Enzyme
Basics
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Basics | Properties
| Coenzymes
and Cofactors
| Isoenzymes |
Enzymes
are
Proteins [but it has been found
that RNA molecules (called ribozymes) having catalytic
properties in the cells have also been discovered (see an
article entitled: Enzymatic
RNA Molecules and the Replication of Chromosome Ends
from the Howard Hughes Medical Institute)],
Synthesized by all living organisms
including man,
Synthesized in the cell and important
in cell functions,
Speed up reactions, that is,
- Under
normal conditions, the activities of many enzymes are held
constant by a balance between synthesis and breakdown of
the reactions,
- But
in pathological conditions, certain cellular enzymes are
secreted into plasma, thus altering 'the balance,' and this
forms the basis of diagnostic enzymology (see Diagnostic
Enzymology in Veterninary Practice, first paragraph),
for instance,
a) Salivary
and serum levels of an enzyme may be increased or decreased
by diseases that lead to increased cell leakage, to increased
amounts available for release or decreased rate of enzyme
breakdown, or
b)
Changes in enzyme activity can thus be related to bodily
disorders as well as in diagnosis, prognosis and in assessing
therapy effectiveness (see Hepatocellular
Leakage Enzymes from Cornell U.)
.
![](../images/CitrateSynthase.gif)
Structure of Citrate Synthase from GSU.edu.
The structure shows the active site crevice with
substrate bound to it.
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Important
Enzyme Terms
Active
site: Active
site is a crevice or pocket formed by a group of amino
acid side chains belonging to residues forming the
pocket. Some or all of these side chains belong to
residues separated from each other in the amino acid
sequence. A portion of this crevice or pocket defined
by certain side chains is designated as substrate binding site (SBS) and an adjacent portion
which contains side chains involved in catalysis is
known as the catalytic site (CS). SBS may be hydrophobic or hydrophilic
depending on the complementary structure of substrates,
the CS is usually hydrophilic.
Substrate: A physiological
compound that binds to the active site of the enzyme
and is converted to product(s). |
Inhibitor: A compound
that binds at or near the active side of the enzyme and thereby
alters the rate of catalyzed reaction. Competitive inhibitors
have structures similar to the substrate. Structures of non-competitive
and end-product inhibitors are generally different from structure
of the substrate.
Allosteric activators
and inhibitors: These
compounds alter the activity of multimeric allosteric enzymes;
Their binding sites on the enzyme are different from the substrate
binding site and their effect on the enzyme activity is through
a distal conformational effect on the SBS.
Specific activity:
This term refers to enzyme activity of an enzyme under specified
conditions of substrate concentration, temperature, pH, ionic
strength etc. It is defined as micromoles of substrate converted
to product per minute per mg of the protein under specified
conditions. In clinical chemistry, these numbers are usually
expressed as ukat/mg or mkat/mg
protein. Specific activity is used to compare the level of
enzyme in different test samples.
Turnover number:
This number represents the absolute catalytic efficiency of
an enzyme. It is defined as umoles of substrate converted
to product per sec per mole of the active
site of the enzyme. This latter part is important because
a polymeric enzyme containing a number of subunits may have
to be compared with another enzyme which may be monomeric.
Optimum pH: Enzymes show optimum activity in a certain
pH range which varies with different enzymes. At a pH one
unit below or above this value the enzymes are only partially
active. At pH values far removed from optimum, the enzymes
can be denatured and lose their activity. Denaturation means
breakdown of non covalent bonds like H bonds and/or electrostatic
and hydrophobic interactions. Disulfide formed by cysteines
can be reduced by reducing agents like dithiothreitol (DTT).
While overall charge on the enzyme is important, denaturation
of the active site structure results in loss of activity.
Effect of temperature:
Like non enzymatic reactions, enzyme activity increases
with increase in temperature usually doubling of rate
with every 10 degree centigrade rise in temperature.
However, there is limit to this increase as enzyme active
site is denatured as the temperatures rise above 37
degrees (for vertebrates). Some bacteria which survive
in high temperature geysers have more temp. stable enzymes.
The denaturation at elevated temperatures results from
the breakdown of primarily H bonds. |
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DEPARTMENT
OF BIOCHEMISTRY AND MOLECULAR BIOLOGY, 185
South Orange Avenue, Newark, NJ 07103-2714.
Phone: 973-972-4750.
FAX: 973-972-5594. For information, contact Dr. Kumar:
kumarsu@umdnj.edu
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