Ubiquitin is a small regulatory protein found in almost all tissues of eukaryotic organisms. The cell attaches short chains of Ubiquitin molecules to proteins, which labels them for destruction and subsequent recycling. The Ubiquitin tag directs proteins to the proteasome, which is a large protein complex in the cell that degrades unneeded proteins back into their amino acid constituents. These are then reused to synthesize new proteins. The constant recycling of proteins not only ensures damaged proteins are removed quickly but also allows rapid regulation of enzyme levels in the cell.
Structurally, Ubiquitin features all of the major structural features of typical proteins including two a-helices a curved b-sheet. Its small size (76 amino acids) makes it one of the most studied proteins for protein folding and dynamics.
Potassium channels form potassium-selective pores that span cell membranes. They are the most widely distributed type of ion channel found in virtually all living organisms. The four identical subunits are situated in a four-fold symmetrical manner around a central pore, which allows potassium ions to pass freely. At the top of the structure, formed by four loops lining the pore, a selectivity filter is situated which prevents other ions (such as sodium ions) from passing. The correct ions are detected by their size and charge. Note that that no active pumping of ions occurs; it merely allows passive conductance of ions down the con-centration gradient between the two sides of the membrane.
The KcsA is an archetypal membrane protein with eight tightly packed membrane-spanning a-helices. The four short helices in the center where the chain crosses half the membrane and then returns to the top are a more unusual feature.
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