PDE6 Has Evolved to Meet the Special Demands of the Central Effector of Visual Transduction

The exquisite light sensitivity and temporal resolution of the visual signaling pathway in retinal photoreceptors requires precise regulation of PDE6 in rod and cone outer segments. On the one hand, spontaneous activation of PDE6 must be minimized in the dark-adapted state to enhance the absolute light sensitivity and to avoid unnecessary consumption of metabolic energy through a futile cycle of cGMP synthesis and breakdown. On the other hand, visual excitation following photic stimulation must generate a very rapid (millisecond) light response that requires immediate acceleration of cGMP breakdown by activated PDE6. Two features that are unique to the PDE6 family of PDEs—the extrinsic regulation of activity by the inhibitory γ-subunit and the extraordinary catalytic efficiency of the PDE6 active site—explain how this member of the PDE superfamily serves as the central effector of visual transduction.

In dark-adapted rod outer segments, electrophysiological measurements of “dark noise” suggest that only 1 of 5,000 PDE6 molecules is spontaneously active at any moment [152]. This observation is consistent with measurements of the amount [73] and binding affinity of the γ-subunit for the PDE6 catalytic dimer [86, 142, 153], which together serve to ensure that nonactivated PDE6 exists as a nonactivated tetramer (αβγ2). Direct measurements of the basal activity of PDE6 in rod outer segment suspensions indicate that only 1 of 2,200 PDE6 molecules is spontaneously active in the darkadapted state [154]. On light activation, displacement of the γ-subunit by activated transducin relieves the inhibitory constraints on the PDE6 holoenzyme and triggers the rapid decline in cGMP levels (discussed in detail in next section).

Full removal of the γ-subunit from the catalytic dimer by various means (proteolysis [85]; polycationic proteins [78]; or γ-subunit extraction in a complex with activated transducin [155]) results in a PDE6 catalytic dimer that hydrolyzes cGMP with about 1,000-fold greater catalytic efficiency than the closely related PDE5 enzyme (Table 1).

The kcat/KM value of 4 × 108 M−1s−1 for catalysis of cGMP by bovine rod PDE6 [142] or amphibian rod PDE6 [154] approaches the diffusion-controlled limit for a bimolecular

collision [156], making activated PDE6 a nearly perfectly designed catalyst for degrading cGMP in photoreceptor outer segments during light stimulation [51]. cAMP is a very poor substrate for PDE6 catalysis; the specificity constants (kcat/KM) for cGMP and cAMP differ by about 100-fold, primarily due to differences in the KM values (Table 1). Wide variations in the literature for the KM for PDE6 [40] are now understood to have resulted from restricted substrate diffusion in those experiments when PDE6 activity was measured in rod outer segment membrane preparations [52, 157].

Table 1. Catalytic properties of human PDE5 and bovine rod PDE6 catalytic dimer

cAMP cyclic adenosine monophosphate, cGMP cyclic guanosine monophosphate, PDE phosphodiesterase