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New Mechanistic Insight from Substrate and Product Bound Structures of the Metal-dependent Dimethylsulfoniopropionate Lyase DddQ
Structural highlights
FunctionDDDQ_RUELI Able to cleave dimethlysulfonioproprionate (DMSP) in vitro, releasing dimethyl sulfide (DMS). DMS is the principal form by which sulfur is transported from oceans to the atmosphere (PubMed:24395783, PubMed:24967457). The real activity of the protein is however subject to debate and it is unclear whether it constitutes a real dimethlysulfonioproprionate lyase in vivo: the very low activity with DMSP as substrate suggests that DMSP is not its native substrate (PubMed:24760823).[1] [2] [3] Publication Abstract from PubMedThe marine microbial catabolism of dimethylsulfoniopropionate (DMSP) by the lyase pathway liberates approximately 300 million tons of dimethyl sulfide (DMS) per year, which plays a major role in the biogeochemical cycling of sulfur. Recent biochemical and structural studies of some DMSP lyases, including DddQ, reveal the importance of divalent transition metal ions in assisting DMSP cleavage. While DddQ is believed to be zinc-dependent primarily on the basis of structural studies, excess zinc inhibits the enzyme. We examine the importance of iron in regulating the DMSP beta-elimination reaction catalyzed by DddQ as our as-isolated purple-colored enzyme possesses approximately 0.5 Fe/subunit. The UV-visible spectrum exhibited a feature at 550 nm, consistent with a tyrosinate-Fe(III) ligand-to-metal charge transfer transition. Incubation of as-isolated DddQ with added iron increases the intensity of the 550 nm peak, whereas addition of dithionite causes a bleaching as Fe(III) is reduced. Both the Fe(III) oxidized and Fe(II) reduced species are active, with similar kcat values and 2-fold differences in their Km values for DMSP. The slow turnover of Fe(III)-bound DddQ allowed us to capture a substrate-bound form of the enzyme. Our DMSP-Fe(III)-DddQ structure reveals conformational changes associated with substrate binding and shows that DMSP is positioned optimally to bind iron and is in the proximity of Tyr 120 that acts as a Lewis base to initiate catalysis. The structures of Tris-, DMSP-, and acrylate-bound forms of Fe(III)-DddQ reported here illustrate various states of the enzyme along the reaction pathway. These results provide new insights into DMSP lyase catalysis and have broader significance for understanding the mechanism of oceanic DMS production. New Mechanistic Insight from Substrate- and Product-Bound Structures of the Metal-Dependent Dimethylsulfoniopropionate Lyase DddQ.,Brummett AE, Dey M Biochemistry. 2016 Nov 8;55(44):6162-6174. Epub 2016 Oct 28. PMID:27755868[4] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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