MEMBRANE PROTEINS - Types and Functions

Membrane proteins are those proteins that are either a part of or interact with biological membranes. They make up around a third of human proteins and give difference kinds of membranes their unique properties. They help with both facilitated diffusion and active transport, connect cells together, participate in signal transduction, and act as markers for cell identification. Proteins carry out most of the specific functions of membranes, so the amount/types of proteins vary between different membranes. Membranes can be up to 75% protein by mass!
Membrane proteins can be integral/intrinsic or peripheral/extrinsic. Integral membrane proteins are a permanent part of the membrane, while peripheral proteins are only transiently associated with either the membrane or integral proteins, with hydrophobic, electrostatic, or other non-covalent interactions.
There are several different kinds of integral proteins. Integral monotopic proteins are attached to only one of the two leaflets of phospholipids making up the membrane and don’t span across. There are also transmembrane proteins and lipid-anchored proteins. Transmembrane proteins are those that span the lipid bilayer, and can be bitopic, spanning across the membrane once, or polytopic, spanning across it more than once. Lipid-anchored proteins are those which are covalently attached to lipids embedded in the lipid bilayer. For example, GPI, or glycosylphosphatidylinositol, is a glycolipid that gets attached to a protein’s C-terminus during post-translational modification. It acts as an anchor for proteins to the outer leaflet of the plasma membrane. Both integral and peripheral proteins can be post-translationally modified (e.g. fatty acids, diacylglycerol, prenyl chains, or GPI).
Recall that cellular membranes are made up of a phospholipid bilayer, which consists of two leaflets of phospholipids. These phospholipids have polar heads which are hydrophilic, or water-loving, and non-polar fatty acyl tails that are hydrophobic, or water-hating. Polar substances like to interact with other polar substances and non-polar substances hang out with other non-polar substances. This really attests to the power of hydrogen bonding. Water molecules want to interact so badly that anything non-polar getting in the way of their hydrogen bonding results in decreased entropy. The result is what’s called the hydrophobic effect. Hence, phospholipids in water will spontaneously form lipid bilayers – minimizes contact between polar and nonpolar molecules, maximizes hydrogen bonding, and maximizes entropy.
This is also why transmembrane proteins are amphipathic – which means that they have regions which are hydrophilic and regions that are hydrophobic. The hydrophilic regions are exposed to water on either side of the membrane, while the hydrophobic bits are happily interacting with the hydrophobic tails of lipid molecules in the interior of the bilayer. As a result, transmembrane proteins are stuck permanently into the cell membrane and are very hard to isolate. To get them out, you need to add a detergent, which is amphipathic and will disrupt the lipid bilayer.
There are two basic types of transmembrane proteins: α-helical proteins, and β-barrel proteins. Note that while helix bundle proteins are found in all types of biological membranes, beta-barrel proteins are only found in the outer membranes of gram-negative bacteria, mitochondria, and chloroplasts – evidence for the endosymbiotic theory.
Transmembrane protein structure can be predicted using a hydropathy plot - hydrophobicity index on Y axis, amino acid number on X axis. The amino acids making up a protein are localized according to polarity within its final structure in such a way that the polar amino acids face the outside aqueous solutions and the nonpolar amino acids are adjacent to the lipid bilayer.
Transmembrane proteins can be classified by topology - based on the position of N- and C-termini, as well as start-transfer and stop-transfer sequences. Type I is a single transmembrane pass with the N-terminus on the extracellular side of the membrane. Type 2 is also a single transmembrane pass but the N-terminus is on the cytosolic side of the membrane.
Often, transmembrane proteins function as gateways, allowing specific substances to pass across the membrane. They can undergo conformational changes as they do this. They might participate in facilitated or active transport. Facilitated transport is spontaneous passive transport of substa nces via transmembrane proteins. Active transport requires energy and may be necessary, for instance, if a substance is being carried across the membrane against its chemical or electrical gradient.
In animal cells, most transmembrane proteins are glycosylated. These sugar residues are always present on the noncytosolic leaflet of the membrane. As a result, the cell surface is covered in carbohydrates, which form what’s called the “cell coat”.