Sulfite reductases are important enzyme

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...

s in the process of sulfur metabolism

Sulfur metabolism

Sulfur metabolism is vital for all living organisms as it is a constituent of a number of essential organic molecules like cysteine, methionine, coenzyme A, and iron-sulfur clusters. These compounds are involved in a number of essential cellular processes such as protein biosynthesis or the...

. They catalyze the reduction of sulfite

Sulfite

Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...

 to hydrogen sulfide

Hydrogen sulfide

Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...

 and water. Electrons for the reaction are provided by a dissociable molecule of either NADPH, bound flavins, or ferredoxin

Ferredoxin

Ferredoxins are iron-sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co...

s.

SO₃^2- (sulfite) + electron donor H₂S (hydrogen sulfide) + oxidized donor + 3 H₂O

Sulfite reductases belong to the family of oxidoreductase

Oxidoreductase

In biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule to another...

s, and are found in archaea

Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, fungi, and plants. They are grouped as either assimilatory or dissimilatory sulfite reductases depending on their function, their spectroscopic properties

Spectroscopy

Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...

, and their catalytic properties. This enzyme participates in selenoamino acid

Selenocysteine

Selenocysteine is an amino acid that is present in several enzymes .-Nomenclature:...

 metabolism and sulfur assimilation

Sulfur assimilation

Sulfur is an essential element for growth and physiological functioning of plants. However, its content strongly varies between plant species and it ranges from 0.1 to 6 % of the plants' dry weight. Sulfates taken up by the roots are the major sulfur source for growth, though it has to be reduced...

. It employs two covalently coupled cofactors

Cofactor (biochemistry)

A cofactor is a non-protein chemical compound that is bound to a protein and is required for the protein's biological activity. These proteins are commonly enzymes, and cofactors can be considered "helper molecules" that assist in biochemical transformations....

 - an iron sulfur cluster and a siroheme

Siroheme

Siroheme is a heme-like prosthetic group used by some enzymes to accomplish the six-electron reduction of sulfur and nitrogen...

- which deliver electrons to the substrate via this coupling.

The systematic name of this enzyme class is hydrogen-sulfide:acceptor oxidoreductase. Other names in common use include assimilatory sulfite reductase, assimilatory-type sulfite reductase, and hydrogen-sulfide:(acceptor) oxidoreductase.