Molecule of the Month: Nanowires

Cells are experts at moving electrons, one at a time, to the places where they are needed. They achieve this type of precision electronics by using specialized carriers of electrons. These carriers can be small molecules, like NAD and FAD, that pick up electrons in one place and deliver them to another place. Small proteins like cytochrome c and ferredoxin do a similar thing, using a heme cofactor or iron-sulfur clusters to carry the electron during delivery. Cells also build more complex circuits by arranging several electron carriers in a row. These carriers often include metal ions, such as heme cofactors or iron-sulfur clusters. They are arranged in line inside the protein and electrons hop from one to the next. For example, the three large protein complexes of the respiratory electron transport chain have these types of tiny electrical wires.

The protein assembly shown here, called HDCR (hydrogen-dependent carbon dioxide reductase, PDB ID 7qv7), allows some bacteria to use hydrogen as an energy source. The assembly includes four types of subunits. One of the subunits (HydA2) is an enzyme that specializes in extracting electrons from hydrogen. Another subunit (FdhF) is an enzyme that adds these electrons to carbon dioxide to form formic acid. The remaining two subunits (HycB3 and HycB4) form a nanowire that connects the two enzymes, delivering electrons between the two. The nanowire is composed of a string of iron-sulfur clusters.

Inside these bacterial cells, HDCR forms long helical filaments, which may then further assemble into large bundles or rings with thousands of iron-sulfur clusters. This may have several advantages. It helps to stabilize the structure of the small nanowire subunits into a large, stable structure. The filament also creates a large interconnected network of many copies of the enzymes, so the continuous internal nanowire can deliver electrons to any part of the filament, not simply between neighboring subunits, or possibly even store electrons if needed.

Amazingly, Geobacter bacteria are able to use metal-containing minerals as part of their energetic toolbox. They build long nanowires to conduct electrons to these minerals. The ones shown here (PDB ID 6nef/6ef8, 7tfs, 7lq5/8d9m) are composed of long strings of cytochrome proteins, each with a collection of electron-carrying heme cofactors.