![]() ![]() Β-bend structure consists of at least 4 amino acids in which nth amino acid is hydrogen bonded with (n+3)th amino acids.β-bend reverse the direction of polypeptide chain and helps it to form globular (spherical ) structure.In β-sheet hydrogen bond may be inter chain or intra chain but they are always inter chain in α-helix.The axial distance between two nearest amino acid is 3.5 Å in contract with 1.5 Å in α-helix.Polypeptide backbone in β-sheet is extended rather than being tightly coiled as in α-helix.Β-sheet differ from α-helix in many characteristics as given below Therefore, it is also known as β-pleated sheet. The surface of β-sheet is not straight but it is pleated.R-group of amino acids are alternately projected above and below the plane of β-sheet.In β-sheet structure, two polypeptide backbone are linked with each other by H-bond which are formed between –CO and –NH group.It can also be formed by folding of same polypeptide chain.It is formed between two different polypeptide chains which are placed parallel or antiparallel to each other.β-Sheet is the most stable form of secondary structure of protein.Aminoacids glycine and proline bring bend in polypeptide chain and disrupt α-ghelix.Similarly aminoacids with bulky R-group disrupt α-helix by steric interference. For examples, the aminoacids with charged R-group disrupt α-helix by electrostatic repulsion or by formation of ionic bond. R-group of aminoacid in α-helix are projected outward to minimize steric hindrance.Therefore, -C=O and –NH group of all aminoacids are hydrogen bonded in α-helical structure. In α-helix, -C=O group of each aminoacid is hydrogen bonded with –NH group of other aminoacid which is situated four aminoacid ahead.Therefore, the vertical distance between two nearest aminoacids is 1.5Å which is called Identity period of α-helix. Vertical length of each helix is known as pitch which is 5.4 Å. Each helix in α-helix structure contains 3.6 aminoacids residues.α-helix is a right handed helical structure formed by twisting of polypeptide chain.There are three commonly occurring secondary structure.Formation of secondary structure involve local folding of polypeptide chain. ![]() Aminoacids that are located near to each other interacts to form regular arrangement called secondary structure.However, bonds between –Cα and –NH and –Cα and –CO rotate freely during folded structure formation. Due to partial double bond character between –CO and –NH group, they do not rotate during formation of folded secondary and tertiary structure.The peptide bond occurs in TRANS-configuration and have partial double bond character.Since, most of α-COOH and α-NH2 group formed peptide bond, they are not available for other bonding except Hydrogen bonding.The end at which –COOH group is free is called C-terminal and other end at which –NH2 group is free is called N-terminal of polypeptide chain.However, two aminoacids which are situated at either end of polypeptide chain have either –COOH group free or –NH2 group free. In polypeptide chain α-COOh group and α-NH2 group of most aminoacids are involved in formation of peptide bond.The Peptide bond linked successive aminoacids in polypeptide chain. α-COOH group (carboxyl group) of one aminoacid is linked with α-NH2 group (amin0 group) of other aminoacid by peptide bond.Primary structure of protein refers to the sequence and arrangement of aminoacids in polypeptide chain. ![]() Proteins are structurally organized into four level primary structure, secondary structure tertiary structure and quaternary structure.Note that because of processes such as the post-translational modifications to proteins we still need protein sequencing and I believe that we currently rely too heavily on DNA sequencing. This is because it is now much easier to sequence DNA. ![]() Instead, since it has been worked out (mostly) how DNA codes for protein, we usually infer the protein sequence from the DNA sequence. However, it is now relatively rare to directly determine protein sequence! The very first protein sequence (bovine insulin) was determined by Fredrick Sanger in 1951-2 (note that this was more than a decade before the first nucleotide sequence). There are many different techniques for directly determining protein sequences - this wikipedia article is a decent introduction: There are also methods that have been developed to remove amino acids one at a time.īy combining theses techniques it is possible to directly determine protein sequences. This is a great question, but actually quite complicated so I'm not going to try to give a complete answer - I have given some useful links below if you wish to learn more.Įach amino acid has unique chemical properties that can be used to tell them apart. ![]()
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