3D Cathepsin B (IMAGE) Max-Planck-Gesellschaft Caption 3D structure of Cathepsin B. In the "closed" conformation, an occluding loop (rigid segment: beige; flexible segment: red) blocks access to the substrate site. The propeptide binding shifts the flexible part of the loop in such a manner that the reactive partners of cathepsin B can reach the substrate site. Credit <i>Science</i> Express, DOI: 10.1126/science.1229 Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.