Structure of the catalytic domain of the matrix metalloproteinase MMP-3. The ribbon diagram was created using WebLab Viewer software based on a crystal structure analysis by Gomis-Rüth et al. . Strands (sI–sV) and helices (hA–hC) are labelled in black; the catalytic zinc (centre), coordinated by three histidine residues (pink), and the structural zinc (Zn) (top) are labelled in black and white, respectively; and the NH2 terminus (N) and COOH terminus (C) are labelled in red. The active site cleft and the characteristic methionine (Met) turn are indicated with arrows.