As mentioned above, the prefix glyco means ‘glucose’, so a glycoprotein is a small amount of a carbohydrate (sugar) attached to a large amount of protein. If the molecule is called a proteoglycan, then there is more sugar (glyco) than protein. Glycoproteins are also found on the external surface of the plasma membrane and act as a cell markers.
Function of Plasma Membrane Proteins
The proteins that are associated with the plasma membrane have an expansive range of roles.
- Structural Elements
- Cell Adhesion Molecules
- Structural Proteins – Theses link cytoskeleton and membrane to maintain cell shape, e.g., microvilli, red blood cells. The characteristic shape of the red blood cell is due to an extensive cytoskeleton that pulls the cell membrane into a biconcave disc shape. In diseases such as hereditary spherocytosis, defects in cytoskeletal proteins produce abnormally shaped red blood cells that are unable to move normally through the circulatory system.
- Cell Adhesion Molecules – Form part of the cell-to-cell connections holding tissues together. Membrane-spanning proteins link the cytoskeleton to the extracellular matrix. The most common fibrous protein that attaches a cell to adjacent cells is collagen!
- Enzymes – Membrane associated enzymes act as any other enzymes do but are fixed to the plasma membrane. Chemical reactions can take place on either membrane face, i.e. on the extracellular or intracellular surface. For example, enzymes on luminal surface in small intestine cells (extracellular) digest peptides and carbohydrates. Enzymes on the intracellular surface, such as adenylyl cyclase, play an important role in signal transduction.
- Receptors – These act as receivers for the body’s chemical signaling system, with each receptor being specific for a certain type or family of signal molecule. A ligand is any molecule binding to a receptor. Ligand binding usually triggers another membrane event, this can be signal transduction (e.g., hormone binding) or directly lead to an ion channel opening or closing (ionotropic effect).
- Transporters – Many molecules require the use of transporters to cross cell membranes. Most lipophobic (can also be termed hydrophilic) molecules, such as smaller carbohydrates, amino acids, peptides, proteins, and charged particles such as ions, must have assistance from membrane proteins in order to get into or out of cells.
All of the above listed functions of plasma membrane proteins are very important. In the next stage that follows, however, we are going concentrate on the role of plasma membrane proteins as transporters in the body and the various mechanisms by which they move molecules from one side of the plasma membrane to the other.
There are 2 Categories of Protein Transporters: Protein Channels and Protein Carriers
Protein channels are well named; they are much like little water-filled channels, forming a passageway that directly links the ECF to ICF. The narrow diameter of protein channels restricts passage through them to small sized molecules, mostly water (H2O) and ions (K+, Na+, Cl– and Ca2+). Electrical charges lining the inner channel may restrict the movement of some molecules; therefore they can be very specific as to what they allow to travel through them. This mode of transport is very fast, much faster than protein carriers because there is no need for the binding of the substrate as in protein carriers.
Open channels spend most time in the open configuration and are also called pores. Other channels are gated and spend most time in a closed state.
Three Types of Gated Ion Channels:
The protein channels that have gates that can open or close are called gated ion channels. There are three types of gated channels that we will explore, and they differ in the ‘trigger’ that opens or closes the gate, they are:
- Chemically Gated Channels: triggered by specific ligands (chemicals) to open or close channel.
- Voltage-Gated Channels: triggered by electrical changes across cell to open or close channel.
- Mechanically Gated Channels: triggered by distention or physical force to open or close channel.
Some gated channels remain open and the molecules leak across the channel, these are often called “leaky channels”. The normal permeability of cells to Na+ and K+ is due to such leak channels.
The second type of protein transporters are called protein carriers. These never form a direct or continuous passage between the ECF and the ICF. They have a binding site (like enzymes) and will only transport specific molecules that match this site. Once the molecule binds to the site, the protein carrier undergoes a conformation (shape) change. It can rotate, or close one end while it opens the opposite, thus carrying the molecule across membrane. This mode of transportation is slower than protein channels, as they need to bind the substrate and change shape while moving substrates.
A perfect example is glucose. Glucose has a MW of 180, so it is a larger molecule, but not massive like starch or albumin. It is also a polar molecule, meaning it is soluble (mixes) in water. Amino acids are another good example of molecules moved by carriers.
Properties of Protein Carrier Mediated Transport
Because of the way that protein carriers work, their transport exhibits saturation, specificity, and competition.
Originally it was thought that membrane proteins all floated freely within the lipid layer of the membrane. However, it has been shown that some proteins are immobile, held in place by cytoskeleton proteins. Restriction of protein movement allows membrane polarity, which can be seen in transporting epithelia. Other proteins are mobile and move under the direction of cytoskeleton. For example, rhodopsin, the protein pigment that absorbs light in the retina, rotates in place, somersaulting at a rate of 60° every 10 seconds.