Question

A synthetic 20 amino acid polypeptide named Betanova was designed as a small soluble molecule that would theoretically form stable beta-sheet structures in the absence of disulfide bonds. NMR of Betanova in solution indicates that it does, in fact, form a three-stranded antiparallel beta-sheet. Given the sequence of Betanova below where would you predict lie the turns in this sequence.
Draw the structure of Betanova showing strands and turns:
Betanova RGWSVQNGKYTNNGKTTEGR

Answer 1

Answer:

The β-Turns in this sequence are two and include the Glycine (G), the two anterior amino acids, and the one posterior amino acid.

You will find the molecule image in the attached file.

Explanation:

β-Turns are structural elements that intervene in the orientation of the polypeptide backbone. These are short sequences of amino acids, with a characteristic conformation, that imposes a sharp 180º turn on the polypeptide chain. They are very abundant in globular proteins, such as Betanova, connecting the two adjacent antiparallel segments of the sheet.    

They are mostly composed of four amino acids. In most cases, the third amino acid is usually Glycine, and in other cases, the second amino acid is Proline. In any case, the first residue of the sequence forms a hydrogen bridge with the last residue of the sequence, linking the amide-group of the last amino acid with the carbonyl-group of the first amino acid in the sequence.    

According to the given sequence of the protein, we can find two specific regions in the molecule that contains glycine (G). Proline is not present in Betanova. Following the theory, in most turns, Gly seems to be the third amino acid of the short sequence. So, we can assume that the first two amino acids that are located before the G and the amino acid located after the G, are the ones that compose the β-Turns.

RGWSVQNGKYTNNGKTTEGR      

• The first turn (QNGK) turns the direction of the polypeptide 180º, going forward to the opposite side.
• The second turn (NNGK) turns again the direction of the polypeptide, driving the last strain to the other side.          

In the attached image, you will find the structure of Betanova showing strands (black arrows) and turns (red arrows).