@entity599 Isotope Effects of Dissimilatory XXXX Reductase.

@entity15372 @entity9735 @entity15373 @entity599 @entity72566 @entity42910 @entity597
The precise interpretation of environmental @entity599 isotope records requires a quantitative understanding of the biochemical controls on @entity599 isotope fractionation by the principle isotope-fractionating process within the S cycle, microbial @entity597 reduction (MSR). Here we provide the only direct observation of the major ( @entity15373 / @entity42910 ) and minor ( @entity15372 / @entity42910 , @entity72566 / @entity42910 ) @entity599 isotope fractionations imparted by a central enzyme in the energy metabolism of @entity597 reducers, dissimilatory @entity9735 reductase (DsrAB). Results from in vitro @entity9735 reduction experiments allow us to calculate the in vitro DsrAB isotope effect in @entity15373 / @entity42910 (hereafter, [Formula: see text]) to be 15.3    2   , 2  . The accompanying minor isotope effect in @entity15372 , described as [Formula: see text], is calculated to be 0.5150    0.0012, 2  . These observations facilitate a rigorous evaluation of the isotopic fractionation associated with the dissimilatory MSR pathway, as well as of the environmental variables that govern the overall magnitude of fractionation by natural communities of @entity597 reducers. The isotope effect induced by DsrAB upon @entity9735 reduction is a factor of 0.3-0.6 times prior indirect estimates, which have ranged from 25 to 53    in (34)  DsrAB. The minor isotope fractionation observed from DsrAB is consistent with a kinetic or equilibrium effect. Our in vitro constraints on the magnitude of [Formula: see text] is similar to the median value of experimental observations compiled from all known published work, where (34)   r-p = 16.1    (r-p indicates reactant vs. product, n = 648). This value closely matches those of MSR operating at high @entity597 reduction rates in both laboratory chemostat experiments ([Formula: see text] 17.3    1.5   , 2  ) and in modern marine sediments ([Formula: see text] 17.3    3.8   ). Targeting the direct isotopic consequences of a specific enzymatic processes is a fundamental step toward a biochemical foundation for reinterpreting the biogeochemical and geobiological @entity599 isotope records in modern and ancient environments.
