Lecture 9 (Redox, Reduction Potentials & Electron Carriers)

What is oxidation vs reduction?

• Oxidation = electrons lost

• Reduction = electrons gained

What are redox reactions?

Coupled oxidation and reduction reactions that occur simultaneously

What is an oxidant vs reductant?

• Oxidant causes oxidation and is reduced

• Reductant causes reduction and is oxidized

Why is electron movement central to biological energy?

Electron transfer between species with different electron affinities creates electromotive force (emf) that can store/release energy and perform work

Why is glucose oxidation exergonic?

• Glucose is reduced

• As it is oxidized, electrons flow to O2 (exergonic) because O2 has higher electron affinity than carrier intermediates

How does mitochondrial electron flow drive ATP synthesis?

• Electron flow pumps H+ across inner mitochondrial membrane → proton-motive force

• ATP synthase uses H+ flow to make ATP

How can an overall redox reaction be analyzed?

Split it into oxidation and reduction half-reactions

What is a conjugate redox pair?

Electron donor/acceptor pair in a half-reaction

What is the electronegativity order?

H < C < S < N < O

How does carbon become oxidized?

• Formal loss of electrons

• Often loss of H or gain of O

What key carbon facts should you know?

C has atomic number 6, is tetravalent (able to form up to 4 covalent bonds), and has 4 valence electrons

What are signs of biological oxidation?

• Loss of electrons

• Loss of hydrogen

• Or addition of oxygen

What are the 4 biological electron-transfer modes?

• Electrons

• H atoms

• Hydride ions (:H−)

• Direct combination with O2

What is a reducing equivalent?

One electron equivalent transferred as e−, H atom, hydride (:H− = 2e−), or oxygen involvement

What is reduction potential, E?

Affinity of an electron acceptor for electrons in a redox pair (measured in volts)

What is biochemical standard reduction potential, E°′?

E measured under biochemical standard conditions:

• 55 M water

• 1 M solutes

• 101.3 kPa gases

• pH 7

• 298 K

How do electrons flow by E°′?

From lower E°′ to higher E°′

What does a more positive E°′ mean?

Greater electron affinity

What is n in ΔG°′ = −nFΔE°′?

Number of electrons transferred

What does the Nernst equation describe?

Actual E depends on concentrations of electron acceptor and donor

Why is biological glucose oxidation controlled?

Energy is released in stages so it can be captured as ATP

What is a coenzyme?

Low-molecular-weight compound required for catalytic activity

What are the universal reversible redox coenzymes?

• NAD+/NADH

• NADP+/NADPH

• FMN/FAD

What are other electron carriers?

• Lipid-soluble quinones

• Iron-sulfur proteins

• Cytochromes (contain heme groups)

What are NAD+ and NADP+?

• Soluble electron carriers

• NAD+ = nicotinamide adenine dinucleotide

• NADP+ = phosphorylated NAD+

What vitamin makes NAD(P)+?

Niacin, vitamin B3

How are NAD+ and NADP+ reduced?

By hydride transfer: NAD(P)+ + 2e− + H+ → NAD(P)H

Do NAD(P)+ coenzymes stay bound?

• No

• They dissociate after each reaction cycle

What is NAD+ mainly used for?

• Catabolism (oxidation breakdown of organic molecules)

• NAD+/NADH mainly in mitochondria

What is NADPH mainly used for?

• Anabolism/reduction (e.g. synthesis of complex molecules)

• NADP+/NADPH mainly in cytosol

What are FAD and FMN?

• Flavin redox coenzymes:

  • FAD = flavin adenine dinucleotide

  • FMN = flavin mononucleotide

How are FAD/FMN bound in enzymes?

Tightly bound prosthetic groups of flavoproteins

Do FAD/FMN dissociate each redox cycle?

• No

• They remain bound and do not dissociate