inverse agonist (negative antagonist) A DRUGthat
acts at the same RECEPTORas that of an AGONIST, yet
produces an opposite effect.
See alsoANTAGONIST.
inversion A chromosomal aberration or mutation,
occurring during meiosis or from mutagens, that involves
detaching a chromosome segment, rotating it 180
degrees, and then reinserting it in its original location.
invertebrates Animals that do not have a backbone
or notochord. They are cold-blooded and have solid,
external skeletons or hydrostatic skeletons. They include
small protozoans, sponges, corals, tapeworms, spiders,
mollusks, and others. Most animals on the planet (95
percent of all animal species) are invertebrates, and the
majority of those, excepting insects, are aquatic.
in vitro fertilization Fertilization outside the body,
whereby a male sperm and female egg are combined in
a laboratory. The embryo is then artificially transferred
into a female’s uterus.
ion An atom that acquires a charge by either gaining
or losing an electron.
ion channel Enables ions to flow rapidly through
membranes in a thermodynamically downhill direc-
tion after an electrical or chemical impulse. Their
structures usually consist of four to six membrane-
spanning DOMAINs. This number determines the
size of the pore and thus the size of the ion to be
transported.
See alsoION PUMP.
ionic bond A chemical bond or link between two
atoms due to an attraction between oppositely charged
(positive-negative) ions.
ionophore A compound that can carry specific ions
through membranes of cells or organelles.
ion pump Enables ions to flow through membranes
in a thermodynamically uphill direction by the use of
anenergy source such as ATP or light. They consist of
sugar-containing heteropeptide assemblies that open
and close upon the binding and subsequent HYDROLYSIS
ofATP, usually transporting more than one ion toward
the outside or the inside of the membrane.
See alsoADENOSINE TRIPHOSPHATE;ION CHANNEL.iron-responsive element Aspecific base SEQUENCE
in certain MESSENGER RNAs that code for various pro-
teins of iron METABOLISM, which allows REGULATIONat
TRANSLATIONal level by the IRON-RESPONSIVE PROTEIN.iron-responsive protein (IRP) A protein that responds
to the level of iron in the cell and regulates the biosyn-
thesis of proteins of iron METABOLISMby binding to the
IRON-RESPONSIVE ELEMENTon MESSENGER RNA.
See alsoREGULATION.iron-sulfur cluster Aunit comprising two or more
iron atoms and bridging sulfide LIGANDs in an IRON-
SULFUR PROTEIN. The recommended designation of a
CLUSTERconsists of the iron and sulfide content, in
square brackets, e.g., [2FE–2S], [4FE–4S]. The possible
oxidation levels are indicated by the net charge exclud-
ing the ligands, for example a [4Fe–4S]2+or [4Fe–4S]1+
or [4Fe–4S]2+;1+cluster.
See alsoBRIDGING LIGAND.iron-sulfur proteins Proteins in which nonheme iron
is coordinated with cysteine sulfur and usually also with
inorganic sulfur. Divided into three major categories:
RUBREDOXINs; simple iron-sulfur proteins, containing
only IRON-SULFUR CLUSTERs; and complex iron-sulfur
proteins, containing additional active redox centers such
as FLAVIN, molybdenum, or HEME. In most iron-sulfur
proteins the clusters function as electron transfer groups,
but in others they have other functions such as catalysis
of hydratase/dehydratase reactions, maintenance of pro-
tein structure, or REGULATIONof activity.
See alsoCOORDINATION.IRP SeeIRON-RESPONSIVE PROTEIN.IRP 181