The enzyme sirohydrochlorin ferrochelatase (EC 4.99.1.4)

catalyzes Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycl ...

the following reaction: : siroheme + 2H⁺

\rightleftharpoons

sirohydrochlorin + Fe²⁺ This enzyme belongs to the family of

lyase In biochemistry, a lyase is an enzyme that catalyzes the breaking (an elimination reaction) of various chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bon ...

s, to be specific the "catch-all" class of lyases that do not fit into any other sub-class. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivial ...

of this enzyme class is siroheme ferro-lyase (sirohydrochlorin-forming). The enzyme is also known as SirB and present in all plants and nitrate and sulfate assimilating/dissimilating bacteria. Siroheme is a co-factor of both assimilatory and dissimilatory nitrite and sulfite reductases. Siroheme is synthesized from the central

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 ...

molecule uroporphyrinogen III, which forms the first branch-point of tetrapyrrole biosynthetic pathway, the other branch being the heme/chlorophyll branch. The siroheme branch consists of three steps:

methylation In the chemical sciences, methylation denotes the addition of a methyl group on a substrate, or the substitution of an atom (or group) by a methyl group. Methylation is a form of alkylation, with a methyl group replacing a hydrogen atom. These t ...

dehydrogenation In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. A ...

, and ferrochelation, with the last step carried out by sirohydrochlorin ferrochelatase. Sirohydrochlorin ferrochelatase is a class II chelatase, i.e. it does not require ATP for its activity unlike class I chelatases such as Mg-chelatase. In ''E. coli'', all three steps of siroheme biosynthesis are carried out by a single multifunctional enzyme called CysG, while in yeast ''Saccharomyces cerevisiae'' the last two steps are carried out by a bifunctional enzyme called Met8p. CysG and Met8p share common folds but are unrelated to SirB and constitute the so-called class III chelatase. SirB belongs to CbiX family protein and the plant SirB is half the length of bacterial SirB and aligns with its N- and C-terminal halves suggesting that the longer form evolved from the gene duplication and fusion of the shorter form. Sirohydrochlorin ferrochelatase in all

land plants The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as sist ...

and certain

green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ( Embryophytes) have emerged deep in the Charophyte alg ...

, but not

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

or other algae, consists of an

iron sulfur cluster Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in fro ...

, which can switch between Fe-2Sand Fe-4Sforms depending on the

redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...

status of the cellular milieu. Although it is not clearly determined what role this switching of the cluster might play, it is postulated to be involved in a critical redox regulation of siroheme biosynthesis.

References

EC 4.99.1 Enzymes of unknown structure {{enzyme-stub