Siroheme (or sirohaem) is a
heme
Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver.
In biochemical terms, heme is a coordination complex "consist ...
-like
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 ...
at the active sites of some
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...
s to accomplish the six-
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
reduction of
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formul ...
and
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at sevent ...
. It is a cofactor at the active site of
sulfite reductase, which plays a major role in
sulfur assimilation
Sulfur assimilation is the process by which organisms obtain sulfur, an essential element for growth and metabolism of most organisms. Biologically, sulfur is often encountered in its most oxidized form as sulfate or the most reduced form as hy ...
pathway, converting
sulfite
Sulfites or sulphites are compounds that contain the sulfite ion (or the sulfate(IV) ion, from its correct systematic name), . The sulfite ion is the conjugate base of bisulfite. Although its acid (sulfurous acid) is elusive, its salts are wide ...
into
sulfide
Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
, which can be incorporated into the organic compound
homocysteine
Homocysteine is a non-proteinogenic α-amino acid. It is a homologue of the amino acid cysteine, differing by an additional methylene bridge (-CH2-). It is biosynthesized from methionine by the removal of its terminal Cε methyl group. In th ...
.
Biosynthesis
Like all
tetrapyrrole
Tetrapyrroles are a class of chemical compounds that contain four pyrrole or pyrrole-like rings. The pyrrole/pyrrole derivatives are linked by ( =- or -- units), in either a linear or a cyclic fashion. Pyrroles are a five-atom ring with four ca ...
s, the macrocyclic ligand in siroheme is derived from
uroporphyrinogen III
Uroporphyrinogen III is a tetrapyrrole, the first macrocyclic intermediate in the biosynthesis of heme, chlorophyll, vitamin B12, and siroheme. It is a colorless compound, like other porphyrinogens.
Structure
The molecular structure of uroporp ...
. This
porphyrinogen
In biochemistry a porphyrinogen is a member of a class of naturally occurring compounds with a tetrapyrrole core, a macrocycle of four pyrrole rings connected by four methylene bridges. is methylated at two adjacent pyrrole rings to give
dihydrosirohydrochlorin, which is subsequently oxidized to give
sirohydrochlorin. A ferrochelatase then inserts iron into the macrocycle to give siroheme.
See also
*
Ferredoxin-nitrite reductase
*
Hydrogensulfite reductase
*
Nitrite reductase (NAD(P)H)
References
Further reading
*
Iron enzymes
Sulfur enzymes
Sulfur metabolism
Tetrapyrroles
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