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D-Amino acids exist widely in microbes, plants, animals, and food and can be applied in pharmaceutical, food, and cosmetics. Because of their widespread applications in industry, D-amino acids have recently received more and more attention. Enzymes including D-hydantoinase, N-acyl-D-amino acid amidohydrolase, D-amino acid amidase, D-aminopeptidase, D-peptidase, L-amino acid oxidase, D-amino acid aminotransferase, and D-amino acid dehydrogenase can be used for D-amino acids synthesis by kinetic resolution or asymmetric amination. In this review, the distribution, industrial applications, and enzymatic synthesis methods are summarized. And, among all the current enzymatic methods, D-amino acid dehydrogenase method not only produces D-amino acid by a one-step reaction but also takes environment and atom economics into consideration; therefore, it is deserved to be paid more attention.

meso-Diaminopimelate dehydrogenase (meso-DAPDH) is a good candidate for one-step synthesis of d-amino acid from 2-keto acids. Our previous research revealed the classification of meso-DAPDH family and showed that type II meso-DAPDH, such as the meso-DAPDH from Symbiobacterium thermophilum (StDAPDH), could catalyze reductive amination. In this article, seven residues of StDAPDH, which are highly conserved in each subfamily but are different between two subfamilies, were targeted to explore the relationships between structure and function. Determination of kinetic parameters showed that the amino acid residues, including P69, K159, V68, S90, V14, and V156, played very important roles in the catalytic function of StDAPDH. Molecular dynamics simulation revealed that these point mutations reduced the productive conformations by the newly formed or eliminated interactions between the residues and ligands. These results strengthen our understanding of the catalytic mechanism and evolution of meso-DAPDH and can aid future endeavors in enzyme engineering.

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chitin deacetylase (cDA) can hydrolyse the acetamido group of chitin polymers to produce chitosans, which are used in various fields including the biomedical and pharmaceutical industries, food production, agriculture, and water treatment. cDA represents a more environmentally-friendly and easier to control alternative to the chemical methods currently utilised to produce chitosans from chitin; however, the majority of identified CDAs display activity toward low-molecular-weight oligomers and are essentially inactive toward polymeric chitin or chitosans. therefore, it is important to identify novel cDAs with activity toward polymeric chitin and chitosans. in this study, we isolated the bacterium Rhodococcus equi F6 from a soil sample and showed that it expresses a novel CDA (ReCDA), whose activity toward 4-nitroacetanilide reached 19.20 U/mL/h during fermentation and was able to deacetylate polymeric chitin, colloidal chitin, glycol-chitin, and chitosan. Whole genome sequencing revealed that RecDA is unique to the R. equi F6 genome, while phylogenetic analysis indicated that RecDA is evolutionarily distant from other cDAs. in conclusion, RecDA isolated from the R. equi F6 strain expands the known repertoire of CDAs and could be used to deacetylate polymeric chitosans and chitin in industrial applications.Chitin is the second most abundant biopolymer after cellulose and is mainly obtained as a waste product of the seafood industry at a relatively low cost 1 . The chitin derivative, chitosan, is a linear polysaccharide comprised of β-(1→4)-linked glucosamine and N-acetyl glucosamine units which are randomly arranged within the chitosan polysaccharide chain 2 . Due to its ability to dissolve in dilute acids, chitosan is more useful than its crystalline precursor chitin in various industrial applications 3 , including the biomedical and pharmaceutical industries, food production, agriculture, and water treatment 4 .Chitosan occurs naturally and is mainly found in the cell walls of certain fungi, the exoskeletons of certain insects (such as the abdominal wall of termite queens), and in some yeasts 5,6 . Although chitosan can be extracted from fungal sources 7 , the method is commercially inapplicable as it provides too low a yield at too high a cost. Therefore, chitosan is still obtained by treating marine-derived chitin with thermo-alkaline 8 , a method that is inexpensive and results in high yields but is environmentally unsafe and difficult to control, producing a heterogeneous range of products depending on the degree of deacetylation 9 . The enzymatic production of chitosan using microbial chitin deacetylase (CDA; EC 3.5.1.41), which can hydrolyse the acetamido group in chitin polymers to produce chitosans 10 , is an environmentally-friendly process that is easy to control and results in a high yield of homogeneous end products 11 .CDAs have been identified in marine and soil bacteria, several fungi, a few insects, and at least one virus 12,13 . Fungal CDAs exist mostly as N-glycosylated (20-70%) glycoproteins, w...

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Divergent evolution exists in the meso-diaminopimelate dehydrogenase (meso-DAPDH) family. Taking meso-DAPDH from Corynebacterium glutamicium ATCC13032 (CgDAPDH) and Symbiobacterium thermophilum IAM14863 (StDAPDH) as examples, the two subtypes differ in terms of their amination ability toward 2-keto acids. The significant structural difference between these two subtypes is an indel loop containing 16 amino acid residues, which exists in CgDAPDH. However, the function of the indel loop has still not been reported to date. Herein, mutagenesis, including loop insertion, loop deletion, and alanine replacement, and molecular dynamics were employed to investigate the function of the loop in CgDAPDH and the evolution of meso-DAPDH. We unveiled the pivotal role of the indel loop during the catalytic cycle of CgDAPDH, especially the R180 residue. The hydrogen-bond interactions formed between R180 and E262 maintain the correct conformation of the catalytic cavity and the catalytic H152 residue. In addition, the indel loop forms a closed hydrogen-bond network with the upstream L176 and downstream H193 residues, which stabilizes the hydrogen-bond network formed by the amino acid side chains throughout the entire protein. Our results demonstrate that the indel loop is not the driving force for the divergent evolution of meso-DAPDH. However, the indel loop affects the release pathway for meso-DAP and pyruvic acid and, more significantly, the release of meso-DAP. Overall, we have identified the importance of the indel loop in CgDAPDH and have excluded its role in the divergent evolution of the meso-DAPDH family.