**Core Concept**
Isoenzymes, also known as isozymes, refer to different molecular forms of an enzyme that catalyze the same chemical reaction but have distinct structures, functions, or regulatory properties. These variations arise from differences in the amino acid sequence, gene expression, or post-translational modifications, allowing the same enzyme to adapt to different physiological conditions or tissue environments.

**Why the Correct Answer is Right**
Isoenzymes are crucial for maintaining tissue-specific functions and responding to various physiological demands. For instance, lactate dehydrogenase (LDH) has multiple isoenzymes, including LDH-1 (heart muscle), LDH-2 (placenta), LDH-3 (skeletal muscle), and LDH-4 (liver), each with distinct kinetic properties and subcellular localizations. The presence of these isoenzymes enables the enzyme to optimize its activity according to the specific requirements of the tissue in which it is expressed.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoenzymes are identical in their primary structure, which is incorrect. Isoenzymes are characterized by differences in their amino acid sequences, gene expression, or post-translational modifications.

**Option B:** Isoenzymes are always encoded by different genes, which is not true. Some isoenzymes can arise from the same gene through alternative splicing or post-translational modifications.

**Option C:** Isoenzymes have the same kinetic properties, which is incorrect. Isoenzymes often exhibit distinct kinetic properties, such as altered substrate specificity or catalytic efficiency, allowing them to adapt to different physiological conditions.

**Option D:** Isoenzymes are exclusive to specific tissues, which is an oversimplification. While some isoenzymes are highly tissue-specific, others can be expressed in multiple tissues or under different physiological conditions.

**Clinical Pearl / High-Yield Fact**
When interpreting enzyme assays, it is essential to consider the tissue specificity of the isoenzymes being measured. For example, elevated levels of LDH-1 in the blood can indicate myocardial infarction, while increased levels of LDH-2 may suggest placental damage.

**Correct Answer:** C. Isoenzymes have the same chemical reaction but distinct structures, functions, or regulatory properties.