the crystal structure of parvalbumin containing six helices, A–F) in which certain amino
acids are invariant consists of two helices enclosing the Ca
binding loop. As a model,
the fore±nger and the thumb of the right hand can resemble the two helices (e.g. E and
binding domain of parvalbumin), and the bent mid±nger, the
enclosed loop, hence the EF-hand.
A transmembrane protein using the downhill gradient of one ion as an energy source
to transport another ion against its gradient across the membrane (e.g. Na
A transmembrane protein using ATP or another nucleotide triphosphate as an energy
source to transport an ion against its gradient across the membrane.
Intracellular small molecules (e.g. cyclic nucleotides or inositol polyphosphates) or ions
such as Ca
, indispensable for the transduction of signals converting extracellular
stimuli (e.g. hormones
primary messengers) into intracellular responses.
Calcium is of pivotal importance for many biological processes. It may have a rather
static, structure-stabilizing role, or it may participate as one of the second messengers
of the cell in signal transduction pathways, ful±lling a more dynamic function. This
is made possible by some speci±c properties of the Ca
ion (e.g. high dehydration
rate, great ﬂexibility in coordinating ligands, largely irregular geometry of the
coordination sphere). The control of calcium homeostasis is of central importance
to the organism, involving an exchange of the mineral between the skeleton (as the
major calcium reservoir), the intestine, and the kidney (as the organs of calcium
absorption or reuptake), from the extracellular ﬂuid (ECF) and intracellular calcium,
respectively. This highly integrated process consists of a number of hormonally
controlled feedback loops and an elaborate system of channels, exchanger, and
pumps to control Ca
ﬂuxes into and out of cells. This article describes the different
roles of calcium in the regulation of biological functions and the proteins involved
in these processes.
Calcium is one of the most common ele-
ments on earth, and it is the ±fth most
abundant element of the human body.
Next to its central role in cellular func-
tions as one of the second messengers,
calcium is a major constituent of the
skeleton. It has a stabilizing function in
shells, bones, and teeth and therefore is an
old component of organisms, the record