Beta-Oxidation of Palmitic Acid: Steps and Energetics

Palmitic acid (C16:0) is a 16-carbon saturated fatty acid and the most common fatty acid in the body. Its complete oxidation proceeds through activation, mitochondrial transport, 7 cycles of beta-oxidation, and entry of the products into the TCA cycle.

Phase 1 - Activation (Cytosol)

Palmitate is first converted to palmitoyl-CoA in the cytosol by long-chain fatty acyl-CoA synthetase (thiokinase) on the outer mitochondrial membrane:

Palmitate + CoA + ATP → Palmitoyl-CoA + AMP + PPi

The pyrophosphate (PPi) is immediately hydrolyzed by pyrophosphatase, making the reaction irreversible. This step costs 2 ATP equivalents (ATP → AMP + PPi).

Phase 2 - Transport into Mitochondria (Carnitine Shuttle)

The inner mitochondrial membrane is impermeable to CoA. Long-chain acyl groups are ferried across by the carnitine shuttle:

CPT-I (carnitine palmitoyltransferase I, outer membrane): transfers the acyl group from CoA to carnitine, forming acylcarnitine. This is the rate-limiting step, inhibited by malonyl-CoA (fatty acid synthesis signal).
Carnitine-acylcarnitine translocase: moves acylcarnitine into the matrix in exchange for free carnitine.
CPT-II (inner membrane): transfers acyl group from carnitine back to CoA in the matrix, regenerating palmitoyl-CoA inside the mitochondrion.

Phase 3 - The Four Reactions of Each Beta-Oxidation Cycle

Each turn of the spiral shortens the acyl chain by 2 carbons and produces 1 acetyl-CoA, 1 FADH2, and 1 NADH.

Four steps of beta-oxidation (Lippincott Fig. 16.17)

Step	Reaction	Enzyme	Product
1. Oxidation	Acyl-CoA → trans-2-Enoyl-CoA	Acyl-CoA dehydrogenase (FAD-linked)	FADH2
2. Hydration	trans-2-Enoyl-CoA + H2O → L-3-Hydroxyacyl-CoA	2,3-Enoyl-CoA hydratase	--
3. Oxidation	L-3-Hydroxyacyl-CoA → 3-Ketoacyl-CoA	L-3-Hydroxyacyl-CoA dehydrogenase (NAD+-linked)	NADH
4. Thiolysis	3-Ketoacyl-CoA + CoA → Acyl-CoA (2C shorter) + Acetyl-CoA	β-Ketothiolase (thiolase)	Acetyl-CoA

The shortened acyl-CoA then re-enters the cycle.

Phase 4 - Number of Cycles for Palmitate

Palmitate has 16 carbons. Each cycle cleaves off 2 carbons:

7 cycles are needed to fully degrade palmitoyl-CoA
This produces 8 acetyl-CoA, 7 NADH, and 7 FADH2

Energetics: ATP Yield

Energy yield from palmitoyl-CoA oxidation (Lippincott Fig. 16.18)

Source	Number	ATP per molecule	Subtotal
FADH2 (from beta-oxidation)	7	1.5 (via CoQ/Complex II)	~14 ATP
NADH (from beta-oxidation)	7	2.5 (via Complex I)	~17.5 ATP
Acetyl-CoA (via TCA cycle)	8	10 each	~80 ATP

Note: The diagram above uses slightly older values (2 ATP per FADH2 = 14, 3 ATP per NADH = 21, 12 ATP per acetyl-CoA = 96, total = 131 ATP gross). Modern P/O ratios give ~129 net by a different calculation, but the classic biochemistry course value is:

Gross ATP = 131 Minus activation cost = -2 ATP Net ATP from palmitate = 129 ATP

Summary Table

Parameter	Value
Starting material	Palmitoyl-CoA (16C)
Number of beta-oxidation cycles	7
Acetyl-CoA produced	8
FADH2 produced	7
NADH produced	7
ATP from FADH2	14
ATP from NADH	21
ATP from 8 acetyl-CoA (TCA)	96
Gross ATP	131
Cost of activation	-2
Net ATP	129

Key Regulatory Points

CPT-I is the rate-limiting step; malonyl-CoA (the first committed intermediate of fatty acid synthesis) inhibits it - this prevents futile cycling between synthesis and degradation.
The acetyl-CoA/CoA ratio regulates thiolase: high ratio slows degradation.
Insulin (fed state) promotes fatty acid synthesis, raising malonyl-CoA and inhibiting beta-oxidation. Glucagon/low insulin (fasted state) promotes beta-oxidation.

Sources: Lippincott's Illustrated Reviews: Biochemistry, 8th ed. (Figs. 16.17, 16.18); Ganong's Review of Medical Physiology, 26th ed.

This is a shared conversation. Sign in to Orris to start your own chat.