Radicals offer the possibility of unique and efficient chemical transformations, if properly controlled. Nature can provide this control in the form of enzymes, and this review provides an overview of the types of enzymes characterized so far, their current applications in biotechnology, and future directions to better exploit these fascinating catalysts.

Nowadays, identification of the active-site motif in a new enzyme is highly possible through sequence alignment and comparison. Although the active site is usually secured in a well-formed pocket, it is also accessible to externally added reagents. By mutating an active-site residue into cysteine, chances of obtaining useful Hg-derivatives for crystal structure determination are greatly increased.

Severe acute respiratory syndrome (SARS)-associated coronavirus (CoV) has disappeared, but might re-emerge in the future, together with other new and existing CoV. The 3CL protease of SARS-CoV, a key enzyme for viral replication, is a promising drug target for inhibitor molecules in order to combat CoVs. This review summarizes current developments and research on inhibitors against proteases and the engineering of the mutant protease as an endopeptidase.

As a highly sought carotenoid with wide applications, the growing market demand for bio-derived astaxanthin, and the insufficient efficiency of astaxanthin biosynthesis drive the advances in metabolic engineering of native astaxanthin producers and heterologous hosts. Recent research progresses in metabolic engineering of astaxanthin biosynthesis by yeasts are reviewed and possible future strategies proposed.

The extremophilic xylanases are the key enzymes in the degradation of the polysaccharide xylan and have been applied as bio-catalysts in a wide variety of hemicellulose- and cellulose-based industries. Here, the latest developments of alkaline-stable and heat-stable xylanases are discussed in order to better understand their catalytic mechanisms and provide an engineering basis.

The use of wild type proteins under industrial conditions has limitations, low thermostability in particular. This review summarizes recent advances in the factors influencing protein thermostability, highlights its effective improvement strategies, and discusses future prospects in this field.

The utilization of phytase has provided great beneficial effects by increasing the nutritional values of animal feeds and reducing the supplementation of inorganic phosphorus. To gain insight into the molecular mechanism of phytases through three-dimensional protein structures is an intense area of research. In this review, the up-to-date status of phytase structural and engineering studies is summarized.

Enzymes in detergents, optimized by genetic engineering, remove natural stains by forming complexes with the substrates (stains) and hydrolyze them. Industrial production of the enzymes takes place in fermenters 40 to 125 m³ in size. Products are coated granules and liquids. The challenges of safe manufacture of large quantities of enzymes are explained.

Tertiary alcohols are an important functionality in many active pharmaceuticals and fine chemicals. Biotechnological processes are therefore required to synthesize the alcohols in high yields and enantiomeric purity; however, generally applicable methods are lacking so far. Already identified enzymatic methods are presented and their advantages and disadvantages are discussed with a view to possible applications.

Efficient cellulose degradation is essential for the production of biofuels from renewable sources. This review discusses the results from recent years in the field of cellulase engineering, highlighting both achievements and limitations. The focus is on designer cellulosomes and strategies to increase the stability and activity of cellulases.