Carotenoids are not only essential to the organisms that produce them - namely plants, and certain bacteria, algae and fungi - but are also required in animals as a source of pro-vitamin A (or retinol), even though they cannot produce the carotenoids themselves but must obtain them through their diet. Plants primarily use carotenoids as photosynthetic accessory pigments associated with chlorophyll in the membrane, with the yellow, orange and red carotenoid pigments absorbing light in the blue region. They act to regulate the flow of energy in the photosynthetic system by either contributing energy for use in photosynthesis through light absorption, or by removing excess energy from the system to modulate energy flow by quenching singlet oxygen (photoprotection). Carotenoids can also provide photoprotection in the animals that consume them, such as found with lutein in the human eye, where it serves to protect against UV damage.
Even though carotenoids play an important role in photosynthesis, leaves appear green because of the dominant chlorophyll pigment present – however, as temperatures begin to drop in autumn, the chlorophyll starts to break down, revealing the more dramatic colours of the carotenoid (and anthocyanin) pigments. Carotenoids are also visible in fruits and vegetables, such as carrots, tomatoes, peppers, cantaloupe, watermelon, guava and beetroot, to name but a few. Sometimes, the carotenoid pigments can be visible in the animals that ingest them. Flamingos have feathers ranging from pink to crimson, where the carotenoids obtained through the flamingos’ diet of water plants gives the feathers their colouration.
Over 600 carotenoids have been identified, displaying an enormous range of structural types. Such diversity arises from the ability of plants, algae, fungi and bacteria to produce specific carotenoids to meet their own individual requirements. However, only about 10% of these can be used in animals, primarily as precursors of vitamin A. As such, animals are limited to accumulating and modifying dietary carotenoids through a limited repertoire of metabolic transformations to meet their needs as best as they can. The most prominent carotenoids used by humans are alpha-carotene, lycopene, lutein, beta-cryptoxanthine, and especially beta-carotene.
In both plants and animals, carotenoids can be broken down into functionally important molecules, including apocarotenoids in plants that function as hormones, pigments, flavours, floral scents and defence compounds, and retinoids in animals that function as vitamins, visual pigments and signalling molecules. Carotenoids are cleaved by various enzymes, known collectively as carotenoid oxygenases.
In humans, beta-carotene can be cleaved into retinoids (including retinol, or vitamin A) in the liver and intestine by enzymes known as carotenoid oxygenases, using two pathways:
Ř Central cleavage using beta-carotene-15,15’-monooxygenase (EC 18.104.22.168), whereby beta-carotene is cleaved at its central 15,15’-double bond to yield two retinal molecules, which are then converted to two molecules of retinol (vitamin A).
Ř Eccentric (or asymmetric) cleavage that splits beta-carotene at double bonds other than the central one, yielding beta-apocarotenals of different chain lengths and carotenoic acids, which can then be converted to one molecule of retinol.
Under normal physiological conditions, central cleavage is the predominant pathway.