Homo-/Heterodimeric Substrates Bias the Type and Multiplicity of Hydroxamic Acid Chelators Assembled by a NIS Synthetase DesD.

The siderophore synthetase DesD generates hydroxamic acid chelators with applications in metal-based radiopharmaceuticals and sequestering toxic or commodity metals. DesD has been used in chemoenzymatic syntheses with native substrates, with less focus on non-native substrates. This study investigated masking the modest activity of the non-native substrate N-hydroxy-N-glutarylcadaverine (2) by forming a heterodimer with the native DesD substrate N-hydroxy-N-succinylcadaverine (1). Recombinant DesD from Salinispora tropica (StDesD) was evaluated with combinations of homo- and heterodimers of 1 and 2 (3-6) as substrates, including N-to-C positional isomers. Chemoenzymatic reactions using heterodimers of 1 and 2 (5, 6) showed similar substrate consumption and product types to the native 1 homodimer (3), with substrate consumption about three times greater than 2 alone, demonstrating the success of the masking approach. Furthermore, dimeric substrates ablated the ability of StDesD to generate the odd-numbered trimeric hexadentate macrocycle desferrioxamine E (DFOE) as its native major product, instead generating the even-numbered tetrameric macrocycles of dimeric substrates (3, 5, 6) as major products. While the StDesD upper limit of iterations per substrate was similar for monomeric and dimeric substrates, the latter generated chelators with unprecedented cavity sizes and denticities, including an icosadentate chelator that formed a 3:1 metal:ligand complex with Ga(III).