Nếu chúng ta tưởng tượng một tờ trong mặt phẳng của trang này, chúng ta có thể có cả hai dây chuyền đang chạy trong cùng một hướng, từ C-ga cuối ở phía trên cùng của trang để ga cuối-N tại botton. Ngoài ra, chúng ta có thể có hai chuỗi chạy theo hướng ngược nhau đối với vị trí của họ Nand C-Termini | SECONDARY STRUCTURE 61 Figure Three common forms of p turn. polypeptide an intramolecular p sheet both types are found in natural proteins. If we imagine a p sheet within the plane of this page we could have both chains running in the same direction say from C-terminus at the top of the page to N-terminus at the botton. Alternatively we could have the two chains running in opposite directions with respect to the placement of their N-and C-termini. These two situations describe structures referred to as parallel and antiparallel -pleated sheets respectively. Again one finds both types in nature. p Turns A third common secondary structure found in natural proteins is the p turn also known as a reverse turn hairpin turn or p bend . The p turns are short segments of the polypeptide chain that allow it to change direction that is to turn upon itself. Turns are composed of four amino acid residues in a compact configuration in which an interamide hydrogen bond is formed between the first and fourth residue to stabilize the structure. Three types of p turn are commonly found in proteins types I II and III Figure . Although turns represent small segments of the polypeptide chain they occur often in a protein allowing the molecule to adopt a compact three-dimensional structure. Consider for example an intramolecular antiparallel p sheet within a contiguous segment of a protein. To bring the two strands of the sheet into register for the correct hydrogen bonds to form the polypeptide chain would have to change direction by 180 . This can be accomplished only by incorporating a type I or type II p turn into the polypeptide chain between the two segments making up the p sheet. Thus p turns play a very important role in establishing the overall three-dimensional structure of a protein. Other Secondary Structures One can imagine other regular repeating structural motifs that are stereo-chemically possible for polypeptides. In a series of adjacent type III p
