Sulfur, one of the four essential macronutrients, is required by all forms of life and passes through ecosystems through the biogeochemical sulfur cycle[1]. A deeper understanding of this cycle is critical to understanding the role sulfur plays in life, especially in marine systems.
Homotaurine is a naturally-occurring organosulfur compound produced by marine algae and found in low concentrations in seawater[2]. Whilst the metabolism of its structural isomer taurine is well-studied, comparatively little is known about the fate of homotaurine in the environment. Despite their structural similarities, many enzymes that utilise taurine as a substrate cannot efficiently metabolise homotaurine.
Here, we report the discovery of a catabolic pathway for homotaurine in Burkholderia sp. CT39-3, a soil bacterium capable of using homotaurine as a sole carbon source for growth. Genomic and proteomic analysis revealed a set of enzymes homologous to the gamma-aminobutyric acid (GABA) shunt and the Krebs cycle.
Binding of homotaurine to the ABC substrate binding protein HtaA was confirmed using isothermal titration calorimetry and X-ray crystallography. Kinetic analysis demonstrated that the pathway’s first enzyme, HtaE, is a PLP-dependent aminotransferase that utilises both homotaurine and GABA. X-ray crystallography was used to solve the 3D structure of HtaE in both the internal and external aldimine forms using the inhibitor aminooxysulfonic acid, revealing the coordination of the sulfonate group and accommodation of the additional oxygen atom within the active site.