Explain Proteins as carriers?

A large variety of compounds are carried in the blood between tissues and organs of the body. Some of the compounds require specific protein for their transport. Not only is this specific protein necessary for the transport of compounds insoluble in blood, but it is also necessary to protect these compounds from further reactions that take place during the transport process. Some of the membrane proteins are carriers and some are both carriers and enzymes. Both intracellular and extracellular carriers have been identified.

The plasma proteins which can have a carrier function are the albumin and the α- and β-globulins. The best studied of the plasma carriers are those associated with the transport of lipid (called Hpoproteins), since these lipoproteins (carriers plus lipids), when levels are elevated, appex to be related to the development of a variety of diseases. These lipoproteins comprise of about 3% of the plasma proteins. They are the loose associations of such lipids as phospholipids, triacylglycerols and cholesterols and represent an example of how proteins function as carriers. The lipids they carry are either from the diet or are synthesized de liovo in tissues, such as the liver. The β-globulin proteins carry these lipids to such sites as muscle or adipose tissue, where they are either used or stored. The release of the lipid from the protein carrier is a complicated process. In adipose tissue, the lipoprotein is attached to a membrane receptor site-an enzyme, lipoprotein lipase cleaves the lipid from the protein. The lipid is then picked up by another protein called a lipid binding protein and is carried to the interior of the cell for storage. The β-globulin protein carrier, once free of its lipid, returns to the liver or intestinal mucosa and is recycled. The plasma lipids, phospholipids, acylglycerols, cholesterol, cholesterol esters and free fatty acids are usually transported as loosely associated lipid-protein complexes. At least three different proteins have been identified. Albumin usually transports the free fatty acids, whereas the α and β-globulins transport the phospholipids, acylglycerols and cholesterols. The different lipoprotein complexes can be separated and identified on the basis of their antigenicity, their electrophoretic mobility and their density. The low density or P-lipoproteins contain the β-peptide, cholesterol and some phospholipids. The majority of phospholipids are carried as a-lipoproteins. With age, the lipid content of the plasma tends to rise and the rise is reflected almost entirely as an increase in β-lipoproteins. As the density of the lipoproteins decreases, the molecular weight and complexity of the lipid it carries decreases. The α-lipoproteins carry mainly (up to 60%) acylglycerols. These glyceroles are usually those synthesized in the body rather than coming from the diet. The dietary acylglycerols are usually carried as chylomicrons. These particles are the largest and least dense of the lipid-protein complexes.

In addition to serving as carriers of lipids, some of the globulins in the plasma can combine with iron and copper, as well as, with other divalent- cations. These combinations are called 'metalloproteins'. The globulins serve to transport these cations from the gut into the tissues where they are used. The monovalent cations, sodium and potassium, do not need carriers but most other minerals do. Many hormones and vitamins require transport or carrier proteins to take them from their point of origin to their active site. In addition, there are intracellular transport proteins such as the lipid binding proteins that are responsible for the transport of materials between the various cellular compartments. Lastly, there are transport proteins which carry single molecules. The classic example is haemoglobin, the red cell protein, responsible for the transport of oxygen from the lungs to every oxygen- using cell in the body. From carrier function, we move on to the regulatory function of proteins.