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Enzyme Basics
| 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,
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Under normal conditions, the activities of many enzymes are held
constant by a balance between synthesis and breakdown of the reactions,
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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.)
.
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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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