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Biological Oxidation and Reduction
Biological Oxidation and Reduction
ATP isn't the only energy currency cells use — a huge share of cellular energy moves through electrons carried by NAD+ and FAD.
ATP transfers energy through phosphate bonds, but that's not the only currency cells use. A huge share of cellular energy moves through electrons — captured, carried, and delivered by dedicated molecules. This subtopic covers redox fundamentals and the two major electron carriers of metabolism: NAD+ and FAD.
Key Takeaways
Oxidation is loss of electrons; reduction is gain of electrons — they always occur together in redox reactions ("LEO the lion goes GER").
Half reactions separate the oxidation and reduction portions of a redox reaction to track electron movement clearly.
NAD+ accepts 2 electrons + 1 proton to become NADH, which diffuses to the electron transport chain to help drive ATP synthesis.
FAD accepts 2 electrons + 2 protons to become FADH2, and typically stays tightly bound to its enzyme (a flavoprotein) rather than dissociating.
ATP carries energy through phosphate bonds; NADH and FADH2 carry it through electrons.
Oxidation and Reduction: LEO the Lion Goes GER
Redox reactions are central to metabolism because much of cellular energy is carried in the form of high-energy electrons.
Oxidation is the loss of electrons.
Reduction is the gain of electrons.
A helpful mnemonic: "LEO the lion goes GER" — Loss of Electrons is Oxidation, Gain of Electrons is Reduction.
Oxidation and reduction always occur together. If one molecule loses electrons, another molecule must gain them. For this reason, reactions involving electron transfer are called redox reactions.
Splitting Redox Reactions into Half Reactions
In biological systems, it's often helpful to separate a redox reaction into half reactions — one representing the oxidation portion, one representing the reduction portion, each written independently to track electron movement clearly.
Worked example: Consider a reaction where a three-carbon compound reacts with NADH and a proton to form a reduced three-carbon product and NAD+. Split into half reactions:
Reduction half reaction: the substrate gains 2 electrons and 2 protons, forming the reduced product.
Oxidation half reaction: NADH loses 2 electrons and a proton to form NAD+.
This separation makes the electron flow explicit — and that electron flow is what carries energy.
Electron Carriers: NAD+ and FAD
Many oxidation-reduction reactions in metabolism involve electron carriers — specialized molecules that temporarily accept high-energy electrons and then deliver them elsewhere in the cell.
NAD+ and NADH
NAD+ is one of the most important electron carriers. During metabolic pathways such as glycolysis and the citric acid cycle, NAD+ accepts two electrons and one proton to become NADH — a reduction of NAD+.
NADH doesn't hold onto its electrons indefinitely. Instead, it transports them to the electron transport chain in the mitochondria, where they move through a series of protein complexes. The energy released during that movement is used to generate a proton gradient that ultimately drives ATP synthesis.
FAD and FADH2
FAD is another major electron carrier. Like NAD+, it accepts electrons during metabolic reactions; when reduced, it becomes FADH2.
One important distinction: NAD+ typically accepts two electrons and one proton, while FAD can accept two electrons and two protons. Because of this, FAD is especially useful in reactions that remove two hydrogen atoms from a substrate — such as the formation of a double bond.
Another key difference is binding behavior. NAD+ binds to an enzyme, accepts electrons, and then dissociates. FAD, in contrast, is usually tightly bound to the enzyme that uses it — it doesn't leave the protein after accepting electrons.
NAD+ vs. FAD: Key Differences
Feature | NAD+ / NADH | FAD / FADH2 |
|---|---|---|
Electrons accepted | 2 | 2 |
Protons accepted | 1 | 2 |
Binding behavior | Binds enzyme, then dissociates | Stays tightly bound to enzyme |
Typical use | General reductions (glycolysis, citric acid cycle) | Reactions removing 2 H atoms (e.g., forming a double bond) |
Flavoproteins: Why FAD Stays Put
Proteins that contain FAD as a tightly bound cofactor are called flavoproteins.
