Higher level structure

Thus far we have concentrated on the primary structure (amino acid sequence) of a polypeptide. Higher level protein structure can be described at various levels, i.e. secondary, tertiary and quaternary  

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Actin monomers assemble to form actin filaments which then associate to form higher level structures like bundles, networks, and gels d mamically in cells. Assembly and disassembly takes place spontaneously with a variety of actin-binding proteins involved in these processes. It has been reported that activated structures preexist as spontaneously fluctuating structures even in the absence of external stimuli. In Listeria monocytogenes an intracellular... 

Proteins are characterized by their primary, secondary, tertiary, and quaternary structures. The primary structure is the sequence of the amino acids in the polypeptide chain that makes up the protein. Secondary structure refers to the hrst folding of the amino acid chain and reflects, for example, disulfide bonds. Tertiary structure (a monomer) is the final folded configuration of the protein that is controlled by the primary and secondary structures and is thermodynamically driven by the relative hydrophobicity of the component amino acids in the structure. Quaternary structure refers to the functional association of several polypeptides (monomers). For example, the final structure of hemoglobin consists of four associated monomers. Any change in the primary structure of a protein often results in changes to aU the higher level structure. Protein structures must be characterized and controlled during the production process. 

In summary, protein structures are formed from a-helices and P-strands which pack together in a limited number of ways to form higher-level structures. These structures constitute the hydrophobic core of proteins and exclude any turns or loops. In contrast, the hydrophilic outer shell of a protein contains turns, loops (c/. [30]), and the hydrophilic portions of a-helices and P-strands. 

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