The diameter of a typical α helix is about 12 Å, which matches the 12 Å wide and 6–8 Å deep major groove of B-form DNA. 

TLS Online TPP Program

#Id: 9300

#Unit 4. Cell Communication and Cell Signaling

Rhodopsin has two components: 

Opsin protein, member of the GPCR A family 

11-cis-retinal light-absorbing pigment

TLS Online TPP Program

#Id: 9301

#Unit 4. Cell Communication and Cell Signaling

TLS Online TPP Program

#Id: 9302

#Unit 4. Cell Communication and Cell Signaling

11-cis-retinal covalently attached to the amino group of lysine residue 296 in the opsin protein

TLS Online TPP Program

#Id: 9303

#Unit 4. Cell Communication and Cell Signaling

The heterotrimeric G protein coupled to rhodopsin, called transducin (Gt)

TLS Online TPP Program

#Id: 9304

#Unit 4. Cell Communication and Cell Signaling

In the absence of light,  bound 11-cis-retinal locks the rhodopsin into its inactive (R) conformation

TLS Online TPP Program

#Id: 9305

#Unit 4. Cell Communication and Cell Signaling

Absorption of light causes rapid photoisomerization of the bound cis-retinal to the all-trans isomer, triggering a conformational change in opsin that activate the Gαt.