyls of aldehydes, saturated C–C bonds, C–N bond of amino acids, amines, nitroalkanes, and thiols [1].

Since primary alcohols can be oxidized to aldehydes and further to carboxylic acids, which are versatile building blocks in organic synthesis, the selective oxidation of primary alcohols using enzymes to produce corresponding aldehydes is important for both fundamental and industrial research. Benzaldehyde, which is important and widely applied in cosmetic, flavor, and pharmaceutical industries [2, 3], is generally prepared by oxidation of toluene or hydrolysis of benzyl chloride. Due to environmental concern, scientific reports have shown that Gluconobacter oxydans can be used for the selective oxidation of benzyl alcohol to obtain benzaldehyde in an organic/aqueous biphasic system to substitute for these environmentally unfavorable processes [4, 5]. Optimization of the immobilization parameters for G. oxydans not only improves the bioconversion of the selective oxidative process but also increases the stability of immobilized cells so that the immobilized cells can be used repeatedly for 10 cycles and a 53.2% of the oxidative activity of the immobilized cells compared with that of free cells was retained [6].