Biocatalysts for the green era

BIOCATALYSTS CAN trigger a whole lot of beneficial effects. Being natural substances, they have their place in current times, when green is the way to go. Engineering researchers from NUS recently made news in the scientific world for their work on biocatalysts - catalysts which are made from natural proteins as opposed to metal catalysts. Their findings were published in Chem. Commun. 2009, 1481-1483. They have demonstrated for the first time, how two different biocatalysts can work together simultaneously under the same set of conditions to effect two separate transformations. Previously, this would have involved two separate processes.

Using this method "tandem biocatalysts in a two-liquid phase system" the team at NUS, led by Assoc Prof Li Zhi, Department of Chemical and Biomolecular Engineering, has managed to achieve high enantioselectivity and yield by selecting the appropriate biocatalysts from different micro-organisms for the chemical processes in their study. The tandem biocatalysts system does away with the need for separation and purification of the intermediary products produced during stages of the chemical reaction. It also allows the circumvention of negative effects that may arise from the accumulation of intermediary products. By removing such products quickly from the aqueous phase through subsequent hydrolysis, any inhibiting effects -- either by toxicity or enzyme inhibition -- are reduced.

This system has been applied for the synthesis of enantiopure pharmaceutical intermediates. As biocatalysts are non-toxic compared to metal catalysts, they are ideal for drug preparation. They are also less expensive and do not contaminate nor pollute the environment. In the preparation of chiral drugs, catalysis is needed to "purify" the chirals which are compounds with both "left-handed" and "right-handed" molecules known as "enantiomers". Though they are mirror images of each other, only one of these enantiomers provides the therapeutic effect. The other "hand" may be inactive or worse, produce toxic effects.

Thalidomide was an example of such a drug. In the 1960s, mothers taking the drug had led to deformed babies. The left-handed enantiomer has sedative effect while the right-handed one induces foetal malformation. The 2001 Nobel Prize in Chemistry was given to the group who championed "asymmetric catalysis" - finding the right catalyst to bring about reactions that would synthesise only the desired enantiomer without its "trouble-making" mirror image. Today, more than 50 per cent of top-selling drugs are in a single enantiomeric form and scientists are still trying to find the best possible way to obtain this.

Novel co-factor recycling system for efficient bioreduction

In collaboration with the Singapore-MIT Alliance, Assoc Prof Li has also developed another efficient method using coupled permeabilised micro-organisms -- one containing an NADPH-dependent keto-reductase and the other a recombinant glucose dehydrogenase -- to produce a pharmaceutical intermediate in a bioreduction reaction. The expensive NADPH required in stoichiometrical amount for most bioreductions and biooxidations as a co-factor was successfully recycled 4,200 times. Their findings were published ina paper in Appl. Environ. Microbiol 2009, 75:687-694.

In permeabilised cell-based approach, the cell membranes are perforated so that exposure of the cell to the substrate would also expose the enzymes. This is a simpler process than isolating enzymes from mico-organisims.

"Our permeabilised cell-based approach shows several advantages over other systems. Compared with whole cells, this enables use of externally added co-factor for efficient catalysis and co-factor recycling. It allows for easy substrate access and product release. Compared with coupling isolated enzymes, permeabilised cells are cheap, easily available in large quantities, remain active longer, stable and reuseable," said Assoc Prof Li.

This method will come in useful in the production of fine chemicals and possibly biofuels. The glucose in sugar cane for example, can be converted to ethanol through microbial reaction. Ethanol is a biofuel alternative for gasoline, and is widely used by lightweight vehicles in Brazil.

Biodegradable plastics as implants

The team is also conducting research on smart materials which have the ability to change shape and then revert to its original shape when triggered by temperature. They published their findings in Macromolecules, 2009, 42, 964-972.

"Such novel materials have biomedical applications. For example, they can be used for medical devices which need to be inserted into the patient's body. A bulky implant can be deformed and inserted into the body through minimally invasive surgery as only a small incision is needed. When inserted, the implant can expand to the desired size and shape, triggered by the body's temperature. Sutures made from such smart materials can also allow optimised tightening of the knot, when catalysed," said Assoc Prof Li.

"Normally biodegradable polymers do not have such behaviour. But we have succeeded in the chemo-enzymatic preparation of novel three-armed co-polyesterurethane with shape-memory effect at body temperature as well as excellent thermoplastic and mechanical properties," said Assoc Prof Li.

The team plans to extend their ressearch in this direction by exploring polymer properties and suitable enzymes to prepare new type of polymers as biocompatible materials and the application of these materials for manufacturing special medical divices and drug delivery.

Assoc Prof Li Zhi (right) and his PhD student Liang Xue(left) have succeeded in the preparation of a novel three-armed co-polyesterurethane with shape-memory effect at body temperature, as well as excellent thermoplastic and mechanical properties.