[2Fe-2S] Designation of a two-iron, two-labile-sul-
fur CLUSTER in a protein, comprising two sulfido-
bridged iron atoms. The oxidation levels of the
clusters are indicated by adding the charges on the
iron and sulfide atoms, i.e., [2Fe-2S]2+; [2Fe-2S]+. The
alternative designation, which conforms to inorganic
chemical convention, is to include the charges on the
LIGANDs; this is more appropriate where the ligands
are other than the usual cysteine sulfurs, such as in
the RIESKE PROTEINs.
See alsoFERREDOXIN.
[4Fe-4S] Designation of a four-iron, four-labile-sul-
fur CLUSTERin a protein. (See[2FE-2S].) Possible oxi-
dation levels of the clusters are [4Fe-4S]3+; [4Fe-4S]2+;
[4Fe-4S]+.
See alsoFERREDOXIN; HIPIP.
feedback inhibition A way for the end product of a
cell’s biosynthetic pathway to stop the activity of the
first enzymes in that pathway, thereby controlling the
enzymatic activity; it stops the synthesis of the product.
Also called end-product inhibition.
FeMo cofactor An inorganic CLUSTERthat is found
in the FeMo protein of the molybdenum-NITROGENASE
and is essential for the catalytic reduction of N 2 to
ammonia. This cluster contains Fe, Mo, and S in a
7:1:9 ratio. The structure of the COFACTORwithin the
FeMo protein can be described in terms of two
cuboidal SUBUNITs, Fe 4 S 3 and MoFe 3 S 3 bridged by
three S2–ions and anchored to the protein by a histi-
dine bound via an imidazole group to the Mo atom
and by a cysteine bound via a deprotonated SH group
to an Fe atom of the Fe 4 S 3 subunit. The Mo atom at
the periphery of the molecule is six-COORDINATEand,
in addition to the three sulfido LIGANDs and the histi-
dine imidazole, is also bound to two oxygen atoms
from an (R)-homocitrate molecule.Fenton reaction
Fe2++ H 2 O 2 →Fe3++ OH.+ OH–
This equation describes the iron-salt-dependent
decomposition of dihydrogen peroxide, generating the
highly reactive hydroxyl radical, possibly via an oxo-
iron(IV) intermediate. Addition of a reducing agent
such as ascorbate leads to a cycle that increases the
damage to biological molecules.
See alsoHABER-WEISS REACTION.fermentation The anaerobic decomposition of com-
plex organic substances by microorganisms such as
bacteria, molds, or yeast (called ferments) on a fermen-
tation substrate that produces simpler substances or
other desired effects, such as the yielding of ethanol
and carbon dioxide from yeast for commercial pur-
poses, the production of ATP and energy production,
and the development of antibiotics and enzymes. Fer-
mentation is used by microflora of the large intestine to
break down indigestible carbohydrates.
Large fermentors are used to culture microorgan-
isms for the production of some commercially valuable
products such as bread, beer, wine, and other bever-
ages.ferredoxin A protein—containing more than one
iron and ACID-LABILE SULFIDE—that displays electron-
transfer activity but not classical ENZYMEfunction.
See alsoHIPIP.ferriheme An iron(III) PORPHYRIN COORDINATION
complex.ferritin An iron-storage protein consisting of a shell
of 24 protein SUBUNITs encapsulating up to 4,500 iron
atoms in the form of a hydrated iron(III) oxide.108 [2Fe-2S]