Question

Which statement is correct about the isoenzymes of LDH?

• A. They have dissimilar polypeptides \
• B. They have same amino acid sequences \
• C. They are synthesized by the same gene \
• D. They are presents in heart muscle

Hint:

Isoenzymes catalyze the same reaction, but they are built from different amino acid chains encoded by separate genes.

Answer 1

The Correct Option is A

Step 1: Concept Isoenzymes (isozymes) are distinct molecular forms of an enzyme that catalyze the exact same chemical reaction but differ in their primary amino acid sequence, structural attributes, and physical properties.

Step 2: Meaning Lactate Dehydrogenase (LDH) is an active tetrameric enzyme built from combinations of two distinct subunit types: H (Heart) and M (Muscle).

Step 3: Analysis * The H and M subunits are encoded by entirely separate structural genes located on different chromosomes. This means they are built from dissimilar polypeptides with non-identical amino acid sequences. Thus, option A is correct, while options B and C are incorrect. * There are five distinct structural arrangements of these tetramers ($\text{LDH}_{1}$ to $\text{LDH}_{5}$). While specific isoenzymes like $\text{LDH}_{1}$ ($H_{4}$) predominate in heart tissue, other isoenzymes are distributed across liver and skeletal muscle, meaning statement D does not uniquely describe the structural nature of LDH isoenzymes as a collective whole.

Step 4: Conclusion The fundamental defining feature of isoenzymes is that they are constructed from structurally distinct or dissimilar polypeptide subunits. This corresponds to option A. Final Answer: (A)