True about isoenzymes is/are:
**Core Concept**
Isoenzymes, also known as isozymes, are different molecular forms of an enzyme that catalyze the same chemical reaction but differ in their amino acid sequence, structure, or regulatory properties. This variability allows isoenzymes to be adapted to different cellular environments and perform specific functions.
**Why the Correct Answer is Right**
Isoenzymes are encoded by different genes or result from alternative splicing of the same gene transcript. They differ in their kinetic properties, such as substrate affinity, catalytic efficiency, and temperature stability. For example, lactate dehydrogenase (LDH) has five isoenzymes, each with a unique tissue distribution and function. The C4 isoenzyme is primarily found in the heart, whereas the M4 isoenzyme is predominantly expressed in the liver.
**Why Each Wrong Option is Incorrect**
**Option A:** Isoenzymes are encoded by the same gene and have identical amino acid sequences. This is incorrect because isoenzymes often arise from genes with distinct regulatory regions or result from alternative splicing.
**Option B:** Isoenzymes have identical kinetic properties and catalytic efficiencies. This is incorrect because isoenzymes typically exhibit distinct kinetic properties, such as substrate affinity and temperature stability.
**Option C:** Isoenzymes are catalytically inactive. This is incorrect because isoenzymes are functional enzymes that catalyze specific chemical reactions.
**Clinical Pearl / High-Yield Fact**
To remember the different isoenzymes of lactate dehydrogenase (LDH), use the mnemonic "Tissue Distribution": LDH-1 (C4) is found in the heart, LDH-2 (M4) is found in the liver, and LDH-5 (H4) is found in the lungs.
**Correct Answer: C. Isoenzymes are encoded by different genes or result from alternative splicing of the same gene transcript.**