Electron-Transfer Metalloproteins

Electron-transfer metalloproteins shuttle electrons through respiration and photosynthesis using heme, iron–sulfur, and copper centres whose potentials and geometries are tuned by the protein.

Definition

Electron-transfer metalloproteins are proteins whose bound metal centres accept and donate single electrons, forming the wiring of respiratory and photosynthetic electron-transport chains.

Scope

This topic covers the metalloproteins that carry out biological electron transfer: cytochromes with heme centres, iron–sulfur proteins such as ferredoxins, and blue (type 1) copper proteins; the factors that set their redox potentials; and the application of Marcus theory to long-range electron tunneling between fixed centres. It treats electron carriers, leaving oxygen carriers and catalytic enzymes to their respective topics.

Core questions

• What metal centres carry out biological electron transfer?
• How does the protein tune a centre's reduction potential?
• How do electrons tunnel rapidly over long distances between centres?
• Why do blue copper proteins have unusual spectra and potentials?

Key concepts

• Cytochromes
• Iron–sulfur clusters
• Blue (type 1) copper centres
• Reduction potential tuning
• Reorganization energy
• Long-range electron tunneling

Key theories

Metal centres for electron transfer

Cytochrome hemes, iron–sulfur clusters, and copper sites cycle between two oxidation states with minimal structural change, an essential feature for fast, reversible electron transfer.

Marcus theory in biology

Marcus and Sutin showed that biological electron-transfer rates depend on the driving force, the reorganization energy, and the donor–acceptor distance, accounting for the speed and directionality of electron-transport chains.

The entatic blue copper site

Blue copper proteins hold copper in a distorted geometry poised between those favoured by the two oxidation states, giving a low reorganization energy, intense colour, and a tuned potential ideal for rapid electron transfer.

Mechanisms

Electrons move between metalloprotein centres by quantum-mechanical tunneling through the intervening protein; the rate is governed by the energy gap, the reorganization energy of the centres and surroundings, and the through-bond and through-space distance separating donor and acceptor.

Clinical relevance

Electron-transfer metalloproteins power respiration and photosynthesis, the energy-converting processes of life, and disruption of these chains underlies mitochondrial dysfunction and oxidative stress; this is reference material, not clinical guidance.

History

The metalloproteins of the respiratory chain were identified through the twentieth century, with Beinert characterizing iron–sulfur clusters and others the cytochromes and copper proteins. Marcus theory, extended by Marcus and Sutin to biology, provided the quantitative framework for the rates of biological electron transfer.

Key figures

• Rudolph Marcus
• Harry Gray
• Helmut Beinert

Seminal works

• marcus1985
• lippard1994
• bertini2007

Frequently asked questions

Why are blue copper proteins so intensely coloured?

The distorted geometry of the blue copper site allows a strong charge-transfer transition between a sulfur ligand and the copper, producing an intense blue colour far deeper than that of ordinary copper complexes.

How can electrons travel so far through a protein?

Electrons tunnel quantum-mechanically through the protein medium between metal centres held at fixed distances; because the protein keeps the centres rigid and close enough and minimizes reorganization, transfer is fast even over distances of a nanometre or more.