(A) IMPDH1 monomer (7rgd) with retinal phosphorylation sites [61] shown in red spheres. (B) Zoomed in view of phosphorylation site S416 showing its proximity to the active site with NAD⁺ in salmon and IMP in gold. (C) Phosphorylation site T159/S160 is between nucleotide binding sites 1 and 2 in the regulatory domain and might disrupt binding at either/both site(s). (D) Surface representation of IMPDH filament and zoom in of the interface between octamers where phosphorylation site S477 is nestled.