In this article we will discuss about the biosynthesis of carotenoid for antioxidant.
Carotenoids are natural pigments, act as colouring agent and predominantly distributed among bacteria, plants and fungi including animals. Over 600 different carotenoids types are known to exist in nature. Their colour range from yellow to red with variations of brown and purple. The typical colours of flowers, fruits and vegetables determined by carotenoids pigments.
Various types of carotenoids like β-carotene, astraxanthin are well known for their antioxidant potential and are vital component of diet for human health. Genetic manipulation of pigmentation of fruits and flowers for colour production is currently an area of platform technology for colour production and arc attractive field for the production of potential antioxidants.
In nature, astraxanthin (3, 3′-dihydroxy β-carotene 4, 4′-dione) is a keto carotenoid believed to boost immune system in human and reduce oral cancer in animals due to their powerful antioxidant property. It is synthesised by bacteria and certain microalgae. Mann (2000) modified carotenoid biosynthesis pathway using metabolic engineering for the production of an economically important red pigment astraxanthin in unicellular green algae Haematococcus pluvialis.
In haematococcus algae the gene crto that encodes β-carotene ketolase, which convert β-carotene to cathaxanthin. Successful production of astraxanthin was accomplished by transferring p-carotene ketolase gene from the algae H. pluvialis to tobacco under the regulation of the tomato pds (phytoene desaturase) promoter.
The transit peptide of PDS from tomato was used to target CRTO polypeptide to plastids. Transgenic tobacco flower exhibit changing the colour of the nector from yellow to red, and it was a clear demonstration of manipulation of carotenoid pathway for the synthesis of coloured agent astraxanthin.
Beta-carotene is also a potential antioxidant belongs to carotenoid group. Carotenoids are isoprenoids which comprises of eight isoprene units. They are indispensible for human health. Carotenoids acts as precursor in the synthesis of vitamin A as vertebrates do not synthesise carotenoids and depend on dietary carotenoid sources for the synthesis of retinal (visual pigment) and retinol (vitamin A).
Carotenoids, as a major supplement for vitamin overcome β-carotene deficiency fighting diseases like diarrhaea, respiratory problem, and prevention of HIV and reducing the risk of coronory artery disease.
Biosynthesis of carotenoids is obtained from isopentanyl pyrophosphate, a common precursor for synthesis of all isoprenoids. Initially three molecules of acetyl CoA participates in isopentanyl pyrophosphate (IPP) synthesis via mevalonic acid.
In the subsequent step another common precursor geranyl geranyl pyrophosphate is produced by the condensation of dimethyl allyl pyrophosphate (DMAPP) with a molecule of IPP and it is followed by addition of two IIP units using GGPP synthase.
Reaction between two molecules of GGPP yields phytoene. This enzyme is dedicated in the biosynthesis of carotenoid biosynthetic pathway. The first C40 hydrocarbon and important intermediate in the biosynthesis of carotenoids phytoene undergoes series of desaturation reaction results in the formation of zeta-carotene (2-carotene) and lycopene.
Four desaturase reactions arc catalysed by two related enzymes: phytoene desaturase (PDS) and 2-carotene desaturases (ZDS). The enzyme introduces double bonds in the phytoene. However, in bacteria and fungi phytoene is converted into lycopene catalysed by single step with the help of single gene product. The pink colour lycopene once it is produced is then cyclizised to yield carotene with two types of rings B and E rings (Fig. 17.8B).
Lycopene undergoes cyclization into carotene is a branch point in carotene pathway. The enzyme lycopene-β-cyclases are responsible for cyclization of lycopene by introducing two β-rings at the ends of linear lycopene molecule. β-carotene with two β-rings is an key end product that serve as a precursor for several other carotenoids in plants like α-carotene, xanthophylls etc.
There have been reports on successful production of carotenoid production in heterologous microorganism by engineering mavalonate pathway to enhance supply of terpenoid precursor. Elucidation of carotenoid biosynthetic pathway in plants and cloning of relevant genes in plants pave the possibilities of carotene content.
Enhanced productions of carotenoids in plants are focused on increased precursor supply without disturbing balance of well controlled metabolic pathway. The key enzyme which is generally targeted for manipulations are phytoene synthase due to its role in directing substrate irreversibility to carotenoids. The constitutive expression of phytoene synthase resulted in increased carotenoid accumulation in transgenic tobacco.
Which generally do not accumulate carotene? Similarly, overexpression of the psy gene in tomato led to substantial accumulation in seed coat and cotyledon. The participation of this enzyme was confirmed by introducing antisense psy gene led to decrease level of this enzyme. Transgenic tomato plants expressing antisense psy gene resulted in reduced accumulation of carotenoids in fruits.
Considerable success in genetic mapping and content of carotenoid was accomplished when switched over to microorganism for choice of selective genes. Enhancement of β-carotene production by manipulation of desaturase activity was noticed in transgenic plants.
In addition, other relevant approaches like antisense can also be utilized for increasing β-carotene in plants. Antisense construct for β-4 hydroxylase which catalyse hydroxylation of β rings of β-carotene to form xanthophyll can be introduced to block this conversion to xanthophyll and thereby increasing β-carotene.