Integral versus peripheral membrane proteins
Integral membrane proteins (those firmly embedded in the membrane) are held there by hydrophobic (water-hating) regions among the lipid fatty acid tails, while peripheral proteins sit on the surface.
Remember
An integral membrane protein is anchored by embedded hydrophobic regions, so it usually needs detergent (a soap-like chemical) to release it, whereas a peripheral protein sits on the surface.
Memory aid
an integral protein is like a nail driven into wood, so it needs a tool to remove it, whereas a peripheral protein is like a sticker on the surface that peels off
Picture the nail for an embedded integral protein and the sticker for a peripheral protein on the surface.
Membrane proteins either sit in the membrane or sit on it. The lipid bilayer is the double layer of phospholipids that makes up the membrane. Its middle is made of oily fatty acid tails, a water-free, hydrophobic region.
Integral proteins are firmly embedded in the bilayer or span it. Part of the chain is made of hydrophobic amino acids, which sit comfortably among the fatty acid tails. Hydrophobic interactions are weak individually, but there are many of them along a membrane-spanning segment, so the anchoring is stable. Many integral proteins cross the whole membrane and are exposed on both sides. Some are embedded in only one half of the bilayer, so the reliable feature is the embedded hydrophobic region rather than crossing the membrane.
Peripheral proteins sit on the inner or outer surface. They are attached to integral proteins or to phospholipids and are not embedded within the lipid, so gentle treatments, such as raised salt concentration or a change in pH, can release them. Integral proteins usually need a detergent (a soap-like chemical that disrupts the lipid bilayer) to be released.
Membrane proteins also act as receptors, channels and transporters. Membrane receptors bind signals such as hormones outside the cell and start responses inside it. Channels are pores that let a specific ion or small molecule pass, and carriers bind a substance and change shape to move it across. Both make the movement of substances controlled and selective.
Key facts
- Integral proteins
- are firmly embedded in the lipid bilayer, or span it, with hydrophobic regions that sit among the fatty acid tails.
- Peripheral proteins
- sit on the inner or outer surface, attached to integral proteins or to phospholipids, and are not embedded within the lipid.
- Defining feature of an integral protein
- an embedded hydrophobic region, because some integral proteins are embedded in only one half of the bilayer.
- Spanning segment
- a membrane-spanning stretch of hydrophobic amino acids is typically about 20 to 25 amino acids long.
- Release
- peripheral proteins can be removed by gentler treatments, whereas integral proteins usually need detergent.
- Jobs
- membrane proteins act as receptors, channels and transporters, letting a cell sense signals and control what crosses its membrane.
Exam traps
- Carbohydrate chains on membrane proteins sit on the outer surface of the cell, where they act in cell recognition, so they do not hold a protein in the bilayer.
- Working as an enzyme at the membrane does not separate integral from peripheral proteins, because both can work as enzymes.
- Release from the membrane after a gentle change in salt concentration or pH is typical of peripheral proteins, which sit on the surface held by weak interactions, so it does not mark a protein as integral.
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