In the conversion of glucose into two molecules of lactate,...

In the conversion of glucose into two molecules of lactate, the NADH generated earlier in the pathway is oxidized to $\mathrm{NAD}^{+} .$ Why is it not to the cell's advantage to simply make more $\mathrm{NAD}^{+}$ so that the regeneration would not be necessary? After all, the cell would save much energy because it would no longer need to synthesize lactic acid dehydrogenase.

Key Concepts

Anaerobic Fermentation

Under conditions where oxygen is scarce, cells rely on fermentation pathways to regenerate NAD+ from NADH. In the conversion of pyruvate to lactate, the enzyme lactate dehydrogenase uses NADH to reduce pyruvate, thereby replenishing NAD+ needed for glycolysis. This process is critical for maintaining energy production even when the electron transport chain cannot operate efficiently.

Enzyme Investment in Metabolic Efficiency

While synthesizing additional enzymes might seem energetically costly, the investment in enzymes like lactate dehydrogenase is minimal compared to the huge benefit of maintaining metabolic flux. Enzymes are highly efficient catalysts that help achieve the essential conversion reactions with minimal energy input. Simply producing more NAD+ chemically would not be as efficient or feasible; enzymatic routes are optimized by evolution to sustain energy production and redox balance in the cell.

NAD+/NADH Redox Balance

The balance between NAD+ and NADH is fundamental to cellular metabolism. During glycolysis, NAD+ is reduced to NADH, and to sustain the ongoing production of ATP, cells must regenerate NAD+ through redox reactions. Without this regeneration, glycolysis would halt because the limited pool of NAD+ would become fully reduced, impeding the energy-yielding reactions of the pathway.

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