Fig. 5.

Structural model of SMS head group selectivity. A: Membrane topology and structural elements that contribute to substrate specificity of SMS enzymes. The invariant His and Asp residues that form the catalytic triad are marked in red. Head group selectivity of SMS enzymes is determined by a single residue adjacent to the catalytic His in exoplasmic loop c, i.e., Asp in SMS1 (marked in green) or Glu in SMS2 and SMSr (marked in yellow), along with structural information in loop b (marked in blue). SAM, sterile α motif. B: Model explaining how structural elements in loops b and c of SMS enzymes cooperate to determine head group selectivity. See text for details.