Isoenzymes and enzyme isoforms
In the organism, there are enzymes called isoemzymes, which catalyze the same reaction, but differ from each other in their physicochemical properties (different affinity to the substrate, KM, sensitivity to inhibitors) and also in their occurrence in tissues. These genetically conditioned differences ( a different sequence of DNA nucleotides), for example, allow a certain regulation of the conditions under which the given reaction will take place in difference tissues.
Isoenzymes catalyzing the conversion of glucose to glucose-6-phosphate (phosphorylation of glucose) are illustrative examples- glucokinase (found in hepatocytes and β-cells pancreas) and hexokinase (localized in other cells of the body). Glucokinase shows a lower affinity for its substrate - glucose (this is expressed by the so-called KM,for glucokinase it is approximately 10 mmol/l). This means that the enzyme-catalyzed reaction takes place if the blood glucose level reaches a sufficient level (ussulay after a meal). With normal glycemia (between meals), glucokinase is not very active. The liver thus leaves enough glucose ffor other tissues that contain hexokinase with a KM value around 0,1 mmol/l. More detailed information can be found on the Embden-Meyerhof-Parnassus pathway page.
In addition to isoenzymes, enzyme isoforms are also found in the body. These multiple forms of enzymes come from the same gene (same sequence of DNA mucleotides), but differ in different post-translation modifications or alternative splicing. As a result, these enzymes can also catalyze different reactions.
