Parallel beta pleated sheets are the beta sheets that have two polypeptide strands running in the same direction. These secondary structures are less stable than antiparallel beta pleated sheets since the hydrogen bonds in parallel beta-sheet are not linear. There are 12 atoms in each hydrogen bonded ring in a parallel beta sheet. In parallel beta sheets, all of the N-termini of polypeptide strands are oriented in the same direction. Antiparallel beta pleated sheets are the second major type of beta sheets of proteins.
In antiparallel beta sheets, the neighbouring two polypeptide strands run in the opposite direction. The number of atoms in each hydrogen bonded ring alternates between 14 and Since hydrogen bonds in an antiparallel beta-sheet are linear, it is more stable than parallel beta sheets.
In antiparallel beta sheets, N-terminus of one strand is adjacent to the C-terminus of the next strand. This arrangement forms the strongest inter-strand stability. Parallel beta pleated sheets have two polypeptide strands running in the same direction while antiparallel beta pleated sheets have two polypeptide strands running in the opposite directions.
So, this is the key difference between parallel and antiparallel beta pleated sheets. What would happen if it was parallel? Interestingly the answer is yes. The problem with parallel strands is that the DNA is not pairing in the known way it does when it is combined anti-parallel or Watson-Crick-pairs. I have found different images illustrating the problems:.
Both images are from this blog post originally from a publication cited there which is not available online. Both pictures show a distorted DNA structure. This article " NMR structure of a parallel-stranded DNA duplex at atomic resolution " shows a more schematic solution of this problem by showing the single pairings.
C and G cannot build up 3 hydrogen bonds as they do this in the Watson-Crick-pairing. This changes in structure disrupts the function of the DNA which for example depends on the reverse strand for proof reading.
Enzymes which replicate, transcribe, repair etc. DNA will not work on this changed structure, as their function depends on the structural features of the DNA. This is wasteful and perhaps even impossible without lots of added sequence. Since most folks accept that the beginnings of life were RNA or RNA-like molecules that were both functional which requires meaningful shapes as well as 'coding' DNA or RNA sequence , if shapes were overly convoluted or could not form, life cannot start.
There is another issue here which is more biological. In general, the enzymes that replicate and "read" DNA run in one direction along the single strand of DNA that they are operating on.
The fact that the other strand of DNA is running in the opposite direction makes it impossible for these enzymes to jump from one strand to the other and ensures that they are operating on the proper information to do their job. If the DNA strands were parallel, the enzymes would be likely to attach at a locus and then randomly read information from either strand, and the information produced would be gobbldygook. So while it is chemically possible to have parallel DNA, a biological system that was based on parallel DNA would be at a big competitive disadvantage and so if they ever arose, they were probably driven extinct by the systems we now see that use anti-parallel DNA.
Sign up to join this community. The best answers are voted up and rise to the top. M lecture about this i was like….. The N-terminus of one beta strand will be opposite the N-terminus of the other beta strand.
The parallel arrangement is less stable because the geometry of the individual amino acid molecules forces the hydrogen bonds to occur at an angle, making them longer and thus weaker. The N-terminus of one beta strand will be opposite the C-terminus of the other beta strand.
In the anti-parallel arrangement the hydrogen bonds are aligned directly opposite each other, making for stronger and more stable bonds. An anti-parallel beta-pleated sheet forms when a polypeptide chain sharply reverses direction. This can occur in the presence of two consecutive proline residues, which create an angled kink in the polypeptide chain and bend it back upon itself.
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