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Lecture 20

Lecture 20:"Membrane Proteins II"

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Biology 2382B
Robert Cumming

Cell Biology Lecture No. 20: Membrane Proteins II Monday March 25 , 2013 Synthesis Of Type IV Proteins: -Type IV proteins have multiple transmembrane domains and the orientation of each hydrophobic helix of the transmembrane domains depends on the positively-charged residues next to the topogenic sequence. Type IV proteins tend to have alternating SA signal sequences and STA signal sequences, often totalling in 7 transmembrane domains. Summary Of Topogenic Sequences: -In Type I proteins, the N-terminal signal sequences are always luminal and the STA signal sequence makes up the transmembrane domain. In Type II proteins, positively-charged residues are on the N- terminal side of the SA signal sequence (with protein’s C-terminus in ER lumen), while in Type III proteins, positively-charged residues are on the C-terminal side of the SA signal sequence (with protein’s N-terminus in ER lumen). In Type IV proteins, a repeating arrangement of SA and STA signal sequences are present. Since the positively charged residues are always present in the cytosol for SA sequences (STA signal sequences do not have these proximal residues), you can deduce the pattern of signal sequences for a multi-transmembrane domain in a Type IV protein. Transport Across Membranes: -As everything the cell needs has to be transported through its semi-permeable membrane, there are many ways that membrane proteins can help with the import and export of large or charged molecules: passive diffusion, facilitated transport (pores, channels, gates, uniporters, etc.), and active transport (ATP pumps, antiporters, symporters, etc.). Passive Diffusion: -Passive diffusion is a form of membrane transport that does not require any membrane proteins. It is for molecules that can freely cross the membrane (e.g. oxygen). Molecules need to be relatively aqueous, but also slightly-soluble in lipids in order to cross the plasma membrane (e.g. ethanol dissolves in water, but has lipophilic characteristics). The partition coefficient (K) is the measure of the preference of a molecule to partition into a hydrophobic environment (how lipid-soluble it is). This partition coefficient can be modelled by the ratio: K = c / c . The higher the partition coefficient, the faster passive diffusion of molecules can occur. In addition to the partition coefficient, passive diffusion depends on the molecule being small and the movement from a high to low concentration gradient. Facilitated Transport: -Unlike passive diffusion, facilitated transport is a form of membrane transport that requires different membrane proteins, driven by a high to low concentration gradient of molecules. Similar to passive diffusion, facilitated transport doesn’t require any energy, but is faster in the transport of molecules across a membrane than passive diffusion. Different membrane proteins functioning as pores, channels, gates, or uniporters, allow for the movement of large and charged hydrophilic substances (e.g. glucose) through a protein-lined pathway so they don’t come into contact with the hydrophobic interior of the membrane. These facilitated transported proteins are both specific (allow only one type of molecule through) and saturable (allow only a certain amount of molecules to cross at a time). The glucose uniporter GLUT1, changes its conformation when one molecule of glucose binds and allows the molecules to cross the membrane. + The K Resting Channel & Gates In Facilitated Transport: -Pores and channels allow for a “hole” in the membrane such that molecules can pass across the membrane through their hydrophilic interior (without contacting the hydrophobic membrane). These pores and channels exclude certain molecules based on size and quite specific to what molecules are + + allowed to pass. The potassium (K ) resting channel is specific for allowing K ions the cross the membrane (by way of a concentration gradient) as only K ions can interact with polar amino acids (oxygen residues) in order to shed its hydration shell (ions in solution are covered by water molecules) and pass through the channel. This K resting channel is used by cells in order to generate a charged gradient across the cell membrane. If the membrane s
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