Characterization of Carboxynorspermidine Dehydrogenase from Two Human Gut Microbes

Abstract

Polyamines are polycationic alkylamines, found in all living organisms, and are essential for a wide array of cellular processes such as cell growth, cell proliferation, and gene expression. Indeed, they are required for cell viability. Increased production of polyamines by cells is correlated with pancreatic and colon cancer. Excess polyamines can be used by cells to promote cell growth and proliferation, accelerating tumorigenesis. In colon and rectal cancer, polyamine production by gut microbes is elevated and associated with human gut microbe dysbiosis. Although polyamine production in the human cells has been investigated thoroughly, the pathways by which human gut microbiota biosynthesize polyamines are less well characterized. In this study, we are investigating carboxynorspermidine dehydrogenase (CASDH), an enzyme that catalyzes the reductive condensation of either diaminopropane or putrescine with L-aspartate semialdehyde to form carboxynorspermidine or carboxyspermidine. We have successfully solved a 3.1 Å crystal structure of CASDH from Clostridium leptum and have diffracting crystals from Bacteroides fragilis CASDH using X-ray crystallography. We have synthesized aspartate semialdehyde to examine CASDH specificity for diaminopropane, putrescince, NADH and NADPH using steady-state kinetic assays. With this structural information and enzyme kinetic data, future work will be focused on the CASDH kinetic mechanism and the amino acid residues that are involved in binding and catalysis.