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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.