The molecular mechanism of muscle contraction has been investigated by X-ray diffraction and scattering. Intense synchrotron X-rays and a novel area detector enabled us to detect a tiny but significant intensity and spacing changes of the weaker actin-based reflections during muscle contraction with high accuracy. Our analyses have shown that the structural changes and extensibility of the actin filament play an important role in force-generation of muscle, necessitating quite significant reevaluation of the current models of crossbridge mechanism. Although the structural changes within crossbridges which produce force in muscle remain to be clarified, our recent X-ray solution scattering has showed distinct alterations of the structure of myosin heads during hydrolysis of ATP. For a real understanding of the force transduction in muscle, we need explicit structural models that explain the coupling between the structural changes in actin and myosin. This special report makes a survey of our recent progress in the X-ray structural reseach of muscle, on occasion for a Japan Crytallographic Society award.
X-ray transmission phase palates are described based on the dynamical theory of X-ray diffraction. Polarization-tunable X-ray optics using transmission phase plates for synchrotron radiation (SR) were proposed and were used to transform horizontal polarization of SR to circular or vertical polarization at a bending-magnet beamline, BL-15C at the Photon Factory (PF) . Combination of x-ray transmission phase plates and X-ray polarization analyzer enabled us to determine full state of polarization in the X-ray spectral region. With this polarization analysis method, polarizations of X-rays from an elliptical multipole wiggler installed on BL-28 at the PF were successfully estimated.
Chiral autocatalysis is the system in which each enantiomer acts as a catalyst for its own production and an anticatalyst for the production of the other enantiomer. Predominant production of one enantiomer would occur if the reaction system involved chiral autocatalysis, since a slight difference between each enantiomer, which arises in the early stage of the reaction by statistical fluctuation could be augmented as the reaction proceeds. Some enantiomeric predominant products could be experimentally obtained by chirally autocatalytic secondary nucleation.
A group of short-chain dehydrogenases/reductases (SDR) has recently been identified. These enzymes utilize NAD (H) or NADP (H) as cofactors and exhibit a wide variety of substrate specificities. Crystal structure analyses have been completed for some of these enzymes, but none of them have targeted the ternary (enzyme-coenzyme-substrate analogue) complexes. We have recently solved the crystal structures of the ternary complex (with NAD (H) ) of 7α-hydroxysteroid dehydrogenase from E. coli and that (with NADP (H) ) of mouse lung carbonyl reductase. These two structures enabled us to discuss the catalytic mechanism and the origin of coenzyme specificities (NAD (H) or NADP (H) ) common to the enzymes of the SDR family.
The endohedral nature of the metallofullerene Y@C82 is described based on recent results of the X-ray structural study for the metallofullerene Y@C82 by the synchrotron powder diffraction experiment. For the structural analysis, the novel method which is the combination of the Rietveld analysis and the Maximum Entropy Method (MEM) is employed to analyse the complicated powder pattern. The obtained MEM charge density reveals that the yttrium atom is trapped within the carbon cage and reside very close to the fullerene cage indicating the strong off-centered nature of the Y@C82 molecule.