Bacteriorhodopsin is a protein used by
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, most notably
halobacteriaIn taxonomy, the Halobacteria are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. They are also called halophiles, though this name is also used for other organisms which live in somewhat less concentrated salt water...
. It acts as a
proton pumpA proton pump is an integral membrane protein that is capable of moving protons across the membrane of a cell, mitochondrion, or other subcellular compartment.-Function:...
, i.e. it captures light energy and uses it to move
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.
Bacteriorhodopsin is an
integral membrane proteinAn Integral Membrane Protein is a protein molecule that is permanently attached to the biological membrane...
usually found in two-dimensional crystalline patches known as "purple membrane", which can occupy up to nearly 50% of the surface area of the archaeal cell.
Bacteriorhodopsin is a protein used by
archaeaThe Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon . They have no cell nucleus or any other organelles within their cells...
, most notably
halobacteriaIn taxonomy, the Halobacteria are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. They are also called halophiles, though this name is also used for other organisms which live in somewhat less concentrated salt water...
. It acts as a
proton pumpA proton pump is an integral membrane protein that is capable of moving protons across the membrane of a cell, mitochondrion, or other subcellular compartment.-Function:...
, i.e. it captures light energy and uses it to move
protonThe proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom but is also stable by itself and has a second identity as the hydrogen ion, H
+...
s across the membrane out of the cell. The resulting proton gradient is subsequently converted into chemical energy.
Bacteriorhodopsin is an
integral membrane proteinAn Integral Membrane Protein is a protein molecule that is permanently attached to the biological membrane...
usually found in two-dimensional crystalline patches known as "purple membrane", which can occupy up to nearly 50% of the surface area of the archaeal cell. The repeating element of the hexagonal lattice is composed of three identical protein chains, each rotated by 120 degrees relative to the others. Each chain has seven
transmembrane alpha helicesTransmembrane domain usually denotes a single transmembrane alpha helix of a transmembrane protein. It is called a "domain" because an alpha-helix in a membrane can be folded independently from the rest of the protein, similar to domains of water-soluble proteins...
and contains one molecule of
retinalRetinal, also called retinaldehyde or vitamin A aldehyde, is one of the three forms of vitamin A. Retinal is a polyene chromophore, and bound to proteins called opsins, is the chemical basis of animal vision. Bound to proteins called type 1 rhodopsins, retinal allows certain microorganisms to...
buried deep within, the typical structure for
retinylidene proteinRetinylidene proteins are a family of proteins that use retinal as chromophore for light reception. Proteins of this family are also called opsins...
s.
It is the retinal molecule that changes its conformation when absorbing a
photonIn physics, a photon is an elementary particle, the quantum of the electromagnetic field and the basic "unit" of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
, resulting in a
conformational changeA macromolecule is usually flexible and dynamic. It can change its shape in response to changes in its environment or other factors; each possible shape is called a conformation, and a transition between them is called a conformational change...
of the surrounding protein and the proton pumping action. It is covalently linked to Lys216 in the
chromophoreA chromophore is part of a molecule responsible for its color.When a molecule absorbs certain wavelengths of visible light and transmits or reflects others, the molecule has a color. A chromophore is a region in a molecule where the energy difference between two different molecular orbitals falls...
by
Schiff baseA Schiff base , named after Hugo Schiff, is a functional group that contains a carbon-nitrogen double bond with the nitrogen atom connected to an aryl or alkyl group—but not hydrogen. Schiff bases are of the general formula R1R2C=N-R3, where R3 is an...
action. After photoisomerization of the retinal molecule, Asp85 becomes a proton acceptor of the donor proton from the retinal molecule. This releases a proton from a "holding site" into the extracellular side (EC) of the membrane. Reprotonation of the retinal molecule by Asp96 restores its original isomerized form. This results in a second proton being released to the EC side. Asp85 releases its proton into the "holding site" where a new cycle may begin.
