Protein , folding patterns four-helix

The shorter chains of the 82- to 86-residue cytochromes C550 (from Pseudomonas ), 6553, " and C555 (from Chlorohiumd ) as well as the longer 112-residue polypeptide of cytochrome C2 from Rhodospirillum rubrum have nearly the same folding pattern as that in mitochondrial cytochrome c. However, the 128-residue chain of the dimeric cytochrome c from Rhodospirillum molischianum forms an antiparallel four-helix bundle (Fig. 16-8). This is the same folding pattern present in the ferritin monomer (Fig. 16-3), hemerythrin (Fig. 2-22), and many other proteins including cytochrome 562 of E. coli ° Cytochrome/, which functions in photosynthetic electron transport, is also a c-type cytochrome but with a unique protein fold. ... 

Proteins are constructed of modular systems or domains. These are portions of the polypeptide chain that can fold independendy into a stable structure. A protein may be just one domain, or may be comprised of many domains. A typical domain may be roughly 25 A in diameter and consist of 100-150 amino acid residues. For example, two a helices, joined by a loop (the helix-loop-helix motif) can give a calcium-binding motif or a DNA-binding motif. The so called Greek key motif consists of four antiparallel P strands arranged in a pattern reminiscent of one found in ancient Greekffiezes (80). The PaP motif consists of two P strands that are parallel but not necessarily adjacent connected by an a helix, which shields the strands from solvent. Some examples of these types of motifs are shown in Fig. 33. 

Figure 13.30. Illustration of the four levels of structure in the potassium channel KcsA protein. Left a small section of the protein showing the primary structure, that is, the sequence of aminoacids identified by the side chains, and the secondary structure, that is, the formation of an a-helix. Right the tertiary structure, that is, the formation of larger subunits conisting of several a helices (in this case each subunit consists of three helices), and the quaternary structure, that is, the arrangement of the three-helix subunits in a pattern with four-fold rotational symmetry around a central axis. The a-helix on the left is the longest helix in each of the subunits, which are each shown in slightly different shades. The ion channel that this protein forms is at the center of structure. (Figure provided by P.L. Maragakis based on data from Ref. [232].)...

---