Metal Ion Roles and the Movement of Hydrogen during Reaction Catalyzed by D-Xylose Isomerase: A Joint X-Ray and Neutron Diffraction Study

SummaryConversion of aldo to keto sugars by the metalloenzyme D-xylose isomerase (XI) is a multistep reaction that involves hydrogen transfer. We have determined the structure of this enzyme by neutron diffraction in order to locate H atoms (or their isotope D). Two studies are presented, one of XI containing cadmium and cyclic D-glucose (before sugar ring opening has occurred), and the other containing nickel and linear D-glucose (after ring opening has occurred but before isomerization). Previously we reported the neutron structures of ligand-free enzyme and enzyme with bound product. The data show that His54 is doubly protonated on the ring N in all four structures. Lys289 is neutral before ring opening and gains a proton after this; the catalytic metal-bound water is deprotonated to hydroxyl during isomerization and O5 is deprotonated. These results lead to new suggestions as to how changes might take place over the course of the reaction

Space Astrometry Missions for Exoplanet Science: Gaia and the Legacy of Hipparcos

Astrometry as a technique has so far proved of limited utility when employed as either a follow-up tool or to independently search for planetary-mass companions around stars in the solar neighborhood. However, the situation is bound to change soon. In this chapter, we provide a brief overview of past and present efforts to detect planets via milli-arcsecond (mas) astrometry, with a special focus on the legacy of the Hipparcos mission. We then focus on the Gaia mission that is poised to become a game changer in the field of exoplanets by unleashing for the first time the power of micro-arcsecond (μas) astrometry. We start by briefly describing the mission status and operation. Next, we address some of the relevant technical issues associated with the precise and accurate determination of astrometric orbits of planetary systems using Gaia data. We then present and discuss the Gaia planet-finding capabilities. We conclude by putting Gaia astrometry in context, illustrating its potential for crucial contributions to exoplanetary science in synergy with other indirect and direct methods for the detection and characterization of planetary systems