Fumarase (or fumarate hydratase) is an enzyme () that catalyzes the reversible hydration/dehydration of fumarate to malate. Fumarase comes in two forms: mitochondrial and cytosolic. The mitochondrial isoenzyme is involved in the Krebs cycle and the cytosolic isoenzyme is involved in the metabolism of amino acids and fumarate. Subcellular localization is established by the presence of a signal sequence on the amino terminus in the mitochondrial form, while subcellular localization in the cytosolic form is established by the absence of the signal sequence found in the mitochondrial variety.
This enzyme participates in 2 metabolic pathways: citric acid cycle and reductive citric acid cycle (CO2 fixation), and is also important in renal cell carcinoma. Mutations in this gene have been associated with the development of leiomyomas in the skin and uterus in combination with renal cell carcinoma (HLRCC syndrome).
Nomenclature
This enzyme belongs to the family of lyases, specifically the hydro-lyases, which cleave carbon-oxygen bonds. The systematic name of this enzyme class is (S)-malate hydro-lyase (fumarate-forming). Other names in common use include: fumarase L-malate hydro-lyase (S)-malate hydro-lyase
Structure
Gene
In humans, the FH gene is localized to the chromosomal position 1q42.3-q43. The FH gene contains 10 exons.
Protein
Crystal structures of fumarase C from Escherichia coli have been observed to have two dicarboxylate binding sites close to one another. These are known as the active site and the B site. These sites are connected by a series of hydrogen bonds and the access to either site is only through an opening near the enzyme surface near the B site. Active site is made up of three domains. Even when no ligand is bound to the active site, the binding pocket created by surrounding residues is sufficient to bind water in its place. Crystallographic research on the B site of the enzyme has observed that there is a shift on His129 between free and occupied states. It also suggests that the use of an imidazole-imidazolium conversion controls access to the allosteric B site.
Subtypes
There are two classes of fumarases, class I and class II. Classification depends on the arrangement of their relative subunits, their metal ion requirement, and their thermal stability. Class I fumarases are change state or become inactive when subjected to heat or radiation, are sensitive to superoxide anion, are iron (Fe2+) dependent, and are dimeric proteins with each subunit consisting of around 120 kD. Class II fumarases, found in prokaryotes as well as in eukaryotes, are tetrameric enzymes with subunits of 200 kD that contain three distinct segments of significantly homologous amino acids. They are also iron-independent and thermally stable. Prokaryotes are known to have three different forms of fumarase: Fumarase A, Fumarase B, and Fumarase C. Fumarase A and Fumarase B from Escherichia coli are classified as class I, whereas Fumarase C is a part of the class II fumarases.
Function
Mechanism
Figure 1 depicts the fumarase reaction mechanism. Two residues catalyze proton transfer and the ionization state of these residues is in part defined by two forms of the enzyme, E1 and E2. In E1, the groups exist in an internally neutralized AH/B: state, while in E2, they occur in a zwitterionic A−/BH+ state. E1 binds fumarate and facilitates its transformation into malate, and E2 binds malate and facilitates its transformation into fumarate. The two forms must undergo isomerization with each catalytic turnover.