Seeing is understanding. In many instances, seeing is the best way to understand natural phenomena. X-ray crystallography is among the most powerful methods to visualize 3D structures of proteins and nucleic acids at the atomic level. In X-ray crystallography, a protein crystal is exposed to X-rays, which generates a specific X-ray scattering pattern, called a diffraction pattern. The diffraction pattern is then used to reconstruct the 3D structure of the protein molecule. In this thesis, the crystallographic method is applied to elucidate the structure of the 3-ketosteroid Δ1-dehydrogenase isoenzyme 1 (Δ1-KSTD1) from Rhodococcus erythropolis SQ1, an organism closely related to Mycobacterium, and the β-xylosidase from the thermophilic bacterium Geobacillus thermoleovorans IT-08 (Xyl). 3-Ketosteroid Δ1-dehydrogenases (Δ1-KSTDs) are pharmaceutically important enzymes as they are applied for the production of Δ1-3-ketosteroidal drugs and hormones. In combination with mutational analysis, the structure of Δ1-KSTD1 clarifies the catalytic mechanism of Δ1-KSTDs. Furthermore, the Δ1-KSTD from Mycobacterium tuberculosis was identified to play an important role in the etiology of tuberculosis. Thus, the structure of Δ1-KSTD1 may also facilitate the design of inhibitors that could possibly be developed into new anti-tuberculosis drugs. Regarding the second subject, β-xylosidases are hemicellulolytic enzymes that are of biotechnological interest for saccharifying hemicellulosic biomass. However, these enzymes are commonly inhibited by monosaccharides produced during the saccharification process. By analyzing the 3D structures of Xyl in complex with its natural inhibitors, we learned how the inhibitors inhibit the activity of the enzyme and offer tentative solutions to overcome the inhibition problems
