| Quaternary structure is formed by interactions between peptide chains
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| Quaternary structure refers to a complex or an assembly of two or more separate peptide chains that are held together by noncovalent or, in some cases, covalent interactions. In general, most proteins larger than 50 kDa consist of more than one chain and are referred to as dimeric, trimeric, or multimeric proteins. Many multisubunit proteins are composed of different kinds of functional subunits, such as the regulatory and catalytic subunits. Hemoglobin is a tetrameric protein (Chapter 4), and beef heart mitochondrial ATPase has 10 protomers (Chapter 8). The smallest unit is referred to as a monomer or subunit. Figure 2.10 indicates the structure of the dimeric protein Cu,Zn-superoxide dismutase. Figure 2.11 is an overview of the primary, secondary, tertiary, and quaternary structures of a tetrameric protein.
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