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Biochemistry Lecture No. 12.docx

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Western University
Biochemistry 3380G

Biochemistry Lecture No. 12: Membrane Proteins nd Tuesday October 2 , 2012 Compositions Of Membranes By Mass: -Proteins comprise about the same composition of the cell membrane as do lipids (by mass). Carbohydrates only account for a small percentage. Functions Of Membrane Proteins: -Membrane proteins can function as transporters, anchors, receptors, or enzymes for the benefit of the cell. Transporters assist in the transport of specific molecules across the cell membrane (as they are polar and can’t cross the membrane by diffusion). Anchors hold certain molecules in a fixed position on side of the membrane or the other. Receptors (high specificity) cause a signal to be generated that often reaches the nucleus. There is much action that is taking place on the cell membrane! Toxins That Target Ion Channels: -The venoms of poisonous animals contain small peptides, some of which bind to ion channels, disrupting their function. As nerve impulses require the movement of ions across the membrane, the effect of the toxin results in the paralysis of the affected organism. Types Of Membrane Proteins: -Membrane proteins can be observed in different arrangements across the cell membrane: either as integral membrane proteins or peripheral membrane proteins. Types of integral membrane proteins include: transmembrane proteins (across the membrane), monolayer-associated proteins (sticks out from hydrophobic region of phospholipid bilayer) and lipid-linked proteins (which are covalently-bonded to lipid groups in the membrane; energetically favourable). Types of peripheral membrane proteins include protein-attached proteins (attached to the membrane only through the interaction of another protein). -Remember, integral proteins are defined as being stuck to the membrane, while peripheral proteins don’t actually make contact with the hydrophobic layer of the cell membrane. Hydrophobic proteins are often very hard to purify because they are quite non-polar and it is necessary to disrupt the membrane in order to get the proteins out. Succinate Dehydrogenase: -Succinate dehydrogenase is a transmembrane protein complex containing both integral subunits (hydrophobic) and per
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