Vanabins
Encyclopedia
Vanabins are a specific group of vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

-binding metalloprotein
Metalloprotein
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. Indeed, about one quarter to one third of all proteins require metals to...

s. Vanabins are found almost exclusively in the blood cells, or vanadocytes of some ascidians and tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...

s (sea squirts). The vanabins extracted from tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...

 vanadocytes are often called hemovanadin
Hemovanadin
Hemovanadin is used to refer to the pale green vanabin proteins found in the blood cells, called vanadocytes, of ascidians and other organisms...

s. These organisms are able to concentrate vanadium to a level more than 100 times higher than in the surrounding seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

. Vanabin proteins seem to be involved in collecting and accumulating this metal ion. At present there is no conclusive understanding of why these organisms collect vanadium, and it remains a biological mystery.

German chemist Martin Henze discovered vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

 in ascidiacea
Ascidiacea
Ascidiacea is a class in the Tunicata subphylum of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid.Ascidians are found all over the world, usually in shallow...

ns in 1911.
Vanabins (also known as vanadium-associated proteins or vanadium chromagen) are a specific group of vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

-binding metalloprotein
Metalloprotein
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. Indeed, about one quarter to one third of all proteins require metals to...

s. Vanabins are found almost exclusively in the blood cells, or vanadocytes of some ascidians and tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...

s (sea squirts). The vanabins extracted from tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...

 vanadocytes are often called hemovanadin
Hemovanadin
Hemovanadin is used to refer to the pale green vanabin proteins found in the blood cells, called vanadocytes, of ascidians and other organisms...

s. These organisms are able to concentrate vanadium to a level more than 100 times higher than in the surrounding seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

. Vanabin proteins seem to be involved in collecting and accumulating this metal ion. At present there is no conclusive understanding of why these organisms collect vanadium, and it remains a biological mystery.

German chemist Martin Henze discovered vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

 in ascidiacea
Ascidiacea
Ascidiacea is a class in the Tunicata subphylum of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid.Ascidians are found all over the world, usually in shallow...

ns in 1911.
Vanabins (also known as vanadium-associated proteins or vanadium chromagen) are a specific group of vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

-binding metalloprotein
Metalloprotein
Metalloprotein is a generic term for a protein that contains a metal ion cofactor. Metalloproteins have many different functions in cells, such as enzymes, transport and storage proteins, and signal transduction proteins. Indeed, about one quarter to one third of all proteins require metals to...

s. Vanabins are found almost exclusively in the blood cells, or vanadocytes of some ascidians and tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...

s (sea squirts). The vanabins extracted from tunicate
Tunicate
Tunicates, also known as urochordates, are members of the subphylum Tunicata, previously known as Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor, others such...

 vanadocytes are often called hemovanadin
Hemovanadin
Hemovanadin is used to refer to the pale green vanabin proteins found in the blood cells, called vanadocytes, of ascidians and other organisms...

s. These organisms are able to concentrate vanadium to a level more than 100 times higher than in the surrounding seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

. Vanabin proteins seem to be involved in collecting and accumulating this metal ion. At present there is no conclusive understanding of why these organisms collect vanadium, and it remains a biological mystery.

German chemist Martin Henze discovered vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...

 in ascidiacea
Ascidiacea
Ascidiacea is a class in the Tunicata subphylum of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide tunicin, as compared to other tunicates which are less rigid.Ascidians are found all over the world, usually in shallow...

ns in 1911.Z. Physiol. Chem. 72, 494–50.Michibata H, Uyama T, Ueki T, Kanamori K (2002). Vanadocytes, cells hold the key to resolving the highly selective accumulation and reduction of vanadium in ascidians. Microscopy Research and Technique. Volume 56 Issue 6, Pages 421 - 434

Vanabins as oxygen carriers

Because of the high concentration of vanadium in the blood, it has been assumed that vanabins are used for oxygen transport
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...

 like iron-based hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

 or copper-based hemocyanin
Hemocyanin
Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule . Oxygenation causes a color change between the colorless Cu deoxygenated form and the blue Cu oxygenated form...

. Unfortunately no scientific evidence can be found to back this hypothesis
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

.

The highest concentration of vanadium found so far, 350 mM
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...

, was found in the blood cells of Ascidia gemmata belonging to the suborder Phlebobranchia
Phlebobranchia
Phlebobranchia is a suborder of sea squirts in the order Enterogona. The group includes both colonial and solitary animals. They are distinguished from other sea squirts by the presence of longitudinal vessels in the pharyngeal basket and by the gonads being surrounded by a loop of gut...

. This concentration is 10,000,000 times higher than that in seawater.

Vanabins accumulate vanadium in the blood cells and produce V(III) species and vandanyl ions (V(IV)) from orthovanadate ions (V(V)), with the use of NADPH
Nicotinamide adenine dinucleotide
Nicotinamide adenine dinucleotide, abbreviated NAD, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups. One nucleotide contains an adenine base and the other nicotinamide.In metabolism, NAD is involved...

 as a reducing agent
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

. Vanabins also transport the reduced vanadium species into the vacuoles of vanadocytes (vanadium-containing blood cells). The vacuoles are kept at a very acidic pH of 1.9, made possible by pumping hydrogen ions into the vacuoles by use of energy intensive H+-ATPase. All the use of NADPH and ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...

 to collect and maintain the vanadium is extremely energy intensive, unusual for oxygen transporting proteins.

Other oxygen transporting proteins have a very low dissociation constant
Dissociation constant
In chemistry, biochemistry, and pharmacology, a dissociation constant is a specific type of equilibrium constant that measures the propensity of a larger object to separate reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into...

 with their metal prosthetic group and bind these groups tightly. Vanabins on the other hand have a moderate dissociation constant and do not tightly bind vanadium. Most importantly, because of this moderate dissociation constant, vanadium is usually found free-floating and separated from any proteins inside the vacuoles. This is completely different from other oxygen transporting proteins.

Vanabins not oxygen carriers

The most convincing evidence against vanadium in use for oxygen transport is that many ascidians and tunicates also have hemocyanin
Hemocyanin
Hemocyanins are respiratory proteins in the form of metalloproteins containing two copper atoms that reversibly bind a single oxygen molecule . Oxygenation causes a color change between the colorless Cu deoxygenated form and the blue Cu oxygenated form...

 in their blood that could be assumed to handle all oxygen transport.

The use of vanabins and vanadium for oxygen transport in ascidians and tunicates is doubtful. Another hypothesized reason for these organisms collecting vanadium is to make themselves toxic to predators, parasites and microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

s.

Technological use of vanabins

The world oceans carry diluted concentrations of rare minerals. Total amount of gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

, silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

, manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

 and platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...

 (among others) in all the world's oceans rivals that of terrestrial mines
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...

, yet because of the low concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...

 of these minerals in sea water, extraction has been considered impractical. Understanding the mechanisms behind how some organisms concentrate rare minerals (such as siderophore
Siderophore
Siderophores are small, high-affinity iron chelating compounds secreted by grasses and microorganisms such as bacteria and fungi...

s) from sea water could make it biotechnologically
Biotechnology
Biotechnology is a field of applied biology that involves the use of living organisms and bioprocesses in engineering, technology, medicine and other fields requiring bioproducts. Biotechnology also utilizes these products for manufacturing purpose...

 possible to grow and farm organisms to filter sea water and extract these minerals, similar to bioleaching
Bioleaching
Bioleaching is the extraction of specific metals from their ores through the use of living organisms. This is much cleaner than the traditional heap leaching using cyanide...

.

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