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I can conceptually grasp the beta pleated anti-parallel sheet by imagining a curved length of string i.e. one polypeptide, starting with the N- terminus on the left and ending with the C-terminus at the end of the string, opposite the N terminus.

I can't do the same for the beta pleated parallel sheet if it is only formed of one polypeptide. How could a fold in a single strand cause the N and C termini to be parallel with each other?

Or is it that parallel sheets are always composed of more than one polypeptide?

Comparison of parallel/antiparallel beta pleated sheets

Would single chain parallel sheet look more like this:

enter image description here

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    $\begingroup$ No, you just need a long linker, imagine perhaps a long Z instead of U on your diagram. $\endgroup$
    – Mithoron
    Aug 22 '20 at 17:51
  • $\begingroup$ Thanks - could you elaborate a little? $\endgroup$
    – Naj
    Aug 22 '20 at 20:33
  • $\begingroup$ Yes, that's the schematic diagram I was thinking about. It's very schematic, though, because this linker would circumvent the sheet altogether, perhaps even contain an alpha helix put onto it (for example). $\endgroup$
    – Mithoron
    Aug 23 '20 at 17:44
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While it is true that some antiparallel beta sheets have neighboring strands that are adjacent in primary sequence, other cases do exist. As for parallel beta sheets, the polypeptide chain between strands could be connected via a loop or a helix if the two strands are close in primary sequence.

The mapping of strands on the primary structure has been studied thoroughly, and there are two types of diagrams to quickly show what is going on. For the "TOPS" diagram, we pretend to look down the strands, and up and down triangles represent strands in one or the other direction:

enter image description here

Source: https://onlinelibrary.wiley.com/doi/pdf/10.1002/pro.3285

Circles represent helices. If you look closely at the connecting lines, you can see whether the connection is above or below the plane of the paper.

The other representation has the sheets in the plane of the paper, e.g.:

enter image description here

Source: https://image.slideserve.com/258316/beta-sheet-topology-diagrams-l.jpg

Take-home messages

  1. Sheets are antiparallel, mixed or parallel.
  2. Most neighboring strands are not adjacent in sequence.
  3. When two neighboring parallel strands are adjacent in sequence, there is a loop or a helix between them (or a strand from a different sheet) to connect them in the proper orientation.
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  • $\begingroup$ The Beta Sheet Topology Diagram helped a lot - thank you :) $\endgroup$
    – Naj
    Aug 24 '20 at 9:04
  • $\begingroup$ +1 for using tops diagrams :) $\endgroup$
    – gilleain
    Aug 24 '20 at 9:42
  • $\begingroup$ @Naj Yes, first comes the hydrophobic collapse, then some shuffling to get secondary structure (main chain interactions) and tertiary stucture (side chain interactions) all sorted out. There is, however, a strong indication which secondary structure a segment of primary sequence will adopt, regardless of the rest of the sequence. For a full answer, ask another question. $\endgroup$ Aug 24 '20 at 16:37
  • $\begingroup$ @KarstenTheis I actually deleted the comment you responded to above, suddenly thinking it was a stupid question. For the sake of any one turning up here and wondering what you were replying to, I asked if main chain interactions (secondary structure) and side chain interactions (tertiary structure) might happen in a non-temporal sequence i.e. not necessarily secondary then tertiary, maybe tertiary then secondary, maybe both at the same time. 'Shuffling' between the two seems to confirm that I was somewhat correct in my hypothesis. New question coming soon :) $\endgroup$
    – Naj
    Aug 25 '20 at 6:38
  • $\begingroup$ @KarstenTheis Note: New question will be regarding "There is, however, a strong indication which secondary structure a segment of primary sequence will adopt, regardless of the rest of the sequence." $\endgroup$
    – Naj
    Aug 25 '20 at 6:45

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