• Quaternary Structure of Lactate Dehydrogenase (LDH):
• LDH is a tetrameric enzyme composed of two distinct polypeptide subunits: $M$ (Muscle) and $H$ (Heart), assembling into five distinct isozymes:

• LDH-1 ($H_4$ / $HHHH$): Heart muscle and erythrocytes ($\gt 90\%$ of cardiac LDH).
• LDH-2 ($H_3M$): Reticuloendothelial system and heart.
• LDH-3 ($H_2M_2$): Lungs and spleen.
• LDH-4 ($HM_3$): Kidneys and pancreas.
• LDH-5 ($M_4$ / $MMMM$): Skeletal muscle and liver.
  • nalysis:

• (HHHH): Correct cardiac LDH isozyme.

• 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

• 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

• Metalloenzyme Co-factors:
• Urease:
• The historical enzyme first crystallized in 1926 by James B. Sumner (Jack bean Canavalia ensiformis urease), proving that enzymes are pure proteins.
• It is a classic Nickel-containing metalloenzyme whose catalytic active site coordinates two essential nickel ($Ni^{2+}$) ions per active subunit to catalyze the hydrolytic cleavage of urea into ammonia and $CO_2$.
• Carbonic anhydrase contains Zinc ($Zn^{2+}$).
• nalysis:
• (Urease): Correct nickel-containing enzyme.

Correct option: (1) Urease