Structural hierarchy quaternary

Figure 11.4 The structural hierarchy in proteins, (a) A segment of primary structure (b) secondary structure illustrated as a segment of alpha helix (c) tertiary structure in which helices are interspersed with coils, and (d) quaternary structure. (Illustration, Irving Geis/Geis Archive Trust. Copyright Howard Hughes Medical Institute. Reproduced with permission.)...

Figure 15.22 The structural hierarchy of proteins. A typical protein s structure can be viewed at different levels. Primary structure (shown as a long string of balls leaving and returning to the picture frame) is the sequence of amino acids. Secondary structure consists of highly ordered regions that occur as an a-helix or a p-sheet. Tertiary structure combines these ordered regions with more random sections. In many proteins, several tertiary units interact to give the quaternary structure.

There are several structural tiers that exist in a folded protein, sometimes referred to as the hierarchy of protein structure. Although different types of proteins can fold in very different ways, this structural hierarchy can be applied generally to all proteins primary, secondary, tertiary, and quaternary structure. 

Many proteins consist of two or more interacting polypeptide chains of characteristic tertiary structure, each of which is commonly referred to as a subunit of the protein. Subunit organization constitutes another level in the hierarchy of protein structure, defined as the protein s quaternary (4°) structure (Figure 5.10). Questions of quaternary structure address the various kinds of subunits within a protein molecule, the number of each, and the ways in which they interact with one another. 

The hierarchy of protein structure is illustrated in hgure 11.4. Here too we have a wealth of structural information. The quaternary structures for many proteins are now known and generally available in databases. As complex as these are, this is not the end of the story. We have atom-by-atom structures for entities as complex as viruses and the ribosome, an intracellular RNA-protein complex and the site of protein synthesis. Modem structural biology continues to provide detailed insights into some of the most complex constracts of nature. We are better off for having these insights. 

Protein domains are conserved regions of a limited number of amino acids that can bind diverse partner molecules to form structures of a higher complexity. The domains have a heterogeneous internal organization that consist of amino acid interactions and comprise multiple functionally distinct sectors or subdivisions. The sectors are regions of proteins that are distinct from the hierarchy of primary, secondary, tertiary, and quaternary structures. 

The structure of any one protein is described in terms of a hierarchy of structures primary, secondary, tertiary, and quaternary. The primary structure is the sequence of amino acids along the polypeptide chain. This is generally synonymous with the covalent structure of the protein, except that any disulfide bridges may alternately be... 

There is a natural hierarchy in proteins (see Figure 15.1) which allows the complex three-dimensional structure to be simplified and categorized as combinations of smaller motifs. At the atom level there are patterns of side-chain interactions at the backbone level we see formation of secondary structure (a helix, P sheet and yS turn) and loop families these combine to give supersecondary structures (e.g. P hairpins) and motifs (e.g. Greek key) and ultimately the whole tertiary and quaternary structure. In this chapter we present an overview of current patterns which are observed... 

Some proteins can be complex, when they contain multiple subunits of polypeptide structural entities. The way in which three-dimensional subunits interact to form the complete functional protein is called the quaternary structure of a protein. This level of hierarchy is possible only if the protein has multiple units. An example is hemoglobin. 

One of the most remarkable properties of self-assembly is its ability to generate exceedingly coinplica ted supramolecular structures from fairly simple components. Perhaps the most elegant embodiment of this phenomenon is protein stmcture. Proteins exhibit at least four hierarchies of stmcture primary, secondary, tertiary, and quaternary stmctures. Primary stmcture describes the covalent connections making up the sequence of amino acids in each strand. Secondary stmcture involves local architectural elements created when portions of a strand... 

Going into the physics of biopoljmiers, the first thing to understand is the hierarchy of their structiu-e. As you must have guessed from the title of this Section, there is a primary, a secondary, and a tertiary structure, and sometimes even a quaternary one. 

There is just one more question we would like to broach to conclude this chapter. We have been using the words primary, secondary, tertiary. And what comes next Sometimes the name quaternary structure is introduced when a few protein globules are stuck together, or when one protein chain forms a number of little globules. Clearly, there is the whole hierarchy of structures There are complexes of chains, these complexes form parts of cells, the cells make up tissues, and so on. 

Before we consider the relation one gene-one polypeptide in more detail, we must become acquainted with certain principles relating to the structure of proteins. There is a whole hierarchy of structural principles, which are referred to as primary, secondary, tertiary, and quaternary structure (Fig. 20). The term primary structure yq(qys to the sequence of the individual amino acids in the polypeptide chain. As already outlined, this amino acid,sequence is laid down in transcription and translation. The bonds which determine the primary structure are the peptide bonds between the different amino acid residues. 

The structure of a protein is generally understood in terms of an organizational hierarchy that consists of protein sequence, local secondary structure, tertiary stmcture, and finally quaternary structure. The study of protein structure in these terms has led to a greater understanding of... 

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