FIGURE 6.

Catalytic mechanism and the SpHCS ternary complex model. A, shown is the proposed catalytic mechanism of HCS with 2-OG and AcCoA depicted as in Fig. 5E and homocitrate illustrated in green. B, shown is a stereoview of the active site of the modeled SpHCS ternary complex with 2-OG and the pantetheine arm of AcCoA rendered as in Fig. 5E. Dashed lines represent coordination of the Zn(II) ion (orange), hydrogen bonding to 2-OG (yellow), hydrogen bonding to the putative catalytic residues (blue), and the direction of the nucleophilic attack from the enolate tautomer of AcCoA to the C2 atom of 2-OG (red). C, in vivo yeast growth assays of the LYS4 mutants implicated in catalysis are shown.