Redox reactions and NADH · Chapter 6

Handing phosphate groups around is one way cells capture energy from food. Another is moving electrons from one molecule to another, and these electron transfers power much of metabolism.

Losing and gaining electrons

Removing one or more electrons from an atom or molecule is oxidation. The name comes from oxygen, which often pulls electrons away from other atoms in chemical reactions. Adding one or more electrons is reduction. That name makes sense once you recall that electrons are negative: adding one lowers, or reduces, the net charge of the atom or molecule.

Free electrons don’t drift around loose in a cell. An electron removed from one atom or molecule must be transferred directly to another, so oxidation and reduction always happen together. A reaction that transfers electrons this way is a redox reaction, short for reduction-oxidation reaction. In general form, with e⁻ standing for an electron:

Ae⁻ + B → A + Be⁻

Molecule A loses the electron, so A is oxidized. Molecule B gains it, so B is reduced.

As a rule, losing electrons leaves a substance with less energy, and gaining them leaves it with more. That’s what makes redox reactions useful: when a cell oxidizes a food molecule, the molecule that picks up the electrons also picks up much of their energy.

When cells oxidize organic molecules such as glucose, many of the electrons removed are picked up by nicotinamide adenine dinucleotide (NAD⁺). NAD⁺ accepts two electrons and a hydrogen ion (H⁺), becoming NADH:

NAD⁺ + 2e⁻ + H⁺ → NADH

The organic molecule is oxidized, and NAD⁺ is reduced. NADH is an energy intermediate: it holds the electrons, and the energy that came with them, until another reaction uses them. When NADH later gives up its electrons and becomes NAD⁺ and H⁺ again, NADH is oxidized.

	Oxidation	Reduction
Electrons	Removed (lost)	Added (gained)
Energy of the substance afterward	Less	More
Example	NADH gives up electrons, becoming NAD⁺	NAD⁺ picks up electrons, becoming NADH

Cells use NADH in two main ways:

Making ATP. The oxidation of NADH is highly exergonic, and cells use the energy it releases to make ATP (Chapter 7).
Building molecules. NADH can hand its electrons to other organic molecules. The extra electrons energize those molecules so they bond to one another more easily, which is why reactions that build larger molecules often need NADH.

Anabolic, or biosynthetic, reactions make larger molecules, including macromolecules (very large molecules such as proteins and nucleic acids). Cells also need to make small organic molecules, such as amino acids and fats, when food doesn’t supply enough of them. They build these by forming covalent bonds between smaller molecules, and that takes energy.

For example, cells make the amino acid glutamate by covalently linking α-ketoglutarate, a product of sugar metabolism, with ammonium (NH₄⁺). A molecule of NADH supplies the energy that drives this reaction forward.

Check yourselfWhen NADH gives up electrons and becomes NAD⁺ and H⁺, has NADH been oxidized or reduced? Did it gain or lose energy?

It has been oxidized, because it lost electrons (oxidation is loss). It also lost energy, since oxidized substances have less energy than reduced ones. Cells capture that released energy to make ATP.