Read this article to learn about the types and techniques of bio-transformations and its product recovery.
Microorganisms possess the capability to enzymatically modify a wide range of organic compounds. Bio-transformations (bi-conversions or microbial transformations) broadly refer to the processes in which microorganisms convert organic compounds into structurally related products. In other words, biotransformation deals with microbial (enzymatic) conversion of a substrate into a product with limited number (one or a few) enzymatic reactions. This is in contrast to fermentation which involves a large number reaction (often complex in nature).
Although there are hundreds of bio-transformations known, only a selected few of them are useful for the synthesis of commercially important products. The significance of bioconversion reactions becomes obvious when the production of a particular compound is either difficult or costly by chemical methods. Further, bio-transformations are generally preferred to chemical reactions because of substrate specificity, stereo specificity and mixed reaction conditions (pH, temperature, and pressure).
The environmental pollution due to biotransformation is almost insignificant or negligible. In addition, it is easy to apply recombinant DNA technology to make desired improvements in bio-transformations. Another practical advantage of bio-transformations is that it is easy to scale-up the processes due to limited number of reactions.
Types of Biotransformation Reactions:
Many types of chemical reactions occur in bio-transformations. These include oxidation, reduction, hydrolysis, condensation, isomerization, formation of new C—C bonds, synthesis of chiral compounds and reversal of hydrolytic reactions. Among these, oxidation, isomerization and hydrolysis reactions are more commonly observed in bio-transformations. Many a times bio-transformations involve more than one type of reaction. A selected list of important biotransformation reactions along with the microorganisms involved is given in Table 22.1.
The conversion time required for biotransformation is related to the type of reaction, the substrate concentration and the microorganism used. In general, oxidation, hydrolysis and dehydration reactions are completed in a few hours.
Sources of Biocatalysts and Techniques for Biotransformation:
A wide variety of biological catalysts can be used for biotransformation reactions. These include growing cells, resting cells, killed cells, immobilized cells, cell-free extracts, enzymes and immobilized enzymes. The most important sources of biocatalysts and the procedures employed for biotransformation are briefly described.
The desired cells are cultivated in a suitable medium. As the growth of the cells occurs (6-24 hours), a concentrated substrate is added to the culture. Sometimes, addition of emulsifiers (Tween, organic solvents) is required to solubilize substrates and/or products e.g. steroid biotransformation. The substrate conversion to product can be monitored by spectroscopic or chromatographic techniques. Biotransformation can be terminated when the product formation is optimum.
The non-growing cells are preferred for biotransformation reactions for the following reasons.
a. Very high concentration of substrate can be used (with high substrate concentration, growing cells stop their growth).
b. Cells can be washed and used and thus there will be no contaminating substances.
c. Conversion efficiency of substrate to product is high.
d. Biotransformation can be optimized by creating suitable environmental conditions (pH, temperature etc.).
e. Product isolation and its recovery are easy.
Bio-transformations can be carried out continuously by employing immobilized cells. Further, the same cells can be used again and again. Several bioconversions with single or multistage reactions are in fact carried out by using immobilized cells e.g. commercial production of L- alanine and malic acid.
Cell-free enzyme systems in the form of immobilized enzymes are most commonly used in bio-transformations, due to the following advantages.
a. No occurrence of undesirable side reactions.
b. The desired products are not degraded.
c. There is no transport barrier across the cell membrane for the substrate or product.
d. The isolation and recovery of the product is simpler and easier.
Several immobilized enzyme systems have been developed for bio-transformations e.g. glucose isomerase, penicillin acylase.
Product Recovery in Bio-transformations:
In most biotransformation reactions, the desired end products are extracellular. The product may be either in a soluble or suspended state. When whole cells are used, they have to be separated and repeatedly washed (with water or organic solvent) as required. The extracted product can be recovered by employing the commonly used techniques-precipitation by salts, extraction with solvents, adsorption to ion-exchangers etc. The volatile products can be recovered by direct distillation from the medium.
Examples of Bio-transformations:
A large number of bio-transformations are described in literature. Of these, only a selected few are important for industrial and commercial purposes. The major limitations with some of the bio-transformations are that either yields are low or the process is expensive or the market itself is very limited.