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ζ-Carotene (zeta-carotene) is a carotenoid. It is different from α-carotene and β-carotene because it is acyclic. [ 1 ] ζ-Carotene is similar in structure to lycopene , but has an additional 4 hydrogen atoms. ζ-carotene can be used as an intermediate in forming β-carotene. [ 2 ]
β-Carotene is composed of two retinyl groups, and is broken down in the mucosa of the human small intestine by β-carotene 15,15'-monooxygenase to retinal,a form of vitamin A. β-Carotene can be stored in the liver and body fat and converted to retinal as needed, thus making it a form of vitamin A for humans and some other mammals.
The central double bond of this tri-cis-ζ-carotene is isomerized by the zeta-carotene isomerase Z-ISO and the resulting 9,9'-di-cis-ζ-carotene is dehydrogenated again via a ζ-carotene desaturase (ZDS). This again introduces two double bonds, resulting in 7,9,7’,9’-tetra-cis-lycopene.
9,9'-dicis-zeta-carotene desaturase (EC 1.3.5.6, zeta-carotene desaturase, ZDS) is an enzyme with systematic name 9,9'-dicis-zeta-corotene:quinone oxidoreductase.
First reported in 1953, it was discovered to occur in small quantities when the fungus Phycomyces blakeseeanus was grown with diphenylamine, a compound that inhibits the synthesis of beta-carotene. [ 2 ]
PDS converts 15-cis-phytoene into 9,15,9'-tri-cis-ζ-carotene through reduction of the enzymes non-covalently bound FAD cofactor. [6] This conversion introduces two additional double bonds at positions 11 and 11' of the carbon chain and isomerizes two adjacent already existing double bonds at positions 9 and 9' from trans to cis .
Phytoene desaturase (zeta-carotene-forming) (EC 1.3.99.29, CrtIa, 2-step phytoene desaturase (ambiguous), two-step phytoene desaturase (ambiguous)) is an enzyme with systematic name 15-cis-phytoene:acceptor oxidoreductase (zeta-carotene-forming). [1] This enzyme catalyses the following chemical reaction
ζ-Carotene isomerase (EC 5.2.1.12, Z-ISO, 15-cis-zeta-carotene isomerase) is an enzyme with systematic name 9,15,9'-tricis-zeta-carotene cis-trans-isomerase. [ 1 ] [ 2 ] This enzyme catalyses the following chemical reaction