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Quaternary Structure of Proteins
All proteins have primary structure and most proteins have secondary and tertiary structure. Larger proteins that consist of two or more polypeptide chains also contain a fourth level of structure called quaternary structure. Quaternary structure refers to the interactions between the different polypeptides that make up the protein. The simplest case of a protein with quaternary structure is a dimer. A dimer consists of two polypeptide units, a trimer consists of three, a tetramer consists of four, and so forth. Generally speaking, the individual polypeptide chains are called subunits and these subunits are usually held together by non-covalent interactions but in some cases can also be held together by covalent bonds called disulfide bonds. The two major categories of proteins inside our body are structural proteins (also called fibrous proteins) and globular proteins. Structural proteins form long fibers that play a role in proving our body and cells with structure. They are found in the cytoskeleton (intermediate filaments), the in our connective tissue such as bone (collagen) and in the hair and nails (alpha-keratin). For instance, alpha keratin consists of two subunits that form right-handed helices that intertwine together to for a supercoil called the alpha coiled coil. The two subunits are together by van der Waals forces, ionic bonds and disulfide bridges. The other category of proteins, called globular proteins, have a relatively spherical shape and have a wide variety of functions. Some of these roles include catalysts, membrane bound proteins, hormones and many more. For instance, hemoglobin is a globular protein found in our blood that has quaternary structure - it consists of four subunits that each contain a special helper group called the heme group. This heme group can carry a single oxygen atom.
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