meso -Diaminopimelate dehydrogenase ( meso -DAPDH) from Symbiobacterium thermophilum (StDAPDH) is the first member of the meso -DAPDH family known to catalyze the asymmetric reductive amination of 2-keto acids to produce d -amino acids. It is important to understand the catalytic mechanisms of StDAPDH and other enzymes in this family. In this study, based on an evolutionary analysis and examination of catalytic activity, the meso -DAPDH enzymes can be divided into two types. Type I showed highly preferable activity toward meso -diaminopimelate ( meso -DAP), and type II exhibited obviously reversible amination activity with a broad substrate spectrum. StDAPDH belongs to type II. A quaternary structure analysis revealed that insertions/deletions (indels) and a loss of quaternary structure resulted in divergence among members of the meso -DAPDH family. A structure alignment of StDAPDH with a representative of type I, the meso -DAPDH from Corynebacterium glutamicum (CgDAPDH), indicated that they had the same folding. Based on sequence and conservation analyses, two amino acid residues of StDAPDH, R35 and R71, were found to be highly conserved within type II while also distinct from each other between the subtypes. Site mutagenesis studies identified R71 as a substrate preference-related residue of StDAPDH, which may serve as an indicator of the amination preference of type II. These results deepen the present understanding of the meso -DAPDH family and provide a solid foundation for the discovery and engineering of meso -DAPDH for d -amino acid biosynthesis. IMPORTANCE The l -form of amino acids is typically more abundant than the d -form. However, the d -form has many important pharmaceutical applications. meso -Diaminopimelate dehydrogenase ( meso -DAPDH) from Symbiobacterium thermophilum (StDAPDH) was the first member of meso -DAPDH known to catalyze the amination of 2-keto acids to produce d -amino acids. Accordingly, we analyzed the evolution of meso -DAPDH proteins and found that they form two groups, i.e., type I proteins, which show high preference toward meso -diaminopimelate ( meso -DAP), and type II proteins, which show a broad substrate spectrum. We examined the differences in sequence, ternary structure, and quaternary structure to determine the mechanisms underlying the functional differences between the type I and type II lineages. These results will facilitate the identification of additional meso -DAPDHs and may provide guidance to protein engineering studies for d -amino acid biosynthesis.

Nowadays, computed tomography (CT) is one of the most common 3D imaging techniques in clinical diagnosis, providing highly accurate information of internal structure and guiding medical treatment. The principle of CT is rendering 3D image of the sample from multiple projections recorded under different angles of view. It is apparent that large projection number will bring a better reconstruction quality.¹ Unfortunately, it will meanwhile deliver a high radiation dose for the patients and increase the probability of cancer.^{2, 3} X-ray integral imaging can provide an alternative to CT in medical imaging as it has the prospect of reconstructing 3D images from one single capture, consequently leading to a lower dose and a shorter exposure time.