Proteins are a type of organic molecule essential to living organisms. They carry out many important functions, including muscle contractions, immune responses and the facilitation of chemical reactions. A molecule known as an amino acid is the building block of proteins. Missing even one type of amino acid in the diet can cause problems with the function of proteins in your body. Milk contains all of the amino acids necessary for human health and is therefore an important human food.
Types
The major types of milk proteins -- known as casein and whey -- are completely unique to milk and cannot be found in any other tissue. They are high-quality sources rich in amino acid content and critical for the growth and development of the nursing young. However, they also digest quite differently. Whey is relatively less digestible than casein in the intestines and may trigger an immune response that results in an allergy.
Components
Both casein and whey are made out of several constituent parts. Casein can be broken down into units known as alpha casein, beta casein, kappa casein and gamma casein that form the structure of the protein. Whey is also made out of four main parts: alpha lactalbumin is an important protein in the synthesis of the milk sugar lactose; beta lactalbumin's function is unknown; immunoglobulins are essentially antibodies used in the immune system to fight foreign agents; and serum albumin is a kind of water-soluble blood protein also found in eggs. In addition to the four main components, whey contains many enzymes, hormones, growth factors and nutrient transporters.
Considerations
Together the main components of casein form a multi-molecular, granular structure known as a casein micelle. A micelle is essential for the absorption of certain vitamins and lipids. However, the structure is somewhat unstable and results in the clot formation known as curd from which cheese derives. In addition to casein molecules, the micelle also contains water and salts -- mainly calcium and phosphorous.
Sources
The major milk proteins are synthesized in epithelial cells -- meaning cells that line the cavities and surfaces of structures throughout the body -- by specific mammary glands. However, this is not true of every component: both immunoglobulin and serum albumin are instead absorbed from the blood. Only a small amount of immunoglobulins are synthesized by white blood cells which reside in the mammary tissue.
Amount
Casein makes up approximately 83 percent of the total milk protein. Whey constitutes the remaining 17 percent. The proteins can be broken down from there into individual components. However, the exact constituency of whey protein found in mammalian secretion is dependent upon such factors as species, the stage of lactation, the presence of intra-mammary infection and others. For example, the presence of an infection may produce more immunoglobulins.
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