enzyme induction The process whereby an
(inducible) ENZYMEis synthesized in response to a spe-
cific inducer molecule. The inducer molecule (often a
substrate that needs the catalytic activity of the
inducible enzyme for its METABOLISM) combines with a
repressor and thereby prevents the blocking of an oper-
ator by the repressor, leading to the translation of the
gene for the enzyme. An inducible enzyme is one whose
synthesis does not occur unless a specific chemical
(inducer) is present, which is often the substrate of that
enzyme.
enzyme repression The mode by which the synthesis
of an ENZYMEis prevented by repressor molecules.
In many cases, the end product of a synthesis chain
(e.g., an amino acid) acts as a feedback corepressor by
combining with an intracellular aporepressor protein,
so that this complex is able to block the function of an
operator. As a result, the whole operation is prevented
from being transcribed into mRNA, and the expression
of all enzymes necessary for the synthesis of the end
product enzyme is abolished.
epimer A DIASTEREOISOMER that has the opposite
CONFIGURATIONat only one of two or more tetrahedral
“stereogenic” centers present in the respective MOLECU-
LAR ENTITY.
epimerization Interconversion of EPIMERs by reversal
of the configuration at one of the “stereogenic” centers.
epitope An alternative term for an antigenic determi-
nant. These are particular chemical groups on a
molecule that are antigenic, i.e., that elicit a specific
immune response. Also called antigenic determinant.epoch A period or date in time, shorter than or part
of an era, and used in geological time tables and to
mark historical events, for example. Usually refers to
an event (mountain building, appearance of a species,
etc.). Also called a series.
See alsoGEOLOGICAL TIME.EPR See ELECTRON PARAMAGNETIC RESONANCE
SPECTROSCOPY.equatorial bonds Those bonds that radiate outwardly
from the center of a molecule around its equator.equilibrium The ideal state in a system where oppos-
ing forces or rates are in balance, resulting in the occur-
rence of no net change.equilibrium, chemical Reversible processes can be
made to proceed in the forward or reverse direction by
the (infinitesimal) change of one variable, ultimately
reaching a point where the rates in both directions are
identical, so that the system gives the appearance of
having a static composition at which the Gibbs energy,
G, is a minimum. At equilibrium, the sum of the chemi-
cal potentials of the reactants equals that of the prod-
ucts, so that
Gr= Gr^0 + RT ln K = 0
Gr^0 = –RT ln K
The equilibrium constant, K, is given by the MASS-LAW
EFFECT.equilibrium constant SeeACIDITY CONSTANT; STA-
BILITY CONSTANT.equilibrium control See THERMODYNAMIC CON-
TROL.equilibrium control 99Enzyme action. In general, an enzyme catalyzes only one reaction
type (reaction specificity) and operates on only a narrow range of
substrates (substrate specificity).