Tetrapyrroles - Carotenoids
Tetrapyrroles are important both in absorbing light and forming metal complexes, including some of the key compounds making energy available to cells. Most are derived via protoporphyrin. This combines with ferrous ions to form heme, which is found throughout the living world as the prosthetic group of various proteins.
The most widespread of these are cytochromes, where heme or derivatives are involved in carrying electrons between metabolism and inorganic agents like oxygen. It also appears in catalases and peroxidases, enzymes to break down peroxide ions, and in many vertebrates, worms, and other animals in red hemoglobins that transport or store oxygen.
Breakdown of heme produces biliverdin. This is common as a waste product, but is also the active group in the light-sensing proteins of some bacteria, a precursor of other bilins, and a blue-green pigment in various animals. Protoporphyrin and related compounds can also be red-brown pigments, but much less often since they act as photosensitizers.
Most photosynthesis relies on green tetrapyrroles with magnesium ions as primary antenna pigments, which transfer light energy to electrons. Anoxygenic bacteria have bacteriochlorophyll aP or other bacteriochlorins, while cyanobacteria, plants, and other algae have chlorophyll a, a phytochlorin. These are accompanied by other light-gathering pigments of varying sorts.
Carotenoids are a large group of pigments derived as terpene dimers. They occur in all photosynthetic organisms, where they are critical in quenching excess energy absorbed by the antenna pigments but may also aid in light-gathering. They are also found as antioxidants in a variety of aerobes, and are a major part of the visual appearance of many organisms.
The majority of carotenoids have 40 carbon atoms, with red lycopene, red-orange γ-carotene, and yellow-orange β-carotene serving as successive precursors for most acyclic, monocyclic, and bicyclic types. These are also precursors for retinal, which combines with proteins to form light-sensing pigments in the eyes of animals, some flagellate algae, and halobacteria.
Most other types are polar, including hydroxy-carotenoids like yellow zeaxanthin and many more specific to different lineages, and redder keto-carotenoids like canthaxanthin and astaxanthins. The latter are especially common in animals, which rely on diet for carotenoids but convert them to different forms, often as esters or variously coloured protein complexes.