Mechanistic insights into the dimethylsulfoniopropionate synthesis enzyme BurB

The organosulfur compound dimethylsulfoniopropionate (DMSP) is an important participant in the global sulfur cycling. DMSP possesses various physiological functions in microorganisms and is also the main precursor of the "climate cooling" gas dimethyl sulfide (DMS). However, studies on the catalytic mechanisms of DMSP synthesis enzymes are limited. BurB, a specific <i>S</i>-methyltransferase involved in the DMSP methylation synthesis pathway, catalyzes the conversion of methionine (Met) to <i>S</i>-methyl-methionine (SMM), with <i>S</i>-adenosyl methionine (SAM) as a methyl donor. Moreover, BurB also represents a new member of the SET (Suppressor of variegation, Enhancer of zeste and Trithorax) domain proteins. Here, we determined the crystal structures of BurB-Met and BurB-SMM-SAM complexes and proposed the catalytic mechanism of BurB based on structural and biochemical analyses. The results offer a better understanding of the catalytic mechanisms of SET domain-containing methyltransferases and provide novel insights into DMSP synthesis.