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Enzyme
Definition:
“Enzymes are proteins that catalyze (i.e. accelerate) chemical reactions.”
Structures and mechanisms:
Ribbon-diagram showing carbonic
anhydrase II. The grey sphere is the zinc cofactor in the active site. Diagram
drawn from PDB 1MOO.
The activities of enzymes are determined by their three-dimensional structure.[9]
Most enzymes are much larger than the substrates they act on, and only a very
small portion of the enzyme (around 3–4 amino acids) is directly involved in
catalysis.[10] The region that contains these catalytic residues, binds the substrate
and then carries out the reaction is known as the active site. Enzymes can also
contain sites that bind cofactors, which are needed for catalysis. Some enzymes
also have binding sites for small molecules, which are often direct or indirect
products or substrates of the reaction catalyzed. This binding can serve to
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increase or decrease the enzyme's activity, providing a means for feedback
regulation.
Like all proteins, enzymes are made as long, linear chains of amino acids that
fold to produce a three-dimensional product. Each unique amino acid sequence
produces a unique structure, which has unique properties. Individual protein
chains may sometimes group together to form a protein complex. Most enzymes
can be denatured—that is, unfolded and inactivated—by heating, which destroys
the three-dimensional structure of the protein. Depending on the enzyme,
denaturation may be reversible or irreversible.
Introduction to Drug Action
Definition:
"All chemicals other than food that affect living processes."
If the affect helps the body, the drug is a
medicine. However, if a drug causes a harmful effect on the body, the drug is a
poison. The same chemical can be a medicine and a poison depending on
conditions of use and the person using it.
Another definition would be:
"Medicinal agents used for diagnosis, prevention, treatment of
symptoms, and cure of diseases."
Sites of Drug Action
l. Enzyme Inhibition:
Drugs act within the cell by modifying normal biochemical reactions. Enzyme
inhibition may be reversible or non reversible; competitive or non-competitive.
Antimetabolites may be used which mimic natural metabolites. Gene functions
may be suppressed
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Enzyme Inhibition
Mechanisms of Drug Actions by Enzyme Inhibition:
a) Direct Enzyme Inhibition:
Although activation of enzymes may be exploited therapeutically, most effects
are produced by enzyme inhibition. Inhibition caused by drugs may be either
reversible or irreversible. A reversible situation occurs when an equilibrium can
be established between the enzyme and the inhibitory drug. A competitive
inhibition occurs when the drug, as "mimic" of the normal substrate competes
with the normal substrate for the active site on the enzyme. Concentration effects
are important for competitive inhibition.
In noncompetitive inhibition, the drug combines with an enzyme, at a different
site other than the active site. The normal substrate can not displace the drug
from this site and can not interact with the active either since the shape of the
enzyme has been altered.
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Among the many types of drugs that act as enzyme
inhibitors the following may be included: antibiotics, acetylchlolinesterase agents,
certain antidepressants such as monoamine oxidase inhibitors and some
diuretics.
b) Suppression of Gene Function:
Many drugs act as suppressors of gene function including antibiotics, fungicides,
antimalarials and antivirals.
Gene function may be suppressed in several steps of protein synthesis or
inhibition of nucleic acid biosynthesis. Many substances which inhibit nucleic acid
biosynthesis are very toxic since the drug is not very selective in its action
between the parasite and host.
c) Antimetabolites:
The strategy of chemotherapy consists of exploiting the
biochemical differences between the host and parasite cells. Metabolites are any
substances used or produced by biochemical reactions. A drug which possesses
a remarkably close chemical similarity (mimic) to the normal metabolite is called
an antimetabolite.
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The antimetabolite enters a normal synthetic
reaction by "fooling" an enzyme and producing a counterfeit metabolite. The
counterfeit metabolite inhibits another enzyme or is an unusable fraudulent end
product which cannot be utilized by the cell for growth or reproduction. Such
antimetabolites have been used as antibacterial or anticancer agents.
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