Figure 8.

Differences in human and Mtb forms of SAHH enzyme near the entrance to the access channel/HCY-binding site. (A) Mtb SAHH bound to ETA and NAD showing a constricted solvent access channel lined by residues from the insertion region. Residues 167–203 from strand-turn-helix insertion are shown in cyan, with Glu186 highlighted. The figures were made using SPOCK (Christopher 1998). (B) Surface of Mtb SAHH binding site with access channel closed off by His363, and ADO enclosed in a buried pocket. (C) Human SAHH complexed with DHCeA and NAD showing a shallower solvent access channel compared to the Mtb SAHH. The surface of the active site has been drawn without His301 to simulate the open form of the channel. His301 would presumably flip out of the channel, allowing the SAH substrate to bind and HCY cleaved from the substrate to be released to solvent.