In fatty acid oxidation, for example, the first dehydrogenation step is catalyzed by a flavoprotein. The bound FAD accepts electrons directly from the fatty acyl substrate, forming FADH2 within the enzyme itself — those electrons are then transferred to the next carrier in the pathway.
Similarly, in the electron transport chain, certain complexes contain flavoproteins that serve as initial electron acceptors. The flavin cofactor temporarily holds electrons before passing them further down the chain.
So while ATP transfers energy through phosphate bonds, NADH and FADH2 transfer energy through electrons. NADH diffuses between enzymes, whereas flavin cofactors remain enzyme-bound within flavoproteins — allowing controlled, stepwise electron transfer.
Common MCAT Mistakes
Reversing LEO GER. Oxidation is loss of electrons, reduction is gain of electrons — not the other way around. The mnemonic exists specifically because this is easy to flip under time pressure.
Assuming NAD+ and FAD accept the same number of protons. NAD+ accepts 2 electrons and 1 proton; FAD accepts 2 electrons and 2 protons. That extra proton is why FAD suits reactions removing two full hydrogen atoms (like double-bond formation).
Treating NAD+ and FAD as interchangeable in binding behavior. NAD+ binds an enzyme, accepts electrons, then dissociates and diffuses away. FAD, as part of a flavoprotein, typically stays tightly bound to the enzyme instead of leaving.
Forgetting oxidation and reduction are inseparable. A redox reaction always has both an oxidation half reaction and a reduction half reaction happening together — one molecule's electron loss is another's electron gain.
MCAT-Style Concept Check
Question: During the first dehydrogenation step of fatty acid oxidation, a flavoprotein removes two hydrogen atoms from a fatty acyl-CoA substrate, forming a double bond. Which statement correctly describes the electron carrier involved in this step?
A) FAD accepts 2 electrons and 1 proton, then dissociates from the enzyme to diffuse to the electron transport chain.
B) FAD accepts 2 electrons and 2 protons, and remains tightly bound to the enzyme as FADH2.
C) NAD+ accepts 2 electrons and 2 protons, then dissociates from the enzyme.
D) FAD accepts 1 electron and 2 protons, then diffuses to the electron transport chain.
Answer: B
Explanation: FAD accepts 2 electrons and 2 protons to become FADH2, and it typically stays tightly bound to its enzyme as part of a flavoprotein rather than dissociating — exactly what happens in the first dehydrogenation step of fatty acid oxidation, where the bound FAD accepts electrons directly from the fatty acyl substrate. Choice A gives FAD the wrong proton count (1 instead of 2) and the wrong binding behavior (dissociation belongs to NAD+, not FAD). Choice C gives NAD+ the wrong proton count — NAD+ accepts only 1 proton, not 2. Choice D gives FAD the wrong electron count (1 instead of 2) and again the wrong behavior — diffusing to the electron transport chain describes NADH, not FAD.
FAQ
What does "LEO the lion goes GER" mean?
It's a mnemonic for redox reactions: Loss of Electrons is Oxidation, Gain of Electrons is Reduction.
What is the difference between NAD+ and FAD as electron carriers?
NAD+ accepts 2 electrons and 1 proton to become NADH, then dissociates from its enzyme and diffuses to the electron transport chain. FAD accepts 2 electrons and 2 protons to become FADH2, and typically stays tightly bound to its enzyme instead of dissociating.
Why are half reactions used to analyze redox reactions?
Splitting a redox reaction into an oxidation half reaction and a reduction half reaction makes the movement of electrons explicit and easier to track, since one molecule's electron loss must match another molecule's electron gain.
What is a flavoprotein?
A flavoprotein is a protein that contains FAD as a tightly bound cofactor. Examples include the enzyme catalyzing the first dehydrogenation step of fatty acid oxidation and certain complexes in the electron transport chain.