Hierarchical Structure of Proteins
Proteins are linear polymers of amino acids linked together by peptide bonds. A protein can have a single polypeptide chain or multiple polypeptide chains. The primary structure of a polypeptide chain is the sequence of covalently linked amino acids that compose the chain. Various, mostly noncovalent interactions between amino acids in the linear sequence stabilize a protein’s specific folded three-
The α helix, β strand and sheet, and β turn are the most prevalent elements of protein secondary structure. Secondary structures are stabilized by hydrogen bonds between atoms of the peptide backbone (see Figures 3-4–3-6).
Protein tertiary structure results from hydrophobic interactions between nonpolar side groups and from hydrogen bonds and ionic interactions involving polar side groups and the polypeptide backbone. These interactions stabilize the folding of the protein, including its secondary structural elements, into an overall three-
Entire proteins or segments of proteins usually fall into one of four broad structural categories: globular proteins, fibrous proteins, integral membrane proteins, and intrinsically disordered proteins.
The exceptional conformational flexibilities of intrinsically disordered proteins contribute to their functions as binding partners, signaling molecules, regulators of other molecules, scaffolds, flexible links between well-
Certain combinations of secondary structures give rise to structural motifs, which are found in a variety of proteins and are often associated with specific functions (see Figure 3-10).
Proteins often contain distinct domains, independently folded regions with characteristic structural, functional, and/or topological properties.
The incorporation of domains as modules in different proteins in the course of evolution has generated diversity in protein structure and function.
The number and organization of individual polypeptide subunits in multimeric proteins define their quaternary structure.
Cells contain large supramolecular assemblies, sometimes called molecular machines, in which all the necessary participants in complex cellular processes (e.g., DNA, RNA, and protein synthesis; photosynthesis; signal transduction) are bound together.
Proteins with similar amino acid sequences generally can be assumed to have similar three-
81
Homologous proteins are proteins that evolved from a common ancestor and thus have similar sequences, structures, and functions. They can be classified into families and superfamilies.