Vitamin A and Metabolites Pathway Suite

Vitamin A, or retinol, is essential for the proper functioning of the organism. Higher organisms cannot synthesize the vitamin and  so they obtain it from the diet as the provitamin or preformed vitamin A of plant or animal sources, respectively. Once taken up, it is processed and/or stored for future processing to serve a large spectrum of functions. Vitamin A, via 11-cis retinal underlies visual phototransduction – a complex signaling cascade in turn prompting others to relay the signal to centers in the brain where processing of visual perception and experience take place. For vision to be sustained, the all-trans isomerized chromophore must be converted back to the cis form which is accomplished through a series of enzyme-catalyzed reactions collectively known as the retinoid or visual cycle.
The retinoic acid metabolite signals through nuclear receptors that, once activated, function as transcription factors to regulate the expression of many genes. Retinol, retinoic acid metabolism and its coupled signaling, 11-cis retinal synthesis and regeneration and the visual phototransduction it initiates, the balance between uptake, storage and use of vitamin A and metabolites are intimately connected and collectively responsible for vitamin A homeostasis in the cell.

Retinol metabolic pathway

Retinoic acid metabolic pathway

Retinoic acid signaling pathway


Higher organisms cannot synthesize vitamin A. They derive it from the diet as the provitamin or preformed vitamin A of plant or animal sources, respectively. Once taken up, it is processed and/or stored for further processing to serve the needs of cells in various tissues.  Click here to explore this important metabolic pathway.
Retinoic acid signals through nuclear receptors to regulate the expression of many genes. All-trans retinoic acid is derived from the irreversible conversion of retinal, in turn derived from the reversible conversion of retinol taken up by cells from the circulation. Click here to explore the metabolic route to this central signaling molecule. Retinoic acid sets in motion nuclear receptors which, once activated, function as transcription factors to regulate the expression of hundreds of genes important for a broad spectrum of physiological processes. Click here to explore this very important signaling pathway.

Visual phototransduction pathway

 

Retinoid cycle metabolic pathway

 

Signaling in visual phototransduction, particularly the one in rod photoreceptor cells, has been extensively investigated and is considered a paradigm for the superfamily of seven trans-membrane (7TM) G-protein coupled receptor (GPCR) mediated cascades. Click here to explore the mechanistic details of this complex signaling network. The retinoid cycle metabolic pathway, also known as the visual cycle, represents the set of enzymatic reactions underlying the regeneration of the vitamin A-derived chromophore 11-cis retinal which assures the continued visual phototransduction response to light. Click here to explore this important metabolic pathway.