Directed evolution has become a full-grown tool in molecular biology nowadays. The methods that are involved in creating a mutant library are extensive and can be divided into several categories according to their basic ideas. Furthermore, both screening and selection can be used to target the enzyme towards the desired direction. Nowadays, this technique is broadly used in two major applications: (industrial) biocatalysis and research. In the first field enzymes are engineered in order to produce suitable biocatalysts with high catalytic activity and stability in an industrial environment. In the latter area methods are established to quickly engineer new enzymes for every possible catalytic step, thereby creating a universal biotechnological toolbox. Furthermore, directed evolution can be used to try to understand the natural evolutionary processes. This review deals with new mutagenesis and recombination strategies published recently. A full overview of new methods for creating more specialised mutant libraries is given. The importance of selection in directed evolution strategies is being exemplified by some current successes including the β-lactam acylases.@en

β-Lactam acylases are crucial for the synthesis of semisynthetic cephalosporins and penicillins. Unfortunately, there are no cephalosporin acylases known that can efficiently hydrolyse the amino-adipic side chain of Cephalosporin C. In a previous directed evolution experiment, residue Asn266 of the glutaryl acylase from Pseudomonas SY-77 was identified as being important for substrate specificity. In order to explore the function of this residue in substrate specificity, we performed a complete mutational analysis of position 266. Codons for all amino acids were introduced in the gene, 16 proteins that could be functionally expressed in Escherichia coli were purified to homogeneity and their catalytic parameters were determined. The mutant enzymes displayed a broad spectrum of affinities and activities, pointing to the flexibility of the enzyme at this position. Mutants in which Asn266 was changed into Phe, Gln, Trp and Tyr displayed up to twofold better catalytic efficiency (k cat/Km)than the wild-type enzyme when adipyl-7- aminodesacetoxycephalosporanic acid (adipyl-7-ADCA) was used as substrate, due to a decreased Km. Only mutants SY-77N266H and SY-77 N266M showed an improvement of both catalytic parameters, resulting in 10- and 15-times higher catalytic efficiency with adipyl-7-ADCA, respectively. Remarkably, the catalytic activity (kcat) of SY-77 N266M when using adipyl-7-ADCA as substrate was as high as when glutaryl-7-aminocephalosporanic acid (glutaryl-7-ACA) was used, and approaches commercially interesting activity. SY-77N266Q, SY-77N266H and SY-77N266M mutants showed a modest improvement in hydrolysing Cephalosporin C. Since these mutants also have a good catalytic efficiency when adipyl-7-ADCA is used and are still active towards glutaryl-7-ACA, they can be regarded as broad substrate acylases. These results demonstrate that the combination of directed evolution for the identification of important positions, together with saturation mutagenesis for finding the optimal amino acid, is a very effective method for finding improved biocatalysts.@en

Residue Phe375 of cephalosporin acylase has been identified as one of the residues that is involved in substrate specificity. A complete mutational analysis was performed by substituting Phe375 with the 19 other amino acids and characterising all purified mutant enzymes. Several mutations cause a substrate specificity shift from the preferred substrate of the enzyme, glutaryl-7-ACA, towards the desired substrate, adipyl-7-ADCA. The catalytic efficiency (k cat/Km) of mutant SY-77F375C towards adipyl-7-ADCA was increased 6-fold with respect to the wild-type enzyme, due to a strong decrease of Km. The kcat of mutant SY-77 F375H towards adipyl-7-ADCA was increased 2.4-fold. The mutational effects point at two possible mechanisms by which residue 375 accommodates the long side chain of adipyl-7-ADCA, either by a widening of a hydrophobic ring-like structure that positions the aliphatic part of the side chain of the substrate, or by hydrogen bonding to the carboxylate head of the side chain.@en

Using directed evolution, we have selected an adipyl acylase enzyme that can be used for a one-step bioconversion of adipyl-7-aminodesacetoxycephalosporanic acid (adipyl-7-ADCA) to 7-ADCA, an important compound for the synthesis of semisynthetic cephalosporins. The starting point for the directed evolution was the glutaryl acylase from Pseudomonas SY-77. The gene fragment encoding the β-subunit was divided into five overlapping parts that were mutagenized separately using error-prone PCR. Mutants were selected in a leucine-deficient host using adipyl-leucine as the sole leucine source. In total, 24 out of 41 plate-selected mutants were found to have a significantly improved ratio of adipyl-7-ADCA versus glutaryl-7-ACA hydrolysis. Several mutations around the substrate-binding site were isolated, especially in two hot spot positions: residues Phe-375 and Asn-266. Five mutants were further characterized by determination of their Michaelis-Menten parameters. Strikingly, mutant SY-77N266H shows a nearly 10-fold improved catalytic efficiency (kcat/Km) on adipyl-7-ADCA, resulting from a 50% increase in kcat and a 6-fold decrease in Km, without decreasing the catalytic efficiency on glutaryl-7-ACA. In contrast, the improved adipyl/glutaryl activity ratio of mutant SY-77F375L mainly is a consequence of a decreased catalytic efficiency toward glutaryl-7-ACA. These results are discussed in the light of a structural model of SY-77 glutaryl acylase.@en