In globular proteins there are

In globular proteins there are areas that have a betasheet structures. Explain if an exhange of histidine residues toalanine residues in these areas will change the functionality ofthese proteins. Why.



Beta Sheets are multiple strands of polypeptides connected toeach other through hydrogen bonding in a sheet-like array. Hydrogenbonding occurs between the NH and CO groups between two differentstrands and not within one strand, as is the case for an alphahelical structure. Due to its often rippled or pleated appearance,this secondary structure conformation has been characterized as thebeta pleated sheet. The beta strands can be arranged in a parallel,anti-parallel, or mixed (parallel and anti-parallel) manner.

   The anti-parallel configuration is the simplest.The N and C terminals of adjacent polypeptide strands are oppositeto one another, meaning the N terminal of one peptide chain isaligned with the C terminal of an adjacent chain. In theanti-parallel configuration, each amino acid is bonded linearly toan amino acid in the adjacent chain.The parallel arrangement occurswhen neighboring polypeptide chains run in the same direction,meaning the N and C terminals of the peptide chains align. As aresult, an amino acid cannot bond directly to the complementaryamino acid in an adjacent chain as in the anti-parallelconfiguration. Instead, the amino group from one chain is bonded toa carbonyl group on the adjacent chain. The carbonyl group from theinitial chain then hydrogen bonds to an amino group two residuesahead on the adjacent chain. The distortion of the hydrogen bondsin the parallel configuration affects the strength of the hydrogenbond because hydrogen bonds are strongest when they are planar.Therefore, due to this distortion of hydrogen bonds, parallel betasheets are not as stable as anti-parallel beta sheet.

The side chains of beta strands are arranged alternately onopposite sides of the strand. The distance between amino acids in abeta strand is 3.5A which is longer in comparison to the 1.5Adistance in alpha strands. Because of this, beta sheets are moreflexible than alpha helices and can be flat and somewhat twisted.The average length of beta sheets in a protein is 6 amino acidresidues. The actual length ranges from 2 to 22 residues.

Ramachandran Plot: Beta strands are found in the purpleregionBeta sheets are graphically found in the upper left quadrant of aRamachandran plot. This corresponds to ? angles of 0 to 180 and ?angles of -180 to 0.

Visual representations in 3D models for beta sheets aretraditionally denoted by a flat arrow pointing in the direction ofthe strand.Loop is everything, but what is alpha helix andbeta-strand does. It is related to SECONDARY structure ofprotein.The Ramachandran Diagram, created by GopalasamudramRamachandran, helps to determine if amino acids will form alphahelices, beta strands, loops or turns. The Ramachandran Diagram isseparated into four quadrants, with angle ? as the x axis and angle? as the y axis. The combinations of torsion angles will put theamino acids in specific quadrants, which determine whether it willform an alpha helix, beta strand, loop, or turn. Those that fall inquadrants 1 and 3 a few times in a row form alpha helices, andthose that repeat in quadrant 2 form beta strands. Quadrant 4 isgenerally disfavored because of steric hindrance. Also, it ismostly impossible because the different torsion angles combinationsin quadrant 4 can’t exist because they cause collisions between theatoms of the amino acids. If the amino acids land in the differentquadrants, with no repeats, then they become loops or turns.Furthermore, the principle of steric exclusion states that twoatoms cannot occupy the same place simultaneously.

For example these are the beta-sheets: Val, Ile, Tyr, Cys, Trp,Phe, Thr.

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