All microorganisms try to make as much energy as they can so that they can grow. This sometimes means outcompeting their neighbors (you know, the stronger, more intelligent and attractive bacteria). This can also mean surviving under some really tough conditions like anaerobic, high temperature or acidic conditions. What strategy they will choose, to make energy, depends on how much energy there is in that stratagem, who they can out-compete and whether this is the best choice in a given environment. The first of these is a very important consideration. For eg, the reason why there are so many aerobes in the world (Almost all the multicellular organisms on earth are aerobes) is because there is a lot of energy to be made from chemically reducing oxygen to water. But there are lots of microorganisms which are very successful without using oxygen.
DMRB (Dissimilatory metal reducing bacteria) use one such unique method to conserve energy. They respire metal oxides to make energy. These are basically minerals contained in different types of rock, like iron oxide or manganese oxide. Unlike oxygen, nitrates or methane or compounds (which are soluble in water or in gaseous form), these are difficult to get inside the cell. So DRMB use “nanowires” to reduce metals outside the cell, and make energy. Nanowires are a complex structure of proteins extending from inside the cell to the outside, moving electrons to the outside to deposit on the metal.
Shewanella oneidensis MR-1 is a bacterium that is capable of making and using these nanowire to reduce metals. It has at least two different nanowire complexes, labelled MtrDEF and MtrCAB.
Figure 1. Nanowire complex in Shewanella oneidensis MR-1
Shewanella‘s strategy requires transfer of electrons across long distances. This is done using certain “electron carriers” called hemes c. (proteins which contain hemes are called cytochromes) For electrons to be transferred from one carrier to another, they have to be within a certain distance of each other(<= 14 Angstroms) according to Marcus theory. There also has to be a driving force moving the electron to the final carrier and eventually the metal oxide. The complex MtrDEF consists of two proteins – MtrD and MtrF – containing ten hemes each, to move electrons. MtrE, the other protein in the complex forms a barrel around MtrD in the inner membrane and helps it make contact with MtrF, which is located in the outer membrane and will take the electrons outside the cell.
Figure 2. Crystal structure of MtrF (PDB code:3PMQ figure made using VMD ) 3PMQ from Clarke et al. (2011 PNAS)
When the crystal structure of the MtrF was obtained, it was clear that the hemes were within 14 Angstrom. Computational studies [Breuer et al. PNAS (2014)] suggested that three possible pathways for the electrons to exit the protein existed. The longest pathway using eight hemes is the most likely one to be used to deposit electrons on the outside but if the binding site on that exit pathway is blocked, one of the others can be used.
Possible applications of DMRB
Aside from the fascinating scientific mysteries involved with nanowires, DMRB also offer a lot of possible future applications. DMRB can be used in bio fuel cells to make them more efficient since they can directly interact with electrodes and do away with mediators. The ability to reduce insoluble metals can also be important in bioremediation of metal waste.