SME
Biochemistry · Problem type
ATP from a fatty acid
Use this for “How many ATP does one stearate (18 carbons) give when it is oxidized completely to CO₂?”
Chart
Total ATP: ATP = 106
This is an example. Type your own values to start a new problem.
n
Carbons in the chain
example
k
Rounds of β-oxidation
calculated
A
Acetyl-CoA made
calculated
NADH
NADH made
calculated
FADH₂
FADH₂ made
calculated
ATP
Total ATP
calculated
Formulas
k
=
n
2
−
1
7
=
16
2
−
1
A
=
n
2
8
=
16
2
NADH
=
k
+
3
×
A
31 = 7 + 3 × 8
FADH₂
=
k
+
A
15 = 7 + 8
ATP
=
2.5
×
NADH
+
1.5
×
FADH₂
+
A
−
2
106 = 2.5 × 31 + 1.5 × 15 + 8 − 2
Assumptions
•
A saturated fatty acid with an even number of carbons; each round of β-oxidation cuts off two carbons as acetyl-CoA and makes 1 FADH₂ and 1 NADH.
•
The last round leaves two acetyl-CoA, so there is one round fewer than the acetyl-CoA.
•
Each acetyl-CoA makes 3 NADH, 1 FADH₂ and 1 GTP in the citric acid cycle; the GTP counts as one ATP.
•
About 2.5 ATP per NADH and 1.5 per FADH₂; activating the acid costs 2 ATP (ATP → AMP).
Step-by-step
WE KNOW
n = 16
FIND
rounds of β-oxidation (k), acetyl-CoA made (A), NADH made (NADH), FADH₂ made (FADH₂), total ATP (ATP)
Step 1 · Find rounds of β-oxidation (k)
Use k = n ÷ 2 − 1
Each round cuts off two carbons; the last round leaves two acetyl-CoA, so one round fewer.
k
=
16
2
−
1
k
=
8
−
1
k
=
7
Step 2 · Find acetyl-CoA made (A)
Use A = n ÷ 2
The chain is cut into two-carbon pieces, each an acetyl-CoA.
A
=
16
2
A
=
8
Step 3 · Find NADH made (NADH)
Use NADH = k + 3 × A
One NADH from each round, and three from each acetyl-CoA in the citric acid cycle.
NADH = 7 + 3 × 8
NADH = 7 + 24
NADH = 31
Step 4 · Find FADH₂ made (FADH₂)
Use FADH₂ = k + A
One FADH₂ from each round, and one from each acetyl-CoA in the citric acid cycle.
FADH₂ = 7 + 8
FADH₂ = 15
Step 5 · Find total ATP (ATP)
Use ATP = 2.5 × NADH + 1.5 × FADH₂ + A − 2
Each NADH gives 2.5 ATP and each FADH₂ 1.5; add one GTP per acetyl-CoA and take off the 2 spent on activation.
ATP = 2.5 × 31 + 1.5 × 15 + 8 − 2
ATP = 77.5 + 22.5 + 8 − 2
ATP = 106
Check
Put the numbers back in. Both sides should match.
7
=
16
2
−
1
✓
8
=
16
2
✓
31 = 7 + 3 × 8 ✓
15 = 7 + 8 ✓
106 = 2.5 × 31 + 1.5 × 15 + 8 − 2 ✓
Topic check
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