File Name: alpha helix and beta pleated sheets .zip
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Protein structure can be discussed at four distinct levels. Below is a Lewis structure of a short segment of a protein with the sequence CHEM cysteine - histidine - glutamate - methionine. Secondary structure is three-dimensional, but is a local phenomenon, confined to a relatively short stretch of amino acids. For the most part, there are three important elements of secondary structure: helices, beta-sheets, and loops. In a helix, the main chain of the protein adopts the shape of a clockwise spiral staircase, and the side chains point out laterally.
This structure occurs when two or more, e. This can happen in a parallel arrangement:. Parallel and anti-parallel arrangement is the direct consequence of the directionality of the polypeptide chain. In anti-parallel arrangement, the C-terminus end of one segment is on the same side as the N-terminus end of the other segment. In parallel arrangement, the C-terminus end and the N-terminus end are on the same sides for both segments. The "pleat" occurs because of the alternating planes of the peptide bonds between amino acids; the aligned amino and carbonyl group of each opposite segment alternate their orientation from facing towards each other to facing opposite directions. 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.
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The crystal structure of CcdB, a protein that poisons Escherichia coli gyrase, was determined in three crystal forms. The protein consists of a five-stranded antiparallel beta-pleated sheet followed by a C-terminal alpha-helix. In one of the loops of the In one of the loops of the sheet, a second small three-stranded antiparallel beta-sheet is inserted that sticks out of the molecule as a wing. This wing contains the LysC proteolytic cleavage site that is protected by CcdA and, therefore, forms a likely CcdA recognition site. A dimer is formed by sheet extension and by extensive hydrophobic contacts involving three of the five methionine residues and the C terminus of the alpha-helix. The surface of the dimer on the side of the alpha-helix is overall negatively charged, while the opposite side as well as the wing sheet is dominated by positive charges.
In the following we will focus on the general aspects of protein secondary structure. The prediction was confirmed when the first three-dimensional structure of a protein, myoglobin by Max Perutz and John Kendrew was determined by X-ray crystallography. To get a better impression of how a helix looks like, only the main chain of the polypeptide is shown, no side chains. There are 3. Each residue is translated 1. Together these groups form a hydrogen bond, one of the main forces in the stabilization of secondary structure in proteins.
Прочитал, - сказал Хейл самодовольно, стараясь извлечь как можно больше выгоды из этой ситуации. - В одном из ваших мозговых штурмов. - Это невозможно. Я никогда не распечатываю свои мозговые штурмы. - Я знаю.
Росио пожала плечами. - Сегодня днем. Примерно через час после того, как его получила. Беккер посмотрел на часы - 11. За восемь часов след остыл. Какого черта я здесь делаю.
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