[4FE-4S], HIPIP, IRON-SULFUR CLUSTER, FEMO COFAC-
TOR, FERREDOXIN, FERRITIN, METALLOTHIONEIN,
NITROGENASE, and RIESKE IRON-SULFUR PROTEIN.
coagulation The clotting of blood.
coal A solid that is composed of tightly bound three-
dimensional networks of hydrocarbon chains and rings,
formed from the decomposition of living things and com-
pressed under high pressure. Used for fuel and heating.
cobalamin Vitamin B12. A vitamin synthesized by
microorganisms and conserved in animals in the liver.
Deficiency or collective uptake of vitamin B12 leads to
pernicious ANEMIA. Cobalamin is a substituted
CORRIN–Co(III) complex in which the cobalt atom is
bound to the four nitrogen atoms of the corrin ring, an
axial group R, and 5,6-dimethylbenzimidazole.The lat-
ter is linked to the cobalt by the N-3 nitrogen atom and
is bound to the C-1 carbon of a ribose molecule by the
N-1 nitrogen atom. Various forms of the vitamin are
known with different R groups such as RCN,
cyanocobalamin; ROH, hydroxocobalamin; RCH 3 ,
methylcobalamin; Radenosyl, COENZYMEB12.
codon A sequence of three consecutive NUCLEOTIDEs
that occurs in MRNA and directs the incorporation of a
specific amino acid into a protein, or represents the
starting or termination signal of protein synthesis.
coefficient of variation A measure of dispersion
around the mean (average).
coenzyme A low-molecular-weight, nonprotein organic
compound (often a NUCLEOTIDE) participating in enzy-
matic reactions as a dissociable acceptor or donor of
chemical groups or electrons.
See alsoENZYME.
cofactor An organic molecule or ion (usually a metal
ion) that is required by an ENZYMEfor its activity. It
can be attached either loosely (COENZYME) or tightly
(PROSTHETIC GROUP).cohesion The force of attraction between molecules
of the same substance that allows them to bind.cohesive force Force that enables something to stick
to itself.coke A hard, dry substance containing carbon that is
produced by heating bituminous coal to a very high
temperature in the absence of air. Used as a fuel for
iron and steel foundries.collagen The most abundant fibrous protein in the
human body (about 30 percent) and the animal king-
dom; shapes the structure of tendons, bones, and con-
nective tissues. There are several types (I, II, III, IV)
that are found in bone, skin, tendons, cartilage, embry-
onic tissues, and basement membranes.colligation The formation of a covalent bond by the
combination or recombination of two RADICALs (the
reverse of unimolecular HOMOLYSIS). For example:
HO.+ H 3 C.→CH 3 OH
See alsoMOLECULARITY.colligative properties Physical properties of solu-
tions that depend on the concentration of solute
molecules or ions in a solution, but not on the kind or
identity of solute particles present in the solution.collision theory A theory of reaction rates that
assumes that molecules must collide in order to react.colloid A heterogeneous mixture in which very small
particles of a substance are dispersed in another
medium. Although sometimes referred to as colloidal
solutions, the dispersed particles are typically muchcolloid 53