Digestion

• Directional Catalysis of LDH Isoenzymes:
• Heart-Type Isoenzyme ($H_4$ / $HHHH$ / LDH-1):
• Possesses a very low $K_m$ (high substrate affinity) for lactate and is allosterically inhibited by high concentrations of pyruvate.
• This kinetic adaptation prevents accumulation of lactic acid in myocardium, actively favoring the oxidation of Lactate $\rightarrow$ Pyruvate:

• The generated pyruvate is immediately funneled into the pyruvate dehydrogenase complex and the TCA cycle to support continuous aerobic ATP production.
• nalysis:
• (Lactate to pyruvate): Correct directional catalysis.

Correct option: (1) Lactate to pyruvate

• Evaluation of Assertion and Reason:
• Assertion (A): The LDH isoenzyme $M_4$ functions optimally in the anaerobic environment of skeletal muscles.
• True. The muscle-type isozyme ($M_4$ / LDH-5) has a high $K_m$ for pyruvate and is not inhibited by high substrate concentrations, operating efficiently during intense anaerobic exercise.
• Reason (R): Skeletal muscles are designed to work anaerobically so as $M_4$. $M_4$ converts pyruvate to lactate, so glycolysis can be operated continuously.
• True. During vigorous muscular contraction when $O_2$ is limiting, $M_4$ rapidly reduces pyruvate to lactate, re-oxidizing $NADH$ back to $NAD^+$. Continuous regeneration of cytosolic $NAD^+$ is obligatory for glyceraldehyde-3-phosphate dehydrogenase, allowing anaerobic glycolysis and ATP production to proceed uninterrupted.
• nalysis:
• Both (A) and (R) are correct, and (R) provides the definitive biochemical explanation for (A). Hence, Option 1 is correct.

• Quantitative Metrics in Downstream Protein Purification:
• Total Activity (Units): $\text{Total Units} = \text{Activity per ml} \times \text{Total Volume}$.
• Because chromatography, precipitation, and filtration steps inevitably involve mechanical losses and denaturation, total enzyme activity always decreases across successive purification stages.
• Specific Activity (Units/mg protein): $\text{Specific Activity} = \frac{\text{Total Activity}}{\text{Total Protein mg}}$.
• As non-target contaminating proteins are selectively removed, the purity of the enzyme rises, causing specific activity to increase.
• nalysis:
• (Decreases): Correct behavior of total activity during purification.

Correct option: (2) Decreases