7 Combinatories and Topology of theβ-Sandwich andβ-Barrel Proteins 131Fig. 7.1.The schematic representation of an immunoglobulin variable domain
(sandwich-like family). (a)β-Sheet strands are numbered sequentially as they are
presented in a sequence. The strands 2, 3, 7, and 8 are shown; (b) Chain fold of
immunoglobulin variable domain of heavy chain (PDB code: 1ine). The drawing is
done using the MOLSCRIPT program [31].β-Sheet strands are shown as ribbons. (c)
Arrangement of the strands in two mainβ-Sheets. The interlocked pairs of strands
(i, i+1) and (k, k+ 1) correspond to the strands #2, 3 and 7, 8
main sheet; (3) strandkis found in one main sheet andk+ 1 – in the other;
(4) strandsiandkare located within the same sheet; they are antiparallel
to each other and linked by hydrogen bonds; (5) likewise, strandsk+1 and
i+ 1 are located within the other main sheet, are antiparallel to each other,
and H bonded. (Fig. 7.1b, c).
These two interlocked pairs form a sandwich-like substructure within SP.
Usually they are found in the middle of the sheets. The number of strands
interposed between strandsiandkvaried from 1 to 10, but in 80% of cases
the number of interposed strands was 2 – 4 (see fig. 7.1c and 7.2b; where the
number of strands between the strandsiandkis equal to 4 and 2, respec-
tively). Two interlocked pairs were detected in 94% of all analyzed structures.
Thus, this investigation led to the discovery of a central feature of the super-
secondary structure that is invariant in almost all SP.