Summarize the enzymatic mechanisms by which phototransduction is terminated after photostimulation.

Phototransduction is the process by which the retina's photoreceptor cells (rods and cones) convert light energy into electrical signals. This occurs through a series of biochemical reactions involving the activation of a G-protein coupled receptor called opsins and the subsequent activation of cyclic guanosine monophosphate (cGMP) dependent phosphodiesterase (PDE). When light activates an opsin, it triggers a cascade of reactions that ultimately leads to a decrease in cGMP levels, causing the closure of cGMP-gated ion channels, hyperpolarization of the photoreceptor cell, and transmission of the signal to other retinal neurons.

The first step in terminating phototransduction is to deactivate activated opsins. This is accomplished by rhodopsin kinase, an enzyme that phosphorylates the opsin, making it a less efficient catalyst for the activation of the G-protein transducin. Then, a protein called arrestin binds to the phosphorylated opsin, effectively blocking its interaction with transducin and stopping further downstream signaling.

Transducin's activity is terminated by the hydrolysis of bound guanosine triphosphate (GTP) to guanosine diphosphate (GDP). A group of proteins called Regulators of G-protein Signaling (RGS) can accelerate this process by acting as GTPase-activating proteins (GAPs), promoting GTP hydrolysis and inactivating transducin. Inactive transducin (with GDP bound) is no longer able to interact with and activate PDE.

To completely terminate phototransduction, the levels of cGMP in the photoreceptor cells must be restored to their initial state. Guanylate cyclase, an enzyme responsible for the synthesis of cGMP, becomes more active when intracellular calcium (Ca2+) levels are low. After the closure of cGMP-gated ion channels during phototransduction, Ca2+ levels inside the cell decrease, promoting guanylate cyclase activity. Additionally, Ca2+-sensitive proteins like guanylate cyclase activating proteins (GCAPs) and recoverin can modulate guanylate cyclase, enhancing its activity to restore cGMP to levels that allow the reopening of cGMP-gated ion channels and ending the process of phototransduction. In conclusion, the termination of phototransduction after photostimulation involves a series of enzymatic mechanisms acting at different levels of the process. These mechanisms include the inactivation of opsins by rhodopsin kinase and arrestin, the termination of transducin via GTP hydrolysis and RGS proteins, and the restoration of cGMP levels by guanylate cyclase and Ca2+-sensitive proteins.