h-bonds patterns in beta-sheet

[Marc Parisien]

Hi All!

  I was looking at the beta-sheet of the Cranbin protein (1CRN). This
beta-sheet is composed of 2 strands (residues 1 to 4 and 32 to 35).
H-bonds partners in the beta-sheets are 1 <-> 35 and 3 <-> 33, like:

A)

    1    <->    35
    2               34
    3    <->    33
    4               32

    Q. Is there any reasons why the h-bonding is such?
    Is there any counter to the possibility of having h-bonds partners 2
<-> 34 and 4 <-> 32 instead of 1 <-> 35 and 3<-> 33, like:

B)

    1             35
    2   <->   34
    3             33
    4   <->   32

  this would inverse the side-chain orientation (the ones on top of the
sheet will now be on the bottom and vice-versa) impacting on the
surrounding side-chains, but if we look at only the sheet itself, is
there a reason why would nature prefer setup A over setup B?


Thanks for your time,
Marc.