Figure 2. The active site.

a, Three-dimensional clustering of key residues identifies the active site. Residues that diminish specific activity by more than 95% when mutated (Extended Data Fig. 1)¹² are drawn in magenta on a ribbon representation. Their high concentration within the dashed region demarcates the active site. b, The active site, depicted as a molecular surface within the ribbon representation. The AdoHcy pocket is cyan; the membrane-exposed cavity is orange. c, Interactions with AdoHcy (sticks with cyan carbon atoms). Dashed lines indicate hydrogen bonds. Water molecules are red spheres. Atom colouring is: nitrogen, blue; oxygen, red; and sulfur, green. The M6–M7 connector is tan. Arrows denote locations of Gly181 and Ser193; parentheses indicate hydrogen bonds with backbone atoms. An inset shows electron density for AdoHcy (simulated annealing omit Fo-Fc map, contoured at 3σ).