You all have to head over to the PNAS website and download the Clarke et al. paper reporting on the crystal structure of MtrF, the MtrC homolog. This cytochrome protein has 10 hemes arranged like a “staggered cross”. To me it looks more like a lightning bolt. It also appears that there is an entry and exit point for electrons… so it is probably not functioning as a capacitor for electrons. There also appears to be a flavin binding site, which supports all the work done in the labs of Gralnick and Bond. The pdb number is: 3PMQ (www.pdb.org). I’m really excited about this paper as it will surely change the way we think about iron reduction, electron transfer to solid substrates, and extracellular respiration.
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May 25, 2011 at 2:43 pm
I just finished reading the paper you recommended. It looks like heme exposure plays a huge role in how the cytochrome can interact with less soluble substrates. You should check out the paper by Khare et al. 2006 (http://www.sciencedirect.com/science/article/pii/S0016703706018643)
This group shows the conformational changes that certain mono-heme cytochromes can take depending on the iron oxide. Wonder if we’ll see similar effects once studies get under way with MtrF and iron oxides.
May 26, 2011 at 11:59 am
You make a good point. There could very well be a shape change upon interaction with solid electron acceptors. Thanks for the reference too.
Clarke et al mentioned that amino acids in two domains (I and IV) of MtrF were more mobile, which they interpret as adding to the disorder of those domains. I wonder if this also means that the protein has some flexibility in these two domains. Domain I (and III) is predicted to interact with electron shuttles like flavins. And domain IV looks like it should interact with MtrB, the outermembrane beta-barrel protein, that connects MtrF to the periplasmic electron donors like MtrA/D. The PDB file is finally available. It’s pretty cool looking.
-c
May 26, 2011 at 7:48 pm
That is a very cool looking structure indeed, especially the two flavin pockets. It’s also interesting how Clarke et al observes this biphasic reduction kinetics with Fe(III)-citrate but not Fe-EDTA or Fe-NTA. I remember a paper by Wang et al., 2008 where they looked at the reduction kinetics of MtrC (http://aem.asm.org/cgi/content/full/74/21/6746). The authors mention this behavior could be indicative of heme groups with different redox potentials interacting differently with the iron or it could involve various reaction sites on the surface of the protein being more or less accessible to the iron. Go forward two years later to this Clarke paper and we see that that hemes 2 and 7 could have a low enough redox potential to interact with soluble iron whereas hemes 1,3,4 and 5 have higher redox potentials that could interact with solid iron oxides. . . then there’s Fe(III)-citrate . . .Clarke et al argues that it’s too bulky to easily access “buried” heme groups (I’m assuming this is referring to hemes 2 and 7) and so reduction proceeds more slowly relative to the other chelated irons. . .