The bacteriorhodopsin molecule is purple and is most efficient at absorbing green light (wavelength 500-650 nm, with the absorption maximum at 568 nm).
The three-dimensional
tertiary structureIn biochemistry and chemistry, the tertiary structure of a protein or any other macromolecule is its three-dimensional structure, as defined by the atomic coordinates.-Relationship to primary structure:...
of bacteriorhodopsin resembles that of
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
rhodopsinRhodopsin, also known as visual purple, is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision...
s, the
pigmentA pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...
s that sense light in the
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
. Rhodopsins also contain retinal; however, the functions of rhodopsin and bacteriorhodopsin are different and there is only slight
homologyIn evolutionary biology, homology refers to any similarity between characteristics of organisms that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology...
in their
amino acidAmino acids are molecules containing an amine group, a carboxylic acid group and one of the twenty R-groups. These molecules are particularly important in biochemistry, where this term refers to alpha-amino acids with the general formula H
2NCHRCOOH, where R is an organic substituent...
sequences. Both rhodopsin and bacteriorhodopsin belong to the 7TM receptor family of proteins, but rhodopsin is a G protein coupled receptor and bacteriorhodopsin is not. In the first use of
electron crystallographyElectron crystallography is a method to determine the arrangement of atoms in solids using an electron microscope. It can complement X-ray crystallography for studies of very small crystals , both inorganic, organic and proteins, such as membrane proteins, that cannot easily form the large...
to obtain an atomic-level
protein structureProteins are an important class of biological macromolecules present in all biological organisms, made up of such elements as carbon, hydrogen, nitrogen, oxygen, and sulphur. All proteins are polymers of amino acids. According to their physical size, proteins are nanoparticles...
, the structure of bacteriorhodopsin was resolved in 1990. It was then used as a template to build models of other G protein-coupled receptors before
crystallographic structuresIn mineralogy and crystallography, a crystal structure is a unique arrangement of atoms or molecules in a crystalline liquid or solid. A crystal structure is composed of a motif, a set of atoms arranged in a particular way, and a lattice exhibiting long-range order and symmetry...
were also available for these
proteinProteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...
s.
Many molecules have homology to bacteriorhodopsin, including the light-driven chloride pump
halorhodopsinHalorhodopsin is a light-driven ion pump, specific for chloride ions, and found in phylogenetically ancient archaea, known as halobacteria...
(for whom the crystal structure is also known), and some directly light-activated channels like
channelrhodopsinChannelrhodopsins are a subfamily of opsin proteins that function as light-gated ion channels. They serve as sensory photoreceptors in unicellular green algae, controlling phototaxis, i.e. movement in response to light. Expressed in cells of other organisms, they enable the use of light to control...
.
All other photosynthetic systems in bacteria, algae and plants use
chlorophyllChlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
s or
bacteriochlorophyllBacteriochlorophylls are photosynthetic pigments that occur in various phototrophic bacteria. They are related to chlorophylls, which are the primary pigments in plants, algae, and cyanobacteria. Groups that contain bacteriochlorophyll conduct photosynthesis, but do not produce oxygen. They use...
s rather than bacteriorhodopsin. These also produce a proton gradient, but in a quite different and more indirect way involving an electron transfer chain consisting of several other proteins. Furthermore, chlorophylls are aided in capturing light energy by other pigments known as "antennas"; these are not present in bacteriorhodopsin based systems. Lastly, chlorophyll-based photosynthesis is coupled to
carbon fixationCarbon fixation refers to any process through which gaseous carbon dioxide is converted into a solid compound. It mostly refers to the processes found in autotrophs , usually driven by photosynthesis, whereby carbon dioxide is changed into sugars...
(the incorporation of
carbon dioxideCarbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state...
into larger organic molecules); this is not true for bacteriorhodopsin-based system. It is thus likely that photosynthesis independently evolved at least twice, once in bacteria and once in archaea.
External links
- Calculated spatial positions of bacteriorhodopsin-like proteins in membrane