Question

In globular proteins there are areas that have a beta sheet structures. Explain if an exhange of histidine residues to alanine residues in these areas will change the functionality of these proteins. Why.

Answer 1

Ans:

Beta Sheets are multiple strands of polypeptides connected to each other through hydrogen bonding in a sheet-like array. Hydrogen bonding occurs between the NH and CO groups between two different strands and not within one strand, as is the case for an alpha helical structure. Due to its often rippled or pleated appearance, this secondary structure conformation has been characterized as the beta 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 opposite to one another, meaning the N terminal of one peptide chain is aligned with the C terminal of an adjacent chain. In the anti-parallel configuration, each amino acid is bonded linearly to an amino acid in the adjacent chain.The parallel arrangement occurs when neighboring polypeptide chains run in the same direction, meaning the N and C terminals of the peptide chains align. As a result, an amino acid cannot bond directly to the complementary amino acid in an adjacent chain as in the anti-parallel configuration. Instead, the amino group from one chain is bonded to a carbonyl group on the adjacent chain. The carbonyl group from the initial chain then hydrogen bonds to an amino group two residues ahead on the adjacent chain. The distortion of the hydrogen bonds in the parallel configuration affects the strength of the hydrogen bond because hydrogen bonds are strongest when they are planar. Therefore, due to this distortion of hydrogen bonds, parallel beta sheets are not as stable as anti-parallel beta sheet.

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

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

Visual representations in 3D models for beta sheets are traditionally denoted by a flat arrow pointing in the direction of the strand.Loop is everything, but what is alpha helix and beta-strand does. It is related to SECONDARY structure of protein.The Ramachandran Diagram, created by Gopalasamudram Ramachandran, helps to determine if amino acids will form alpha helices, beta strands, loops or turns. The Ramachandran Diagram is separated into four quadrants, with angle ? as the x axis and angle ? as the y axis. The combinations of torsion angles will put the amino acids in specific quadrants, which determine whether it will form an alpha helix, beta strand, loop, or turn. Those that fall in quadrants 1 and 3 a few times in a row form alpha helices, and those that repeat in quadrant 2 form beta strands. Quadrant 4 is generally disfavored because of steric hindrance. Also, it is mostly impossible because the different torsion angles combinations in quadrant 4 can't exist because they cause collisions between the atoms of the amino acids. If the amino acids land in the different quadrants, with no repeats, then they become loops or turns. Furthermore, the principle of steric exclusion states that two atoms cannot occupy the same place simultaneously.

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