Influence of stereochemistry on the activity of rapadocin, an isoform-specific inhibitor of the nucleoside transporter ENT1

Mechanisms of Sugar Aminotransferase-like Enzymes to Synthesize Stereoisomers of Non-proteinogenic Amino Acids in Natural Product Biosynthesis

(2,6)-Diamino-(5,7)-dihydroxyheptanoic acid (DADH), a non-proteinogenic amino acid, is converted to 1-azabicyclo[3.1.0]hexane ring-containing amino acids that are subsequently incorporated into ficellomycin and vazabitide A. The present study revealed that the sugar aminotransferase-like enzymes Fic25 and Vzb9, with a high amino acid sequence identity (56%) to each other, synthesized stereoisomers of DADH with (6S) and (6R) configurations, respectively. The crystal structure of the Fic25 complex with a PLP-(6S)-N²-acetyl-DADH adduct indicated that Asn45 and Gln197 (Asn205 and Ala53 in Vzb9) were located at positions that affected the stereochemistry of DADH being synthesized. A modeling study suggested that amino acid substitutions between Fic25 and Vzb9 allowed the enzymes to bind to the substrate with almost 180° rotation in the C5-C7 portions of the DADH molecules, accompanied by a concomitant shift in their C1-C4 portions. In support of this result, the replacement of two corresponding residues in Fic25 and Vzb9 increased (6R) and (6S) stereoselectivities, respectively. The different stereochemistry at C6 of DADH resulted in a different stereochemistry/orientation of the aziridine portion of the 1-azabicyclo[3.1.0]hexane ring, which plays a crucial role in biological activity, between ficellomycin and vazabitide A. A phylogenic analysis suggested that Fic25 and Vzb9 evolved from sugar aminotransferases to produce unusual building blocks for expanding the structural diversity of secondary metabolites.

Nucleoside transporters and immunosuppressive adenosine signaling in the tumor microenvironment: Potential therapeutic opportunities

Adenosine compartmentalization has a profound impact on immune cell function by regulating adenosine localization and, therefore, extracellular signaling capabilities, which suppresses immune cell function in the tumor microenvironment. Nucleoside transporters, responsible for the translocation and cellular compartmentalization of hydrophilic adenosine, represent an understudied yet crucial component of adenosine disposition in the tumor microenvironment. In this review article, we will summarize what is known regarding nucleoside transporter's function within the purinome in relation to currently devised points of intervention (i.e., ectonucleotidases, adenosine receptors) for cancer immunotherapy, alterations in nucleoside transporter expression reported in cancer, and potential avenues for targeting of nucleoside transporters for the desired modulation of adenosine compartmentalization and action. Further, we put forward that nucleoside transporters are an unexplored therapeutic opportunity, and modulation of nucleoside transport processes could attenuate the pathogenic buildup of immunosuppressive adenosine in solid tumors, particularly those enriched with nucleoside transport proteins.

Rapid mild macrocyclization of depsipeptides under continuous flow: total syntheses of five cyclodepsipeptides

A systematic investigation of the flow macrocyclization approaches for five destruxin analogues natural products at three different cyclization point has been reported.