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 Encyclopedia of Physical Science and Technology  EN013D-616  July 27, 2001  12:5







              Protein Structure                                                                           207















                                  ββ motif                                         βαβ motif
                                     (a)                                              (b)































                            αα motif                                       helix-turn-helix motif
                               (c)                                                 (d)

                     FIGURE 10 Ribbon representations of the (a) ββ, (b) βαβ, (c) αα motif, and (d) helix-turn-helix motif. Various forms
                     of the αα motifs are found depending on the manner in which the α-helices associate. (c) Shows the alignment of two
                     helices joined by a short connection. (d) Shows the helix–turn–helix motif associated with calcium binding proteins.

              the α-helices and β-strands. In turn, this has provided  ing these secondary structural elements are segregated),
              insight into the common underlying principles of pro-  and small proteins that are stabilized by metal ligands
              tein structure. Several important databases exist of pro-  or disulfide bonds. Additional classifications have been
              tein structures and tertiary structure classification. These  added to incorporate multidomain proteins, membrane
              include the RSCB (//www.rcsb.org/pdb/), CATH classifi-  proteins, and peptides. Representative members of these
              cation (//www.biochem.ucl.ac.uk/bsm/cath/), and SCOP  families are discussed in the following.
              (//scop.mrc-lmb.cam.ac.uk/scop/) structural databases.
              The first includes all of the coordinates for structures
                                                                E. All α-Proteins
              that have been made publicly available. The second two
              databases contain structural classifications for all the pro-  Proteinsthatfallintothisclasstypicallyconsistofpredom-
              tein deposited in the RSCB. Both of these systems ini-  inately α-helices (>60%), but may contain a small amount
              tially classify proteins into five major groups: all α, all  of β-sheet at their periphery (See CATH Classification).
              β, α/β (where these secondary structural elements alter-  Historically the first two protein structures determined,
              nate through the fold), α + β (where the sections contain-  myoglobin and hemoglobin, belonged to the all-α class.
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