Introduction to Human Nutrition

138 Introduction to Human Nutrition

CH 2 OH

CH 3

H 3 C CH 3 CH^3 CH^3

all-trans-Retinol

CH 3

H 3 C CH 3 CH^3

H 3 C

11-cis-Retinol CH 2 OH

CH 3

H 3 C CH 3 CH^3

H 3 C

11-cis-Retinaldehyde HCO

+H 3 N

NH

C O

N

NH

C O

Lysine residue in opsin

CH 3

H 3 C CH 3 CH^3

H 3 C

Rhodopsin HC

(visual purple)

N

NH

C O

CH 3

H 3 C CH 3 H

C

CH 3 CH 3

Photorhodopsin

Bathorhodopsin

Lumirhodopsin

Metarhodopsin I

Metarhodopsin II

Metarhodopsin III

Opsin

H

C

CH 3

H 3 C CH 3 CH^3 CH^3

all-trans-Retinaldehyde

Light 10 –15 s

45 ps

30 ns

75 μs

10 ms

min

GTP

GDP

Transducin-GDP

Transducin-GTP

Pi

Inactive

phosphodiesterase

Active

phosphodiesterase

cGMP

Na+ channel open

5'GMP

Na+ channel closed

GTPase

O

Figure 8.3 Role of vitamin A and the cyclic GMP cascade in the visual cycle. Retinol isomerase (EC 5.2.1.3), phosphodiesterase (EC 3.1.4.35).