Some of the isoenzymes present in the cell like SOD has different isoforms? What is the use of being in different isoforms? Do they show any substrate specificity?
I would be very grateful if anyone clarify my doubt .
Hi there,
Isoenzymes are variants of an enzyme which catalyze exactly the same reaction. Mainly they exhibit different enzymatic properties in terms of Km and kcat. The most popular example of isoenzymes are those of LDH. They exhibit tissue specific expression which means the lactate pyruvate conversion will be run differently according to the cell where it occurs. So instead of having tissue specific regulation of a single LDH activity (which becomes complicated when considering the diversity of tissues), the different isoforms contribute to the regulation of the reaction.
As Dominique says, isoenzymes are defined in the first instance by the fact that they catalyse the same reaction, and they are likely to have different kinetic properties for this shared substrate, sometimes very different - e.g. hexokinase and glucokinase for glucose. However this does not mean that their specificity range is the same. Indeed sometimes the reaction which defines them as isoenzymes may not be the principal physiological reaction for one of them. A good example is alcohol dehydrogenase. As people searched in different mammalian organs for enzymes that catalysed the oxidation of ethanol they found ADH 'isozymes'that eventually turned out to be sterol dehydrogenases - i.e. the ethanol activity was a side specificity. And another example is LDH, which Dominique mentioned. People usually mention two isoenzyme subunits in man, but there is a third. LDH- X or LDH C is the testicular isoenzyme and as well as working with lactate it works quite well with much larger hydroxyacids which would be rejected by the two more common LDH forms found in skeletal muscle, heart, liver etc.
They catalyze the same reaction, as the previous researchers said. The genes encoding isoenzymes are usually regulated in different ways. I have worked in oxidative stress in yeast for a long time, and I observed that, in stress conditions, the genes encoding isoenzymes of the pentose phosphate pathway are differencially expressed, i.e., only one of the isoenzymes of 6-phosphogluconate dehydrogenase is upregulated (I mean, the mRNA expression), not both.
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