Figure 8.

Hypothesized isomerosynthase and alternate visual cycle in chicken retinas. Schematic drawings of the retinol isomerase and palm-CoA-dependent retinyl ester synthase in chicken retinas. The isomerase catalyzes passive interconversion of atROL and 11cROL. The ester synthase uses palm-CoA as an acyl donor to catalyze formation of an atRE or 11cRE from 11cROL or atROL, respectively. In the isomerosynthase catalytic mode (depicted here as a complex between these enzymes), the 11cROL product of isomerization is directly esterified to drive the formation of 11-cis-retinoids. This results in higher catalytic efficiency than for uncoupled atROL-dependent synthesis of atREs or 11cROL-dependent synthesis of 11cREs.