Protein

Any of a class of nitrogenous organic compounds which have large molecules composed of one or more long chains of amino acids and are an essential part of all living organisms, especially as structural components of body tissues such as muscle, hair, etc., and as enzymes and antibodies.

    Protein, highly complex substance that is present in all living organisms. Proteins are of great nutritional value and are directly involved in the chemical processes essential for life. The importance of proteins was recognized by chemists in the early 19th century, including Swedish chemist Jöns Jacob Berzelius, who in 1838 coined the term protein, a word derived from the Greek prōteios, meaning “holding first place.” Proteins are species-specific; that is, the proteins of one species differ from those of another species. They are also organ-specific; for instance, within a single organism, muscle proteins differ from those of the brain and liver.

    Proteins are involved in every aspect of cellular life. Proteins function to transport materials across membranes, catalyze chemical reactions, organize DNA, support the movement of materials within a cell, and even drive movement of the entire cell. The technique of sodium dodecyl sulfate polyacrylamide gel electrophoresis or SDS-PAGE is a standard technique for the analysis of the protein composition of a sample. The nature of the sample will vary depending on the specific experiment. Proteins differ in the number and type of amino acids that assemble to form a polypeptide chain. 

    The chemistry of the amino acid side chains (R groups) determines protein folding and the overall charge of the protein. SDS-PAGE works by separating proteins based on their relative size. In order to do this, protein folding and the differences in charge of the various proteins must be eliminated. Protein gel samples are treated with the nonionic detergent, sodium dodecyl sulfate (SDS). SDS coats the proteins with a net negative charge, masking any charges normally present on the protein’s surface. The sample is also treated to disrupt protein folding by the addition of chemical reagents such as dithiothreitol (DTT) or beta mercaptoethanol (BME) and heat. DTT and BME function to disrupt any disulfide bonds within the protein. The combination of SDS, DTT/BME, and heat will denature or unfold the protein.