the beta sheet
/ BAY-tuh sheet /
If the alpha helix is a chain coiled into a spring, the beta sheet is a chain laid out flat and folded back and forth, so that several stretches lie side by side like the planks of a boardwalk or the pleats of a paper fan. It is the second of the two great patterns of protein secondary structure.
Each near-straight stretch is called a beta strand, and a sheet is several strands lined up next to one another. The strands are stitched together by hydrogen bonds between their backbones — the C=O of one strand bonding to the N-H of its neighbor, again and again across the sheet. The strands can run the same way (parallel) or in opposite directions (antiparallel), and the sheet usually has a gentle twist or pleat rather than being perfectly flat. The side chains stick up and down out of the sheet, alternating above and below, so one face of a sheet can be water-fearing and the other water-loving.
Beta sheets give proteins flat, strong surfaces and rigid barrels and sandwiches; they are the load-bearing fabric of silk and the walls of many transport proteins. There is also a darker side worth being honest about: when proteins misfold, their strands can stack into long, abnormally stable beta-sheet fibers called amyloid, the kind of aggregate found in Alzheimer's and other diseases. The same hydrogen-bonded sheet that makes silk strong can, gone wrong, make a fiber the body cannot clear.
Spider dragline silk owes its remarkable strength to stacked beta sheets: many strands hydrogen-bonded side by side, then crystallized into tiny tough blocks embedded in a stretchier matrix.
Side-by-side strands, hydrogen-bonded into a sheet, make a strong flat fabric.
Like the helix, the sheet is held by backbone hydrogen bonds — but here they run between separate strands, which may be far apart in the sequence yet close in space. Misfolded sheets (amyloid) underlie several diseases.