BLBS102-c08 BLBS102-Simpson March 21, 2012 12:8 Trim: 276mm X 219mm Printer Name: Yet to Come
8 Enzyme Activities 169Enzymes have different types of specificities. They can be
grouped into the following common types:- Absolute specificity:For example, urease (Sirko and
Brodzik 2000) and carbonate anhydrase (Khalifah 2003)
catalyze only the hydrolysis and cleavage of urea and
carbonic acid, respectively. Those enzymes having small
molecules as substrates or that work on biosynthesis path-
ways belong to this category. - Group specificity:For example, hexokinase prefers D-
glucose, but it also acts on other hexoses (Cardenas et al.
1998). - Stereo specificity:For example,d-amino acid oxidase re-
acts only withd-amino acid, but not withl-amino acid;
and succinate dehydrogenase reacts only with the fumarate
(transform), but not with maleate (cisform). - Bond specificity:For example, many digestive enzymes.
They catalyze the hydrolysis of large molecules of food
components, such as proteases, amylases, and lipases.
They seem to have broad specificity in their substrates; for
example, trypsin acts on all kinds of denatured proteins in
the intestine, but prefers the basic amino acid residues at
theC-terminal of the peptide bond. This broad specificity
brings up an economic effect in organisms; they are not
required to produce many digestive enzymes for all kinds
of food components.
The following two less common types of enzyme specificity
are impressive: (1) An unchiral compound, citric acid is formed
by citrate synthase with the condensation of oxaloacetate and
acetyl-CoA. An aconitase acts only on the moiety that comes
from oxaloacetate. This phenomenon shows that, for aconitase,
the citric acid acts as a chiral compound (Barry 1997). (2) If
people notice the rare mutation that happens naturally, they will
be impressed by the fidelity of certain enzymes, for example,
amino acyltRNA synthethase (Burbaum and Schimmel 1991,
Cusack 1997), RNA polymerase (Kornberg 1996), and DNA
polymerase (Goodman 1997); they even correct the accidents
that occur during catalyzing reactions.Enzymes are RegulatedThe catalytic activity of enzymes is changeable under different
conditions. These enzymes catalyze the set steps of a metabolic
pathway, and their activities are responsible for the states of cells.
Intermediate metabolites serve as modulators on enzyme activ-
ity; for example, at high concentrations of adenosine diphos-
phate, the catalytic activity of the phosphofructokinase, pyruvate
kinase, and pyruvate dehydrogenase complex are enhanced; as
adenosine triphosphate becomes high, they will be inhibited.
These modulators bind at allosteric sites on enzymes (Hammes
2002). The structure and mechanism of allosteric regulatory en-
zymes such as aspartate transcarbamylase (Cherfils et al. 1990)
and ribonucleoside diphosphate reductase (Scott et al. 2001)
have been well studied. However, allosteric regulation is not
the only way that the enzymatic activity is influenced. Covalent
modification by protein kinases (Langfort et al. 1998) and phos-
phatases (Luan 2003) on enzymes will cause large fluctuationsin total enzymatic activity in the metabolism pool. In addition,
sophisticated tuning phenomena on glycogen phosphorylase and
glycogen synthase through phosphorylation and dephosphory-
lation have been observed after hormone signaling (Nuttall et al.
1988, Preiss and Romeo 1994).Enzymes are Powerful CatalystsThe compound glucose will remain in a bottle for years without
any detectable changes. However, when glucose is applied in a
minimal medium as the only carbon source for the growth of
Salmonella typhimurium(orEscherichia coli), phosphorylation
of this molecule to glucose-6-phosphate is the first chemical
reaction that occurs as it enters the cells. From then on, the
activated glucose not only serves as a fuel compound to be
oxidized to produce chemical energy, but also goes through
numerous reactions to become the carbon skeleton of various
micro- and macrobiomolecules. To obtain each end product,
multiple steps have to be carried out. It may take less than 30
minutes for a generation to go through all the reactions. Only
the existence of enzymes guarantees this quick utilization and
disappearance of glucose.ENZYMES AND ACTIVATION ENERGY
Enzymes Lower the Activation EnergyEnzymes are mostly protein catalysts; except for the presence
of a group of ribonucleic acid-mediated reactions, they are re-
sponsible for the chemical reactions of metabolism in cells. For
the catalysis of a reaction, the reactants involved in this reaction
all require sufficient energy to cross the potential energy bar-
rier, the activation energy (EA), for the breakage of the chemical
bonds and the start of the reaction. Few have enough energy to
cross the reaction energy barrier until the reaction catalyst (the
enzyme) forms a transition state with the reactants to lower the
EA(Fig. 8.1). Thus, the enzyme lowers the barrier that usually
prevents the chemical reaction from occurring and facilitates the
reaction proceeding at a faster rate to approach equilibrium. The
substrates (the reactants), which are specific for the enzyme in-
volved in the reaction, combine with enzyme to lower theEAof
the reaction. One enzyme type will combine with one specific
type of substrate, that is, the active site of one particular enzyme
will only fit one specific type of substrate, and this leads to the
formation of and enzyme–substrate (ES) complex. Once the en-
ergy barrier is overcome, the substrate is then changed to another
chemical, the product. It should be noted that the enzyme itself
is not consumed or altered by the reaction, and that the overall
free-energy change (G) or the related equilibrium also remain
constant. Only the activation energy of the uncatalyzed reaction
(EAu) decreases to that of the enzyme-catalyzed reaction (EAc).
One of the mostimportantmechanisms of the enzyme function
that decreases theEAinvolves the initial binding of the enzyme
to the substrate, reacting in the correct direction, and closing of
the catalytic groups of the ES complex. The binding energy, part
of theEA, is required for the enzyme to bind to the substrate
and is determined primarily by the structure complementarities