Isoenzymes
are:
* Polymeric
and consist of two or more polypeptide chains. These isoenzymes
catalyze the same chemical reaction.
* Products of separate genes.
* Structurally
different - i.e. have different amino acid sequence and likely
a different net charge.
* Can be detected by gel electrophoresis and subsequent
specific enzymatic staining - because changes in electric
charge affect the migration rate of proteins in an electric
field, isoenzymes. Methods other than gel electrophoresis
like quantitation using monoclonal antibodies are also used.
* May have different Km and Vmax but
the Keq remains unaltered. Since enzymes only enhance reaction rates but not the equilibrium,
isoenzymes
How are they useful in Clinical Medicine?
May be tissue specific in which case
they can be used as markers for tissue damage and for diagnosis
and prognosis of the disease.
Some important enzymes
as diagnostic tools: Plasma
levels of isoenzyme patterns of the following enzymes change
following damage to these tissues:
Lactate
Dehydrogenase (LDH)
Creatine Kinase (CK)
Cholinesterase
Gamma-glutamyltransferase
Alkaline phosphatase (AP)
Alpha-amylase |
Myocardial
infarction
Myocardial infarc., brain, skeletal muscle
Liver, erythrocytes
Liver damage
Bone disease
Intestinal obstruction |
Other useful
information: Allozymes
are allelic variants of enzymes encoded by structural genes.
When due to mutation an amino acid has been replaced, the
net electric charge of the protein may have been altered.
For each enzyme usually two or more loci can be distinguished
that have been termed isoloci. Therefore, allozyme variation
is also referred to as isozyme variation. Allozymes have
been applied in many population genetic issues, including
mating structure, (sub)population structure and population
divergence. The main drawback of allozymes is their low
abundance and low level of polymorphism. Allozymes are particularly
useful at the level of conspecific populations and closely
related species. For other discussions see The
Measurement of Genetic Variation
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