primitive change One of the conceptually simpler
molecular changes into which an ELEMENTARY REAC-
TIONcan be notionally dissected. Such changes include
BOND rupture, bond formation, internal rotation,
change of bond length or bond angle, bond
MIGRATION, redistribution of charge, etc.
The concept of primitive changes is helpful in the
detailed verbal description of elementary reactions, but
a primitive change does not represent a process that is
by itself necessarily observable as a component of an
elementary reaction.
prior equilibrium SeePRE-EQUILIBRIUM.
probability The statistical measure of likelihood.
prodrug Any compound that undergoes BIOTRANS-
FORMATION before exhibiting its pharmacological
effects. Prodrugs can thus be viewed as DRUGs contain-
ing specialized nontoxic protective groups used in a
transient manner to alter or to eliminate undesirable
properties in the parent molecule.
See alsoDOUBLE PRODRUG.
product-determining step The step of a STEPWISE
REACTIONin which the product distribution is deter-
mined. The product-determining step may be identical
to, or occur later than, the RATE-CONTROLLING STEPon
the REACTION COORDINATE.
product-development control The term is used for
reactions under KINETIC CONTROLwhere the SELECTIV-
ITYparallels the relative (thermodynamic) stabilities of
the products. Product-development control is usually
associated with a TRANSITION STATEoccurring late on
the REACTION COORDINATE.
See alsoSTERIC-APPROACH CONTROL; THERMODY-
NAMIC CONTROL.
promoter The DNA region, usually upstream to the
coding SEQUENCEof a GENEor OPERON, that binds and
directs RNA polymerase to the correct transcriptional
start site and thus permits TRANSCRIPTIONat a specific
initiation site. (In catalysis, a promoter is used differ-
ently: a cocatalyst is usually present in much smaller
amounts than the catalyst.)promotion SeePSEUDOCATALYSIS.propagation SeeCHAIN REACTION.prosthetic group A tightly bound, specific non-
polypeptide unit in a protein that is determining and
that is involved in the protein’s biological activity.
See alsoCOFACTOR.protein A molecule composed of many amino acids
and with a complex structure, e.g., immunoglobulin,
casein, etc.
See alsoAMINO ACID.protein phosphatase(phosphoprotein phosphatase)
An enzyme that removes a phosphate group from a
protein by the use of hydrolysis; opposite effect of a
protein kinase.proteoglycan A type of glycoprotein with high car-
bohydrate content; component in the extracellular
matrix of animal cells. Composed of one or more gly-
cosaminoglycans; large, long polysaccharide chains
covalently linked to protein cores.proteomics The study of proteins—their location,
structure, and function—through the combination of
high-resolution protein separation techniques with
mass spectrometry and modern sequence database min-
ing tools.protic SeePROTOGENIC SOLVENT.protogenic solvent Capable of acting as a PROTON
(HYDRON) donor that is strongly or weakly acidic (asprotogenic solvent 221