HEN r LEY CLASS 



yo:^ 



stcricall) unhiiulciccl, aiul {oiisccjucnlly only the loiiiis ;tll-/rr/;/.s, \)-(is, 

 \''^-cis, and 9, l'i-clir/.v were exjK'ciccl. In actuality, all the visual pig- 

 ments so lar analyzed possess instead the sterically hindered Il-r/5 

 isomer. This isomer is actually remarkably stable, since it seems to 

 lie in "a potential energy valley walled off by high potential barriers." 

 In other Avords, it recjuires a high activation energy either to lorm it 

 or to reisomerize it. When rhodopsin or a similar pigment is bleached, 

 however, and retinene is freed, it is in the form of the 'A\\-lrans isomer; 

 and before it can be reconverted to rhodopsin it must be reisomerized 

 to the ll-r/.v configuration. A cycle of cis-lrans reisomerization is 

 therefore an integral part of the visual pigment system. 



^Vald emphasizes that the only thing light does in any visual 

 system, so far as we know^ is to isomerize retinene. All other reactions 

 are ordinary "dark" reactions. A suggestive scheme to account for 

 the behavior of the system when bleached by light begins with W-cis 

 retinene firmly attached to a site on the opsin surface which it fits 

 sterically, and perhaps w ith the aldehyde group of retinene bonded in 

 a protonated Schift base linkage to an amino group of the protein. 

 Further assumptions must be made to allow for the shift of the 

 maximal absorption so far toward the red as it actually is (retinene, 

 380 ui/a; rhodopsin, 500 m^u.) . Absorption of a quantum of light by 

 rhodopsin isomerizes the retinene to the 2i\\-trans configuration. It 

 no longer fits the opsin, hydrolyzes off, and exposes reactive groups 

 that may trigger the visual excitation (Fig. 15) . 



If this explanation is correct, it woidd be expected that any agent 

 that isomerizes retinene and destroys the good fit between it and the 

 opsin Avoidd bleach the visual pigment. It tiuns out that this is so. 



Rhodopsin 



Lumirhodopsin 



Metorhodopsin 



Retinene 



/ >-20«C 



Bleaching 



Fig. IT). The action of liglit on riiodopsin. 1 he absoiption of light by rhodopsin 

 isomerizes its W-cis chiomophore to tlie all-/)7/?;.s configuration, yielding as first 

 product the M-trniis chromoprotein hiini-rhodopsin. This labilizes the ])rotcin, 

 opsin, which rearranges to a new configuration, yielding a second all-/iv/)/.s chromo- 

 protein, meta-rhodopsin. This second process exposes reactive groups on opsin 

 — two — SH gioups. and one proton-binding group, symbolized in the figiue with 

 a negative charge — and may be responsible for triggering \isual excitation, \erte- 

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

 and all-trans retinene, the process that corresponds to bleaching. 



