This is a cartoon representation of the structure
of human protein m-Ras, a member of an important family of small GTPases. For reference, the
PDB entry for this structure is 1X1R. Now we will highlight a single beta strand
as a cartoon ribbon with the backbone atoms as sticks and the sidechain atoms as lines.
Notice how the sidechains stick out of the wide, flat sides of the ribbon, while the
backbone amide hydrogen and carbonyl oxygen atoms stick out of the narrow edge. When we put this single strand in the context
of a parallel beta sheet structure, we can highlight the hydrogen bonds stabilizing the
sheet. These appear as dashed lines. In the parallel sheet topology, the hydrogen bonds
may appear to zig-zag a little, because they are slightly out of plane. But two adjacent
strands of this sheet are anti-parallel to each other, and when we examine their hydrogen
bonding pattern, we may notice that the hydrogen bonds appear slightly more even and straight.
In anti-parallel sheets, the hydrogen bonding pattern can be planar. Both parallel and anti-parallel sheets have
the same number of hydrogen bonds per amino acid. This structure illustrates how a single beta
sheet can have both parallel and anti-parallel portions. The strands of the sheet have an
overall twist, such that the entire sheet is shaped like a saddle, sitting between other,
alpha helical portions, of the larger protein.