Metabolic pathway design: multistep pathways in cells are considered efficient primarily for which organizational reason?

Difficulty: Easy

Correct Answer: They localize the pathway’s enzymes together in the same cellular region to facilitate substrate channeling

Explanation:


Introduction / Context:
Cellular metabolism is organized into multistep pathways rather than single giant reactions. A key efficiency gain comes from the spatial organization of enzymes, which can form complexes (metabolons) or colocalize within specific compartments. This architecture reduces diffusion time, protects labile intermediates, and enables coordinated regulation.


Given Data / Assumptions:

  • Pathways involve sequential enzymes.
  • Cells can cluster enzymes or place them within microenvironments (e.g., mitochondria, ER).
  • Efficiency relates to speed, control, and fidelity of intermediate transfer.


Concept / Approach:
When enzymes are localized together, intermediates are passed directly from one active site to the next (substrate channeling). This minimizes loss to competing reactions, decreases effective transit time, and can couple thermodynamically unfavorable steps to favorable ones through proximity and rapid consumption of intermediates.


Step-by-Step Solution:

1) Consider how distance and diffusion limit reaction rates in dilute cytosol.2) Recognize that enzyme clustering reduces intermediate escape and degradation.3) Conclude that colocalization/compartmentation is a central efficiency mechanism.4) Note that many pathways (e.g., TCA cycle, fatty acid synthesis) show physical or compartmental organization.


Verification / Alternative check:
Experimental reconstitution of multienzyme complexes often shows increased flux compared to dispersed enzymes, consistent with channeling and reduced transit times.


Why Other Options Are Wrong:

  • Same substrate for all enzymes: false; each step transforms the substrate into a new intermediate.
  • One enzyme for every step: contradicts the definition of a pathway.
  • Dispersing into different organelles: typically reduces efficiency for a single pathway.
  • No regulation: pathways are highly regulated; localization often enhances regulation.


Common Pitfalls:
Equating pathway efficiency only with thermodynamics; spatial and temporal organization are equally crucial for flux control and robustness.


Final Answer:
They localize the pathway’s enzymes together in the same cellular region to facilitate substrate channeling

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