Krebs (Citric Acid) Cycle: yields and ATP equivalents
The Krebs cycle (also called the citric acid cycle or TCA cycle) is a cyclic pathway that oxidizes
the 2-carbon acetyl group of acetyl-CoA to CO2. In the process, it captures high-energy electrons
in the reduced carriers NADH and FADH2, and produces GTP (or ATP) by substrate-level phosphorylation.
Where the cycle happens
-
Eukaryotes: in the mitochondrial matrix (with succinate dehydrogenase embedded in the inner membrane).
-
Prokaryotes: in the cytosol, with analogous electron transport components in the cell membrane.
Core idea
The cycle begins when acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons).
Through a sequence of reactions, the pathway releases 2 CO2, regenerates oxaloacetate, and conserves energy as:
- 3 NADH
- 1 FADH2
- 1 GTP (≈ 1 ATP)
Standard yields per acetyl-CoA
One “turn” of the Krebs cycle corresponds to one acetyl-CoA entering the cycle. The standard textbook yield is:
Yields per glucose
One glucose molecule produces 2 pyruvate, which are converted to 2 acetyl-CoA.
Therefore, the Krebs cycle runs twice per glucose:
\[
\begin{aligned}
\text{Per 1 glucose:}\quad
\text{NADH} &= 2 \cdot 3 = 6 \\
\text{FADH}_2 &= 2 \cdot 1 = 2 \\
\text{GTP} &= 2 \cdot 1 = 2 \\
\text{CO}_2 &= 2 \cdot 2 = 4
\end{aligned}
\]
Scaling to any amount
If nacetyl is the amount of acetyl-CoA (in moles), the totals scale linearly:
\[
\begin{aligned}
\text{NADH} &= 3\,n_{\text{acetyl}} \\
\text{FADH}_2 &= 1\,n_{\text{acetyl}} \\
\text{GTP} &= 1\,n_{\text{acetyl}} \\
\text{CO}_2 &= 2\,n_{\text{acetyl}}
\end{aligned}
\]
If the input is glucose amount nglc (in moles), then:
\[
\begin{aligned}
n_{\text{acetyl}} &= 2\,n_{\text{glc}}
\end{aligned}
\]
If glucose is provided in grams, convert mass to moles using the molar mass of glucose:
\( M_{\text{glc}} = 180.156\ \mathrm{g\cdot mol^{-1}} \).
\[
\begin{aligned}
n_{\text{glc}} = \frac{m_{\text{glc}}}{M_{\text{glc}}}
\end{aligned}
\]
ATP equivalents from NADH and FADH2 (P/O ratios)
NADH and FADH2 do not directly become ATP in the Krebs cycle. Instead, they deliver electrons to the electron transport chain.
The calculator converts carrier counts to ATP equivalents using user-chosen P/O ratios (ATP per reduced carrier):
\[
\begin{aligned}
\text{ATP}_{\text{eq}}
&= (\text{NADH})\cdot(P/O)_{\text{NADH}}
+ (\text{FADH}_2)\cdot(P/O)_{\text{FADH2}}
+ (\text{GTP})\cdot(eq)_{\text{GTP}}
\end{aligned}
\]
Common textbook defaults: \( (P/O)_{\text{NADH}} \approx 2.5 \), \( (P/O)_{\text{FADH2}} \approx 1.5 \), and \( eq_{\text{GTP}} = 1 \).
Different organisms or assumptions can change these values.
Why CO2 matters
CO2 output is a useful carbon-tracking check. Each acetyl-CoA contributes 2 carbons, and the cycle releases
2 CO2 per turn. Per glucose, the cycle releases 4 CO2.
Important note about “ATP yield”
The Krebs cycle itself directly makes only 1 GTP per acetyl-CoA by substrate-level phosphorylation.
Most energy is conserved in NADH and FADH2, which later drive oxidative phosphorylation.
Therefore, “total ATP from Krebs” depends on the chosen P/O ratios.
