Summary
Different molecular forms of an enzyme that catalyse the same reaction but vary in kinetic properties or tissue location.
In depth
Isoenzymes (isozymes) arise from different genes or splicing and differ in kinetics or regulation. Lactate dehydrogenase has heart (H4) and muscle (M4) forms suited for aerobic or anaerobic work.
Their distribution in blood is used in diagnosis.
The heart isoenzyme LDH-1 (H4) is inhibited by high pyruvate, while muscle LDH-5 (M4) works well in low oxygen to convert pyruvate to lactate. Creatine kinase has MM, MB and BB forms; the MB form rises in blood after a heart attack, and a rise of cardiac troponin is now the preferred marker.
Isoenzymes allow different tissues or developmental stages to tune metabolism: hexokinase and glucokinase both phosphorylate glucose but with different Km. Alkaline phosphatase isoforms help to distinguish liver from bone disease.
Key idea: isoenzymes catalyse the same reaction but are separate proteins, which can be separated by electrophoresis because of their different charges.
Frequently asked questions
What is Isoenzymes?
Different molecular forms of an enzyme that catalyse the same reaction but vary in kinetic properties or tissue location.
Which area of biology covers Isoenzymes, and how advanced is it?
Isoenzymes is one of the 51 topics in Biomolecules & Biochemistry. It is rated Advanced, so it suits learners who want specialist depth. Related topics in this area include Michaelis-Menten Kinetics, Active Site, Enzymes.
Can you give an example of Isoenzymes?
Raised cardiac troponin and CK-MB are markers of a heart attack.
What should I learn before Isoenzymes?
Work through the Beginner topics in Biomolecules & Biochemistry first, such as Water Properties, Carbon Chemistry, Carbohydrates, then return to Isoenzymes. Advanced topics assume the Intermediate vocabulary.
What topics are related to Isoenzymes?
The closest topics to Isoenzymes are Michaelis-Menten Kinetics, Active Site, Enzymes, Monosaccharides. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Isoenzymes?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Isoenzymes. To practise the whole of Biomolecules & Biochemistry, use the Biomolecules & Biochemistry quiz.
How is Isoenzymes different from Michaelis-Menten Kinetics?
Isoenzymes: Different molecular forms of an enzyme that catalyse the same reaction but vary in kinetic properties or tissue location. Michaelis-Menten Kinetics: Models reaction velocity as Vmax times substrate over Km plus substrate, describing enzyme saturation. Read both side by side and note what each one explains.
How should I study Isoenzymes step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Michaelis-Menten Kinetics and explain the difference aloud. Finally, take the Biomolecules & Biochemistry quiz and come back to this page for any question you miss.