Schiff base, protonated

The chromophore, 11-ris-retinal, is linked to the a-amino group of lysine-296 in helix 7 via a protonated Schiff base. Protonated Schiff bases usually absorb light maximally at around 440 nm, but the of rhodopsin is near 500 nm. 

Both in the case of sensory rhodopsin in humans and of bacteriorhodopsin (a heptahelical membrane protein in halobacteria which is not coupled to a G protein) translocation of a Schiff-base proton is the essential step in making the protein functional (reviewed in ref 58). In rhodopsin the conversion of the inactive AH state to the AHI state that binds to the G protein is coupled to proton transfer from the Schiff base to the counterion, Glul 13, and proton uptake from the medium to the highly conserved Glul34, which serves as proton acceptor. Based on that similarity, one could consider sensory rhodopsin as an incomplete proton pump. Furthermore, a property shared by all G-protein-coupled receptors is a triplet, formed by residues 134-136 in rhodopsin, consisting of Glu-Arg-Tyr. The consequences of mutational replacement of Glul34 supports the notion that the state of protonation of this amino add is crudal for activity, and that its protonation triggers the conformational transition of the receptor from the inactive to the active state. 

Ffe. 5.11 TLie MM forms of rhodopsin (R) are in equilibrium with their parent compound Ml and the progeny Mill. The equilibnum between the metastates, Ml, Mila, and Mllb, are attained within a few milliseconds after illumination. The Ml/Mlla transition is accompanied by the intramolecular translocation of the Schiff-base proton to GlullS, located in a region close to the lipid membrane. The subsequent Mlla/Mllb transition is dependent on proton uptake from the aqueous phase and occurs in a watersxposed area. Fast binding of the G protein, transducin, occurs only with Mllb.(Reproduced from ref. 58 with permission of Elsevier Science.)... 

Actually, the reaction is a bit more complex than shown above because it does not take place on the free ketone. Instead, fructose 1,6-bisphosphate undergoes reaction with the side-chain -NH2 group of a lysine residue on the aldolase enzyme to yield an imine (Section 19.9), sometimes called a Schiff base. Protonation of the imine makes it more reactive a retro aldol-like reaction ensues, giving glyceraldehyde 3-phosphate and the imine of dihydroxyacetone phosphate and the imine is then hydrolyzed to yield dihy-droxyacetone phosphate. 

Jager F, Fahmy K, Sakmar TP, Siebert F. Identification of glutamic acid 113 as the Schiff base proton acceptor in the metarhodopsin II photointermediate of rhodopsin. Biochemistry 1994 33 10,878-10,882. 

Figure 10. Coupled consecutive proton transfer pathway across the membrane. The actual number of binding sites is not known. For simplicity, only five binding sites (A1 througji As) are shown. Site A3 designates the Schiff base proton binding site. It is understood that the Schiff base is neutral when deprotonated and carries a positive charge when protonated. The reverse reactions are not shown but are important for the discussion. (Reproduced with permission from reference 82. Copyright 1990.)...

Singh, A. K Sandorfy, C., Fendler, J. H. (1990). Retinylidene Schiff-base protonation in surfactant-solubilized water pools in heptane, J. Chem. Soc., Chem. Commun., p. 233. 

The Schiff-base proton plays an important role in the mechanism of proton translocation during the cycle a deprotonation and reprotonation of the nitrogen atom of the Schiff-base linkage occurs. 

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