Mechanistic Insights into the Anaerobic Degradation of Globally Abundant Dihydroxypropanesulfonate Catalyzed by the DHPS-Sulfolyase (HpsG).

Abstract

2(S)-Dihydroxypropanesulfonate (DHPS) is the main abundant organosulfonate in the biosphere generated by the microbial degradation of the abundant organosulfur species 6-deoxy-6-sulfo-d-glucopyranose (sulfoquinovose, SQ). Massive amounts of DHPS can also be produced by the highly abundant oceanic diatoms. The quantity of degradation DHPS is so large that it has become an important part of the earth's sulfur. The recently characterized O2-sensitive glycyl radical enzyme DHPS-sulfolyase HpsG in anaerobic bacteria was found to be capable of cleaving the C-S bond of DHPS under anaerobic conditions. However, the detailed degradation mechanism is still unclear. Here, on the basis of the crystal structure of HpsG, we constructed the computational model and performed QM/MM calculations to illuminate the anaerobic degradation mechanism of DHPS. Our calculations revealed that the degradation reaction follows an unusual radical-dependent mechanism that does not require a conserved Glu464 to deprotonate the C2 hydroxyl of substrate to promote the C-S cleavage; instead, after the first hydrogen abstraction triggered by the thiyl radical (Cys462), the C-S bond in 2(S)-dihydroxypropanesulfonate can directly collapse. Thus, conserved Glu464 mainly plays a role in stabilizing the substrate and reaction intermediate by forming a hydrogen bond. After the release of the sulfonic acid group from the protein environment, the deprotonated Glu464 spontaneously accepts a proton from the C2 hydroxyl of the substrate radical. Our findings clarified an unusual C-S cleavage mechanism involved in the DHPS degradation reaction catalyzed by GREs.