Vanabins (also known as vanadium-associated proteins or vanadium chromagen) are a group of

vanadium Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...

-binding proteins.

metalloprotein Metalloprotein is a generic term for a protein that contains a metal ion Cofactor (biochemistry), cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-bi ...

s. Vanabins are found almost exclusively in the

blood cells A blood cell, also called a hematopoietic cell, hemocyte, or hematocyte, is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), a ...

, or vanadocytes, of some

tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ca ...

s (sea squirts), including the

Ascidiacea Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" ...

. The vanabins extracted from

vanadocytes are often called

hemovanadin Hemovanadin is a pale green vanabin protein found in the blood cells, called vanadocytes, of ascidians (sea squirts) and other organisms (particularly sea organisms). It is one of the few known vanadium-containing proteins. The German chemist M ...

s. These organisms are able to concentrate vanadium from the surrounding

seawater Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...

, and vanabin proteins have been involved in collecting and accumulating this metal ion. At present there is no conclusive understanding of why these organisms collect vanadium.

Possible function

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 Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the ci ...

like iron-based

hemoglobin Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...

or copper-based

hemocyanin Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2) ...

. Unfortunately no scientific evidence can be found to back this

hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...

. The highest concentration of vanadium found so far, 350 mM, was found in the blood cells of '' Ascidia gemmata'' belonging to the

suborder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

Phlebobranchia Phlebobranchia is a suborder of sea squirts in the class Ascidiacea. Characteristics The group includes both colonial and solitary animals. They are distinguished from other sea squirts by the presence of longitudinal vessels in the pharynge ...

. 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 phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require NAD ...

as a

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth meta ...

. Vanabins also transport the reduced vanadium species into the vacuoles of

vanadocytes A vanadocyte is a specialized type of blood cell found in ascidians (tunicates). These cells are notable for their high levels of vanadium (concentrations 107 higher than that of seawater), which is typically a metabolic poison in other contexts. ...

(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 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 In chemistry, biochemistry, and pharmacology, a dissociation constant (K_D) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex fa ...

with their metal

prosthetic group A prosthetic group is the non-amino acid component that is part of the structure of the heteroproteins or conjugated proteins, being tightly linked to the apoprotein. Not to be confused with the cofactor that binds to the enzyme apoenzyme (eith ...

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. Another possible function is that vanabin containing organisms cleave the core out of

Chlorin In organic chemistry, chlorins are tetrapyrrole pigments that are partially hydrogenated porphyrins. The parent chlorin is an unstable compound which undergoes air oxidation to porphine. The name chlorin derives from chlorophyll. Chlorophylls a ...

based compounds such as the magnesium in

Chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

and replace it with their vanadium center in order to attach and harvest energy via Light Harvesting Complexes. It is believed that the bacteria contained in worm castings, ''

Rhodopseudomonas Palustris ''Rhodopseudomonas palustris'' is a rod-shaped, Gram-negative purple nonsulfur bacterium, notable for its ability to switch between four different modes of metabolism. ''R. palustris'' is found extensively in nature, and has been isolated from ...

'',

do this during its

Photoautotrophism Photoautotrophs are organisms that use light energy and inorganic carbon to produce organic materials. Eukaryotic photoautotrophs absorb energy through the chlorophyll molecules in their chloroplasts while prokaryotic photoautotrophs use chlorophyll ...

mode of metabolism. By this information it may be hypothesized that

Kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. Comprising an estimated 1016 tons of carbon, it is the most abundant source of organic compounds on earth, exceeding the total organic content of living matter 10,000-fold. It ...

is the result of Photoautotrophic bacteria using Vanabin as their method of attaching to chlorophyll.

Vanabins not oxygen carriers

The most convincing evidence against vanadium in use for oxygen transport is that many ascidians and tunicates also have

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

predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

s and

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

History

German chemist Martin Henze discovered

ascidiacea Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" ...

ns in 1911.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

References

External links

* {{Archive url, url=https://web.archive.org/web/20051218024255/http://www.sci.u-szeged.hu/kkk/vanadium4/Michibata.pdf, date=December 18, 2005, title=Characterization of Vanabins, Vanadium-Binding Proteins, Key Substances of Vanadium Accumulation in Ascidians
Transferring vanabin genes to bacteria
Metalloproteins Vanadium compounds Ascidiacea