PHYSL405 Lecture Notes - Lecture 3: Transducin, Rhodopsin, Opsin
7 Nov 2016
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Pde has dark activity = basal activity = very important to maintain the dark [ ] of cgmp at around 1-10 um. Pde activation involves pdey removal (inhibitory subunit) from its binding site on pde. Consequence = enormous activation of pde/ta-gtp complex which rapidly hydrolyzes cgmp. Turnover number of this again reaches thousands of cgmp molecules hydrolyzed per activated. Pde molecule per second (kcat = n103cgmphyd/mol pde/sec) n =1-5 depending on the paper. Consequence = rapid deplete of cytoplasmic cgmp cng channel closure in plasma membrane. Cng has two subunits (stoichiometry 3cnga/1cngb), both of which have a cytoplasmic cgmp binding site, keeping cng open. Depletion of cgmp dissociation of bound cgmp cgmp do(cid:373)ai(cid:374) (cid:862)plugs(cid:863) the catio(cid:374) pore on the inside of the cell membrane. Physical consequence of channel closure = change of conductance of plasma membrane. Hyperpolarization = achieved by continuous extrusion of ca2+ and k+ which changes cation balance inside vs outside.
