**Abstract**

Microbial-catalyzed biotransformations have considerable potential for the generation of an enormous variety of structurally diversified organic compounds*,* especially natural products with complex structures like triterpenoids, flavonoids, steroids, steroidal saponins, and sesquiterpenoids. They offer efficient and economical ways to produce semisynthetic analogues and novel lead molecules. Microorganisms such as bacteria and fungi could catalyze *chemo*-, *regio*-, and *stereo*specific hydroxylations of diverse substrates that are extremely difficult to produce by chemical routes. During recent years, considerable research has been performed on the microbial transformation of bioactive compounds, in order to obtain biologically active molecules with diverse structural features. In green chemistry, biotransformations are an important chemical methodology toward more sustainable industrial processes.

**Keywords:** microorganisms, fungi, bacteria, microbial transformation, natural products, enzymes

### **1. Introduction**

Microbial transformation is regarded as an enzymatic reaction by using the metabolic activities of microorganisms to modify the chemical structures of bioactive substrates for finding the new chemical derivatives with the potent bioactivities and physical-chemical characteristics. It has a number of advantages over chemical synthesis such as higher *stereo*- and *regio*selectivity but is also enantiospecific, allowing the production of chiral products from racemic mixtures. The conditions for biotransformations are mild, and in the majority of cases, they do not require the protection of pre-existing functional groups. Furthermore, some reactions that do not occur when using chemical approaches are easily carried out by microbial transformation. Microbial factories show advantages, for instance, growing rapidly and ease of large-scale production [1–3].

The use of microorganisms may be a highly efficient method of production of these compounds. The reactions involved in biotransformation of organic compounds by whole cells of various microorganisms include oxidation, reduction, hydroxylation, esterification, methylation, demethylation, isomerization, hydrolysis, glycosylation, and hydrogenation [4, 5].

Biotransformation may be carried out with isolated enzyme systems or with intact organism. Although isolated enzyme systems may be more specific and

efficient for certain biotransformation, these reactions may involve isolating the enzyme system, and, for some classes of enzyme-catalyzed reaction, a recycling sequence may be required to regenerate the enzyme [6].

Fungi are playing a prominent role in the catalysis of organic compounds and in the production of commercially and industrially important compounds, because of their ability to catalyze novel reactions [7]. Fungi are commonly used in the industry for production of fermented beverages, foods, physiologically active substances, solvents, organic acids, polysaccharides, antibiotics, etc. Of the zygomycota, *Mucor* and *Rhizopus* are commonly used in the industry. *Rhizopus* strains are important in citric acid production. *Mucor* strains make a significant number of important lipases and catalyze the hydroxylation of a wide range of chemical compounds [2–4].

The use of the microbial model offers a number of advantages over the use of animals in metabolism studies, mainly: (1) simple, easy, and can be prepared at low cost; (2) screening for a large number of strains is a simple repetitive process; (3) the large number of metabolites formed allows easier detection, isolation, and structural identification; (4) newer metabolites can be isolated; (5) utilized for synthetic reactions involving many steps; (6) useful in cases where *regio*- and *stereo*specificity is required; (7) maintenance of stock cultures of microorganisms is simpler and cheaper than the maintenance of cell or tissue cultures or laboratory animals; (8) ease of setup and manipulation; and (9) more reliable and reproducible [8, 9].

The objective of this review is to highlight the importance of microorganisms or enzymes isolated from them in the biotransformation process of natural products or xenobiotic compounds, according to green chemistry or white biotechnology.
