GEORGE WALD 



733 



Rhodopsin 



Lumirhodopsin 



Metarhodopsin 



Retinene 



H* ygb} 



Fig. 4. The action ol light on rhodopsin. The absorption of light Ijy rhodopsin 

 isomerizes its Wcis chromophore to liic M-lrcnis configuration, yielding as first 

 product the ali-/)v///.s chronioprotcin hmiirhodopsin. This labilizes the protein, 

 opsin, which rearranges to a new conliguiation. yickhng a second M-lraus chronio- 

 protcin, metarhodopsin. lliis second process exposes reactive groups on opsin 

 — two — SH groups, and one pr()ton-i)inding group, symbolized in the figure with 

 a negative cluirge — and niav be responsible for triggering visual excitation. Verte- 

 brate metarhodopsins are unstable, and above about — 20°C hydrolyze to opsin 

 and all-/»Y(;/.? retinene. the process that corresponds to bleaching. (From Hubbard 

 and Kropf. 2fi). 



charge on opsin, in intimate association with the retinene side-chain, 

 draws the positive charge up into the conjugated system, so greatly 

 enhancing the resonance forms of the chromophore (21). 



In forming the iso-pigments, the same mechanisms operate; but 

 the fit between the retinene and opsin is less close than with the 11- 

 cis isomer, and the spectrum is correspondingly less enhanced. Not 

 only do all the iso-pigments lie to the short-wavelength side of the 

 corresponding visual pigments (A A„,ax: rod pigments, 15 m,x; cone 

 pigments, 50 m^) , but the special displacements of spectrum induced 

 by cone as compared with rod opsin are much smaller (Table 1) . 



The absorption of a quantum of light by rhodopsin or iso-rhodopsin 

 isomerizes the retinene chromophore to the all-/?Y7;/5 configuration. 

 Now the retinene no longer fits the opsin; in consequence its attach- 

 ment is unstable and it readily hydrolyzes off. This is the essential 

 mechanism of bleaching (Fig. 4) . 



TABLE 1 



.\BSORrTION .MAXI.VIA (\,„ax) OF THE VERTEBRATE VISUAL I'IGMENTS AND ISO-PICMENTS, 

 AND THE DIFFERENCES BETWEEN THEM (/^ Xmax) 



Amax 



Rhodopsin 

 Porphyropsin 

 lodopsin 

 Cyanopsin 



Visual pigment 



500 

 522 

 562 

 620 



Iso-pigment 



(m/i) 



485 

 507 

 510 

 575 



(m/x) 



15 

 15 

 52 

 45 



