substitution reaction A reaction in which atoms are
replaced by other atoms.
substrate A substrate can be a chemical species of par-
ticular interest, of which the reaction with some other
chemical reagent is under observation (e.g., a compound
that is transformed under the influence of a catalyst). It
can also be the chemical entity whose conversion to a
product or products is catalyzed by an ENZYME, or it
could be a solution or dry mixture containing all ingredi-
ents necessary for the growth of a microbial culture or
for product formation. Finally, it can be a component in
the nutrient medium, supplying the organisms with car-
bon (C-substrate), nitrogen (N-substrate), etc.
substrate-level phosphorylation The formation or
synthesis of the energy source ATP(adenosine triphos-
phate) by transferring an inorganic phosphate group to
ADP (adenosine diphosphate).
subunit An individual polypeptide chain in a protein
containing more than one polypeptide chain. Different
types of subunits are frequently designated by α, β, γ, etc.
successor complex The radical ion pair that forms
by the transfer of an electron from the donor D to the
acceptor A after these species have diffused together to
form the PRECURSORor ENCOUNTER COMPLEX:
A + D →(A D) →(A.–D.+)sugar An organic compound that has the general
chemical formula (CH 2 O)n, also called saccharides. All
sugars are carbohydrates, but not all carbohydrates are
called sugars, e.g., starch, cellulose.
suicide inhibition SeeMECHANISM-BASED INHIBITION.
sulfite reductase ENZYMESthat catalyze the reduc-
tion of sulfite to sulfide. All known enzymes of this
type contain SIROHEMEand IRON-SULFUR CLUSTERs.
superacid A medium having a high ACIDITY, gener-
ally greater than that of 100 wt.% sulfuric acid. The
common superacids are made by dissolving a powerful
LEWIS ACID(e.g., SbF 5 ) in a suitable BRONSTED ACID
such as HF or HSO 3 F. (An equimolar mixture of
HSO 3 F and SbF 5 is known by the trade name “magic
acid.”)
In a biochemical context, superacid catalysis is
sometimes used to denote catalysis by metal ions, anal-
ogous to catalysis by hydrogen ions.
Oppositely, a compound having a very high basic-
ity, such as lithium diisopropylamide, is called a
“superbase.”superconductivity The ability of certain kinds of
materials to conduct an electric current with virtually
no resistance and under low temperature.superconductor An intermetallic alloy, element, or
compound that will conduct electricity without resis-
tance below a certain temperature. Once the current
begins to flow in a closed loop of superconducting
material, it will flow forever. There are two types of
superconductors: Type 1 and Type 2. Type 1 or “soft”
superconductors are mainly composed of metals and
metalloids that show someconductivity at room tem-
perature but require incredibly cold temperatures to
slow down molecular vibrations enough to facilitate
unimpeded electron flow. Type 2 or “hard” supercon-
ductors are composed of metallic compounds and
alloys (with the exception of elements vanadium, tech-
netium, and niobium). Superconductors are used in
maglev trains, medical imaging, and electricity genera-
tion, among other applications.supercooled liquids Cooled substances existing as
liquids at a temperature below their freezing point. An
amorphous solid that will continue to flow very slowly
under the influence of gravity.supercritical fluid Highly compressed gases that
combine properties of gases and liquids; a substance at
a temperature above its CRITICAL TEMPERATURE and
pressure (CRITICAL POINT).supercritical fluid 259