Spheroidene is a

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

. It is a component of the

photosynthetic reaction center A photosynthetic reaction center is a complex of several proteins, biological pigments, and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from ...

of certain

purple bacteria Purple bacteria or purple photosynthetic bacteria are Gram-negative proteobacteria that are phototrophic, capable of producing their own food via photosynthesis. They are pigmented with bacteriochlorophyll ''a'' or ''b'', together with various ...

of the ''

Rhodospirillaceae The Rhodospirillaceae are a family of Pseudomonadota. The majority are purple bacteria, purple nonsulfur bacteria, producing energy through photosynthesis; originally all purple nonsulfur bacteria were included here.George M. Garrity, Don J. Bre ...

'' family, including ''

Rhodobacter sphaeroides ''Rhodobacter sphaeroides'' is a kind of purple bacterium; a group of bacteria that can obtain energy through photosynthesis. Its best growth conditions are anaerobic phototrophy ( photoheterotrophic and photoautotrophic) and aerobic chemoh ...

'' and '' Rhodopseudomonas sphaeroides''. Like other carotenoids, it is a tetraterpenoid. In purified form, it is a brick-red solid soluble in

benzene Benzene is an Organic compound, organic chemical compound with the Chemical formula#Molecular formula, molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar hexagonal Ring (chemistry), ring with one hyd ...

. Spheroidene was discovered by microbiologist C. B. van Niel, who named it "pigment Y". It was renamed by

Basil Weedon Professor Basil Charles Leicester Weedon CBE, FRS (18 July 1923 – 10 October 2003) was an organic chemist and university administrator. Using nuclear magnetic resonance spectroscopy, he was the first to map the structures of carotenoid pig ...

, who was the first to prepare it synthetically, and to determine its structure, in the mid-1960s.

Function

Spheroidene is bound to the type II photosynthetic reaction center of purple bacteria, and together with the

bacteriochlorophyll Bacteriochlorophylls (BChl) are photosynthetic pigments that occur in various phototrophic bacteria. They were discovered by C. B. van Niel in 1932. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacte ...

forms part of the

light-harvesting complex In biology, a light-harvesting complex or LHC is an aggregate consisting of proteins bound with chromophores (chlorophylls and carotenoids) that play a key role in photosynthesis. LHCs are arrayed around photosynthetic reaction centers in both pl ...

. Spheroidene has two major functions in the complex. First, it absorbs visible light in the blue-green part of the visible spectrum (320–500 nm), where bacteriochlorophyll has little absorbance. It then transfers energy to the bacteriochlorophyll via singlet–singlet energy transfer. In this manner the reaction center is able to harness more of the visible light spectrum than would be possible with bacteriochlorophyll alone. Second, spheroidene quenches excited singlet states of bacteriochlorophyll by forming a stable

triplet state In quantum mechanics, a triplet state, or spin triplet, is the quantum state of an object such as an electron, atom, or molecule, having a quantum spin ''S'' = 1. It has three allowed values of the spin's projection along a given axis ''m''S = � ...

. This quenching helps to prevent the formation of harmful

singlet oxygen Singlet oxygen, systematically named dioxygen(singlet) and dioxidene, is a gaseous inorganic chemistry, inorganic chemical with the formula O=O (also written as or ), which is in a quantum state where all electrons are Radical (chemistry), spin p ...

. Other functions of spheroidene may include scavenging of singlet oxygen, nonradiative dissipation of excess light energy, and structural stabilization of the photosystem proteins. Spheroidene is thought to exist as the 15,15'-''cis''

isomer In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element (chemistry), element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the exi ...

, and not the all-''trans'' isomer commonly shown in the literature, in native photosynthetic reaction centers.

Biosynthesis

The proteins involved in spheroidene biosynthesis are encoded by a

gene cluster A gene cluster is a group of two or more genes found within an organism's DNA that encode similar peptide, polypeptides or proteins which collectively share a generalized function and are often located within a few thousand base pairs of each othe ...

Geranylgeranyl pyrophosphate Geranylgeranyl pyrophosphate is an intermediate in the biosynthesis of diterpenes and diterpenoids. It is also the precursor to carotenoids, gibberellins, tocopherols, and chlorophylls. It is also a precursor to geranylgeranylated proteins, whic ...

(GGPP) is the precursor to spheroidene and the other carotenoids; two molecules of GGPP condense to form the symmetric tetraterpene

phytoene Phytoene () is a 40-carbon intermediate in the biosynthesis of carotenoids. The synthesis of phytoene is the first committed step in the synthesis of carotenoids in plants. Phytoene is produced from two molecules of geranylgeranyl pyrophosphate ( ...

. This molecule then undergoes three desaturations to form

neurosporene Neurosporene is a carotenoid pigment. It is an intermediate in the biosynthesis of lycopene Lycopene is an organic compound classified as a tetraterpene and a carotene. Lycopene (from the Neo-Latin '' Lycopersicon'', the name of a former toma ...

, which is then hydroxylated, desaturated again, and methoxylated to produce spheroidene. In some species, spheroidene is further oxygenated to produce the ketone spheroidenone.

References

{{reflist Carotenoids Photosynthetic pigments Methoxy compounds

Function

Biosynthesis

See also

References