Vanadium nitrogenase
Encyclopedia
Vanadium nitrogenase is a key enzyme
for nitrogen fixation
found in nitrogen-fixing bacteria, and is used as an alternative to molybdenum nitrogenase
when molybdenum
is unavailable. An important component of the nitrogen cycle
, vanadium nitrogenase converts nitrogen
gas to ammonia, thereby making otherwise inaccessible nitrogen available to plants. Unlike molybdenum nitrogenase, vanadium nitrogenase can also reduce carbon monoxide
to ethylene
, ethane
and propane
but both enzymes can reduce proton
s to hydrogen gas and acetylene
to ethylene
.
as well as the species Rhodopseudomonas palustris
and Anabaena variabilis
. Most of the functions of vanadium nitrogenase match those of the more common molybdenum nitrogenase
s and serve as an alternative pathway for nitrogen fixation
in molybdenum
deficient conditions. Like molybdenum nitrogenase, dihydrogen functions as a competitive inhibitor
and carbon monoxide
functions as a non-competitive inhibitor
of nitrogen fixation. Vanadium nitrogenase has an α2β2Ύ2 subunit structure
while molybdenum nitrogenase has an α2β2 structure. Though the structural genes encoding vanadium nitrogenase show only about 15% conservation with molybdenum nitrogenases, the two nitrogenases share the same type of redox centers. At room temperature, vanadium nitrogenase is less efficient at fixing nitrogen than moldybdenum nitrogenases because it converts more H+
to H2 as a side reaction. However, at low temperatures vanadium nitrogenases have been found to be more active than the molybdenum type, and at temperatures as low as 5 °C its nitrogen-fixing activity is 10 times higher than that of molybdenum nitrogenase. Like molybdenum nitrogenase, vanadium nitrogenase is easily oxidized and is thus only active under anaerobic conditions. Various bacteria employ complex protection mechanisms to avoid oxygen
.
The overall stoichiometry of nitrogen fixation catalyzed by vanadium nitrogenase can be summarized as follows :
showed the ability of vanadium nitrogenase to convert carbon monoxide
into trace amounts of propane
, ethylene
, and
ethane
in the absence of nitrogen through the reduction of carbon monoxide by dithionite
and ATP
hydrolysis . The process of forming
these hydrocarbons is carried out through proton
and electron
transfer in
which short carbon chains are formed and may ultimately allow the production of hydrocarbon fuel from CO
at an industrial scale.
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
for nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
found in nitrogen-fixing bacteria, and is used as an alternative to molybdenum nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
when molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
is unavailable. An important component of the nitrogen cycle
Nitrogen cycle
The nitrogen cycle is the process by which nitrogen is converted between its various chemical forms. This transformation can be carried out by both biological and non-biological processes. Important processes in the nitrogen cycle include fixation, mineralization, nitrification, and denitrification...
, vanadium nitrogenase converts nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
gas to ammonia, thereby making otherwise inaccessible nitrogen available to plants. Unlike molybdenum nitrogenase, vanadium nitrogenase can also reduce carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
to ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
and propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
but both enzymes can reduce proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s to hydrogen gas and acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
to ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
.
Biological Functions
Vanadium nitrogenases are found in members of the bacterial genus AzotobacterAzotobacter
Azotobacter is a genus of usually motile, oval or spherical bacteria that form thick-walled cysts and may produce large quantities of capsular slime. They are aerobic, free-living soil microbes which play an important role in the nitrogen cycle in nature, binding atmospheric nitrogen, which is...
as well as the species Rhodopseudomonas palustris
Rhodopseudomonas palustris
Rhodopseudomonas palustris is a gram-negative purple non-sulfur bacteria, notable for its ability to switch between four different modes of metabolism....
and Anabaena variabilis
Anabaena
Anabaena is a genus of filamentous cyanobacteria that exists as plankton. It is known for its nitrogen fixing abilities, and they form symbiotic relationships with certain plants, such as the mosquito fern. They are one of four genera of cyanobacteria that produce neurotoxins, which are harmful to...
. Most of the functions of vanadium nitrogenase match those of the more common molybdenum nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
s and serve as an alternative pathway for nitrogen fixation
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
in molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
deficient conditions. Like molybdenum nitrogenase, dihydrogen functions as a competitive inhibitor
Competitive inhibition
Competitive inhibition is a form of enzyme inhibition where binding of the inhibitor to the active site on the enzyme prevents binding of the substrate and vice versa.-Mechanism:...
and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
functions as a non-competitive inhibitor
Non-competitive inhibition
Non-competitive inhibition is a type of enzyme inhibition where the inhibitor reduces the activity of the enzyme, by binding not to the active site on the enzyme, but to a different site...
of nitrogen fixation. Vanadium nitrogenase has an α2β2Ύ2 subunit structure
Protein subunit
In structural biology, a protein subunit or subunit protein is a single protein molecule that assembles with other protein molecules to form a protein complex: a multimeric or oligomeric protein. Many naturally occurring proteins and enzymes are multimeric...
while molybdenum nitrogenase has an α2β2 structure. Though the structural genes encoding vanadium nitrogenase show only about 15% conservation with molybdenum nitrogenases, the two nitrogenases share the same type of redox centers. At room temperature, vanadium nitrogenase is less efficient at fixing nitrogen than moldybdenum nitrogenases because it converts more H+
Hydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
to H2 as a side reaction. However, at low temperatures vanadium nitrogenases have been found to be more active than the molybdenum type, and at temperatures as low as 5 °C its nitrogen-fixing activity is 10 times higher than that of molybdenum nitrogenase. Like molybdenum nitrogenase, vanadium nitrogenase is easily oxidized and is thus only active under anaerobic conditions. Various bacteria employ complex protection mechanisms to avoid oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
.
The overall stoichiometry of nitrogen fixation catalyzed by vanadium nitrogenase can be summarized as follows :
- N2 + 12e− + 14H+ + 24MgATP → 2NH4+ + 3H2 + 24MgADP + 24HPO42−
Carbon Monoxide Reduction
Research at the University of California Irvineshowed the ability of vanadium nitrogenase to convert carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
into trace amounts of propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
, ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, and
ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
in the absence of nitrogen through the reduction of carbon monoxide by dithionite
Dithionite
The dithionite anion , is an oxoanion of sulfur formally derived from dithionous acid, H2S2O4.-Chemistry:Dithionous acid has not been detected either as a pure compound or in solution....
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...
hydrolysis . The process of forming
these hydrocarbons is carried out through proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
and electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
transfer in
which short carbon chains are formed and may ultimately allow the production of hydrocarbon fuel from CO
at an industrial scale.