Why acyl-CoA synthetase (thiokinase) activation is effectively irreversible What explains the practical irreversibility of forming acyl-CoA from a fatty acid, CoA, and ATP, and what is the metabolic consequence of this step?

Introduction / Context:
Fatty acids must be “activated” to acyl-CoA before oxidation or incorporation into complex lipids. The activation step is catalyzed by acyl-CoA synthetases (thiokinases) and is essentially irreversible in vivo, ensuring directional flux into downstream pathways.

Given Data / Assumptions:

• Reaction: fatty acid + CoA + ATP → acyl-CoA + AMP + PPi.
• Inorganic pyrophosphatase hydrolyzes PPi → 2 Pi.
• Cellular conditions maintain low PPi, pulling the reaction forward.

Concept / Approach:
The reaction consumes ATP to AMP, equivalent to two high-energy phosphate bonds. Subsequent rapid hydrolysis of pyrophosphate by pyrophosphatase renders the overall process highly exergonic, making the reverse reaction unfavorable.

Step-by-Step Solution:
Synthetase forms acyl-adenylate intermediate and releases PPi.CoA attacks acyl-adenylate to yield acyl-CoA + AMP.Pyrophosphatase hydrolyzes PPi to 2 Pi, dissipating potential for reversal.Net effect: activation is effectively irreversible and commits the fatty acid to subsequent metabolism.

Verification / Alternative check:
Measuring cellular PPi shows it remains very low due to constitutive pyrophosphatase activity, a common device cells use to drive biosynthetic and activation reactions forward.

Why Other Options Are Wrong:
Option c is false: irreversibility applies broadly, not only to even-chain fatty acids.Option e contradicts thermodynamic coupling via PPi hydrolysis.

Common Pitfalls:
Equating “ATP used” with irreversibility without acknowledging the critical role of PPi hydrolysis; overlooking that AMP formation equates to using two phosphoanhydride bonds.

Final Answer:
Both (a) and (b).