Tertiary And Quaternary Structures

Tertiary structure refers to the three-dimensional structure of a polypeptide chain. The three-dimensional structure of multisubunit proteins is then described by the term quaternary structure, which refers to that structure resulting from the interactions between polypeptide chains, frequently through so-called self-association. However, in general, the three-dimensional structure includes all types of structure above the level of primary structure.

Three-dimensional structures of several hundred proteins have been determined by X-ray crystallography, yielding the location and bonding of all the atoms in the polypeptide chains. Detailed three-dimensional protein structure can also be determined by NMR methods (33), which are advantageous because the structure of proteins in solution can be determined in contrast to the requirement of protein crystals in X-ray diffraction measurement. It is now possible to determine the three-dimensional structure of proteins with molecular weights up to 20,000 routinely using NMR, provided that the protein can be obtained in large-enough quantities and is soluble and stable at room temperature over a period of days. This limit of molecular sizes is quickly increasing as described above (28).

Oligomeric proteins and multienzyme complexes such as pyruvate dehydrogenase are representative of the lowest level of macromolecular structural organization. "Su-pramolecular structures," such as ribosomes or the membranous components of the electron-transport chain, are examples of higher levels of macromolecular organization, which is the structural basis of life (3).

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