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 insects and other animals. Protoporphyrin and related compounds can also give red-brown colours but are photosensitizers, so rare except in cases like shells.
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 share chlorophyll a, a phytochlorin. These are accompanied by various other light-gathering pigments.
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 lycopene, γ-carotene, and β-carotene as both common types and successive precursors for other acyclic, monocyclic, and bicyclic types. The rings are tilted and so twist the double bonds from the plane of the others, shifting the colour from red in lycopene to orange-yellow.
Most other types are polar including a variety of hydroxy-carotenoids, for instance yellow zeaxanthin in many groups, and keto-carotenoids, like red-orange canthaxanthin and astaxanthins. The last are especially common in animals, which accumulate or modify carotenoids from their diet, often as esters or in variously coloured protein complexes.
Cyclic carotenoids also serve as precursors for retinal, which combines with proteins called opsins to form light-sensing pigments in halobacteria, some flagellate algae, and the eyes of animals, among others. Depending on the opsin the absorption can be anywhere from red to ultraviolet, with different colours visible to different groups.