Protein structural organization

J. Drenth, B. W. Low, J. S. Richardson, and C. S. Wright, The toxin-agglutinin fold—a new group of small protein structures organized around a 4-disulfide core, J. Biol. Chem., 255 (1980) 2652-2655. 

The architecture of protein molecules is quite complex. Nevertheless, this complexity can be resolved by defining various levels of structural organization. 

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. 

Many enzymes carry out their catalytic function relying solely on their protein structure. Many others require nonprotein components, called cofactors (Table 14.2). Cofactors may be metal ions or organic molecules referred to as coenzymes. Cofactors, because they are structurally less complex than proteins, tend to be stable to heat (incubation in a boiling water bath). Typically, proteins are denatured under such conditions. Many coenzymes are vitamins or contain vitamins as part of their structure. Usually coenzymes are actively involved in the catalytic reaction of the enzyme, often serving as intermediate carriers of functional groups in the conversion of substrates to products. In most cases, a coenzyme is firmly associated with its enzyme, perhaps even by covalent bonds, and it is difficult to... 

Thomson Click Organic Interactive to use interactive animations to view aspects of protein structure. 

Similar residues in the cores of protein structures especially hydrophobic residues at the same positions, are responsible for common folds of homologous proteins. Certain sequence profiles of conserved residue successions have been identified which give rise to a common fold of protein domains. They are organized in the smart database (simple modular architecture research tool) http //smait.embl-heidelberg.de. 

Sequence conservation is, in general, much weaker than structural conservation. There are proteins, which are clearly not related in sequence but are closely related in 3D-stmcture and fold, like heamoglobin and myoglobin, which have similar functions. In many proteins, fold elements like 4-helical bundles are repeated. Classifications of known structural folds of proteins are organized in the SCOP or CATH database see e.g., http //scop.mrc-lmb.cam.ac.uk/scop/. 

Proteins of the cytoskeleton play a central role in the creation and maintenance of cell shapes in all tissues. They serve multiple roles in eukaryotic cells. First, they provide structural organization for the cell interior, helping to establish metabolic compartments. Second, cytoskeletal structures serve as tracks for intracellular transport, which creates and maintains differentiated cellular functions. Finally, the cytoskeleton comprises the core framework of cellular morphologies. 

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