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by Eric Martz
Coronavirus SARS-CoV-2 (responsible for COVID-19) has a spike protein on its surface, which enables it to infect host cells. Initially, proteases in the lungs clip the homo-trimeric spike protein at a unique sequence. This primes it, causing it to extend its receptor binding surface (shown in the above animation), optimizing binding to the host cell's ACE2 receptor (not shown). Next, spike protein initiates fusion of the virus and host cell membranes (not shown), enabling the virus RNA to enter the cell and initiate production of new virions. Knowledge of spike protein's molecular structure and function is crucial to developing effective therapies and vaccines.
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F Wang, Y Gu, JP O'Brien, SM Yi, SE Yalcin, V Srikanth, C Shen, D Vu, NL Ing, AI Hochbaum, EH Egelman, NS Malvankar. Cell 2019 doi: 10.1016/j.cell.2019.03.029
Bacteria living in anaerobic environments (no oxygen) need alternative electron acceptors in order to get energy from their food. An acceptor abundant in the earth's crust is red iron oxide ("rust"), which gets reduced to black iron oxide (magnetite). Many bacteria, such as Geobacter, get their metabolic energy by transferring electrons to acceptors that are multiple cell diameters distant, using protein nanowires. These were long thought to be pili. But when the structure of the nanowires was solved in 2019, to everyone's surprise, they turned out to be unprecedented linear polymers of multi-heme cytochromes. The hemes form an electrically conductive chain in the cores of these nanowires.

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by Eric Martz
The Ramachandran Principle says that alpha helices, beta strands, and turns are the most likely conformations for a polypeptide chain to adopt, because most other conformations are impossible due to steric collisions between atoms. Check Show Clashes to see where non-bonded atoms are overlapping, and thus in physically impossible positions.

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